- Books Name
- A TEXT OF BIOLOGY - CLASS XII
- ACME SMART PUBLICATION
- CBSE Class 12
From Origin of Earth to Origin of Life
1. Evolution is a slow, continuous and irreversible process of change.
2. Origin of Earth: The Big Bang Theory proposes that the universe had an explosive beginning. The universe originated about 20 billion years ago by a big bang (thermonuclear explosion) of a dense entity. About 4.6 billion years ago, our solar system was probably created when the gaseous cloud called Solar Nebula started to collapse under the force of its own gravity, until it became a flattened spinning disc of atoms and particles. Its central region heated up and became a star.
3. Earth is about 4.6 billion years old, and the oldest rocks that have persisted in recognizable form are about 3.8 billion years old. For many years, scientists believed that such ancient rocks did not contain any fossils, but they knew that fossils were simply too small to be seen clearly without an electron microscope.
4. The oldest microfossils discovered so far are that of cyanobacteria that appeared 3.3 to 3.5 billion years ago.
5. Massive limestone deposits called Stromatolites became frequent in the fossil record about 2.8 billion years ago. Produced by cyanobacteria, stromatolites were abundant in virtually all freshwater and marine communities until about 1.6 billion years ago.
6. The fossil records indicate that unicellular protists -the first eukaryotes -appeared about 1.5 billion years ago.
7. Basic unit of evolution is population.
According to recent literature, the first non-cellularforms of life could have originated 3 billion years back. They would have been giant molecules (RNA, protein, polysaccharides etc.) These capsules reproduced their molecules perhaps. The first cellular form of life did not possibly originate till about 2000 million years ago.
- Books Name
- A TEXT OF BIOLOGY - CLASS XII
- ACME SMART PUBLICATION
- CBSE Class 12
THEORIES ON THE ORIGIN OF LIFE
1. Theory of special creation states that the life was created by supernatural power in the form which has not undergone any change. It was given by Father Suarez. God created life in six days from materia prima and man was created by Him on the sixth day. According to this theory, earth is about 4000 years old.
2. Theory of catastrophism was given by Cuvier, according to which after a gap of certain period ( called age), the world undergoes a catastrophe (sudden calamity) which kills almost all the living organisms and then God creates a new generation or new life from inorganic matter.
3. Theory of biogenesis (i.e. life from life, omnis vivum ex. vivo) was proved by Redi, Spallanzani and Pasteur independently. They disproved (refuted) theory of spontaneous generation (abiogenesis). Francesco Redi (1668) proved that flies could not arise from putrefying meat without their eggs. Spallanzani (1767) demonstrated that putrefaction of meat is due to microbes in the air and it can be prevented by boiling and sealing the meat in air tight containers. Pasteur gave a definite proof of life arising from pre-existing life using microbes and sterilization methods. He performed "swan neck flask" experiment.
4. Cosmozoic theory (Theory of panspermia) given by Richter (1865); Helmholtz (1884), Arrhenius (1908) suggested that life reached the earth from some heavenly body through meteorites. Panspermia (primitive form of life, as suggested by Arrhenius, 1908) consisted of spores or seeds (sperms) and microbes that existed throughout universe and produced different forms of life on this earth.
Abiogenic or Chemical Origin of Life
Majority of the scientists are ofthe opinion that life originated from inanimate matter. Since the theory of abiogenic origin or chemical evolution of life is the only one that provides an explanation, which can be tested, most scientists have tentatively accepted it.
1. Alexander I. Oparin (1894-1980), a Russian biochemist, and J.B.S. Haldane (1892-1964), a British scientist, put forward the concept that the first living organism evolved from non-living material. They also suggested that the sequence of events that might have occured. In 1923, Oparin postulated that life originated on Earth at some point of time in the remote past, and under the conditions no longer observed. In his book, The Origin of Life (1938), Oparin submitted" abiogenesis first, butbiogenesis eversince". Oparin's theory is known as primary abiogenesis.
2. According to Oparin and Haldane (1929), spontaneous generation of early molecules might have taken place ifthe earth once had more reducing atmosphere compared to the present oxidising atmosphere. Oparin and Haldane agreed that the primeval Earth contained little, if at all, oxygen. Perhaps, in the primitive atmosphere oxygen in the free gaseous state was virtually absent. Therefore, no degradation of any organic compound arising in the primeval Earth could have taken place.
3. As there was no ozone layer in the atmosphere, any absorption of UV radiations, that is lethal to our present lives, was not possible in the primeval Earth.
4. The early gas cloud was rich in hydrogen, being present in the combined form in methane (CH4), ammonia (NH3) and water vapour (H2O).
5. Moreover, the atmospheric water vapour along with early gas cloud condensed into drops of water and fell as rain that rolled down the rock surfaces and accumulated to form liquid pools and oceans. In the process, erosion of rocks and washing of minerals (e.g., chlorides and phosphates) into the oceans were inevitable. Thus, Haldane's hot dilute soup was produced and the stage was set for combination of various chemical elements.
6. Atmospheric chemicals and those in water produced small precursor molecules, like amino acids, sugars, nitrogenous bases etc. These precursor molecules then combined resulting in the appearance of proteins, polysaccharides and nucleic acids.
7. The energy sources for such reactions of organic synthesis were the UV radiations (solar radiation), cosmic rays, electrical discharges (lightning), intense dry heat (volcanic eruption) and radioactive decay of various elements on the Earth's surface. Once formed, the organic molecules accumulated in water because their degradation was extremely slow in the absence of any life or enzyme catalysts. Such transformation is not possible in the present oxidising atmosphere because oxygen or micro organisms will decompose or destroy the living particle that may arise by mere chance.
Experimental Evidence for Abiogenic Molecular Evolution of Life
Harold C.Urey (1893 -1981), an astronomer, accorded the first adequate recognition of Oparin-Haldane's view on the origin of life in 1952.
Urey asked his student Stanely L. Miller, a biochemist, to replicate the primordial atmosphere as propounded by Oparin and Haldane.
Miller (1953) made the first successful simulated experiment to assess the validity of the claim for origin of organic molecules in the primeval Earth's conditions.
1. Mllier sealed in a spark chamber a mixture of water (H2O), methane (CH4), ammonia (NH3) and hydrogen gas (H2). He made arrangement for insertion of two electrodes to provide electrical energy (simulation of lightning) to the spark chamber. CH4, NH3, H2 were in 2 : 1 : 2 ratio and water vapour at 800ºC. Electric sparks of 75,000 volt was provided to the mixture.
2. The spark chamber was connected to another flask with arrangement for boiling water (provision for evaporation). The other end of the spark chamber was connected to a trap by a tube that passed through a condenser (an arrangement for condensation and collection of aqueous solution, equivalent to rain and Haldane's soup). The trap, in turn, was connected with the flask for boiling water (arrangement for circulation).
3. The control apparatus contained every arrangement except that it was devoid of energy source.
4. After eighteen days, significant amount of the simple organic compounds (monomers), such as amino acids and peptide chains began to appear in the aqueous sample of the experimental set. Amino acids found were alanine, glycine and aspartic acid. Therefore, the obvious inference was that abiotic synthesis of organic monomers occurred in the simulated experimental condition. By analogy, such synthesis could have occurred in the primitive atmospheric condition. Later on many scientists repeated Miller's experiment using slightly different starting materials and UV radiation or other energy sources. All of them could successfully synthesise amino acid and related compounds. With hydrogen cyanide (HCN), even adenine and other nitrogen bases were produced.
Abiotic synthesis of biomolecules is studied under following headings:
1. Chemogeny: Synthesis of organic molecules by chemical reactions.
2. Biogeny: Formation of self replicating biomolecules in broth (primordial hot soup or warm little pond).
3. Cognogeny: Evolution of various forms of life or diversification of existing groups.
Enclosing the Prebiotic Systems
The experiments of Miller and other scientists demonstrate that prebiotic molecules could have been formed under the conditions which most likely existed on early Earth.
Still, the formation of prebiotic soup of small molecules does not necessarily lead to the origin of life.
For origin of life, atleast three conditions needed to have been fulfilled:
1. There must have been a supply of self-replicators i.e., self-producing molecules.
2. Copying of these replicators must have been subject to error via mutation.
3. The system of replicators must have required a perpetual supply of free energy and partial isolation from the general environment.
The high temperature prevailing in early Earth would have easily fulfilled the second condition, that is, the requirement of mutation. The thermal motion would have continually altered the prebiotic molecules.
The third condition, partial isolation, has been attained within aggregates of artificially produced prebiotic molecules.
These aggregates called protobionts can separate combinations of molecules from the surroundings; maintain an internal environment but are unable to reproduce.
Two important protobionts are coacervates and microspheres.
Oparin (1924) observed that if a mixture of a large protein and a polysaccharide is shaken, coacervates form.
Their interiors, which are primarily protein and polysaccharide, with some water, become separated from the surrounding aqueous solution.
The later has much lower concentration of proteins and polysaccharide.
Oparin's coacervates also exhibits a simple form of metabolism.
As these coacervates do not have lipid outer membranes and cannot reproduce, they fail to fulfil the requirement as a candidate of probable precursors of life.
Microspheres were formed when mixtures of artificially produced organic compounds were mixed with cool water.
If the mixture contains lipids, the surface of the microspheres consists of a lipid bilayer, reminiscent to the lipid bilayer of cell membranes.
Sydney Fox (1950) obtained protenoid microspheres.
There is considerable discussion among biologists as to how the first cells may have evolved.
The discovery made in the 1980's that RNA can act like enzyme to assemble new RNA molecules on an RNA template raised the interesting possibility that Coacervates may not have been the first step in the evolution of life.
Perhaps the first macromolecules were RNA molecules, and the initial steps on the evolutionary line were ones leading to more complex and stable RNA molecules.
Later, the stability might have been improved by surrounding the RNA within a coacervate.
Still other scientists reject the notion of 'RNA world', entirely, pointing out that some RNA components are too complex to have been present on the primitive earth.
- Books Name
- A TEXT OF BIOLOGY - CLASS XII
- ACME SMART PUBLICATION
- CBSE Class 12
EVIDENCES OF EVOLUTION
1. EVIDENCES FROM ANATOMY
(i) Homologous organs :
These organs have the similar basic structure and developmental origin.
The organisms which possess such organs are said to have originated from common ancestor.
Consider, for example, the seal's flipper, the bat's wing, the horse's foot, the cat's paw and the human hand.
In plants, a thorn of Bougainvillaea differs from a tendril of Cucurbita in its function, both are located in a similar (axillary) position and have similar origin.
Thorns and tendrils are considered homologous.
Homologous organs show divergent evolution which means that the similar structures developed along different directions due to adaptions to different needs or Adaptive Radiation which is the development of dissimilar functional structures in closely related group of organisms.
(Homology indicates common ancestry. Other examples are comparison of heart and brain of vertebrates)
(ii) Analogous organs :
These organs which are not anatomically similar though they perform similar functions.
For example, the wings of birds and of butterfly look alike, they perform the similar function of flying but they are not anatomically or structurally similar.
Even the wings of birds and bats are also analogous structures which have different origins.
Other examples are flippers of penguin and dolphin (the former is a bird and the later is a mammal); eye of an octopus and the eye of a mammal, both differ in retinal position, still the function is same.
In plants, sweet potato (root modification) and potato (stem modification) is another example of analogy.
Both are meant for storage of food but modifications of different parts of plant.
Now, what is the reason of development of analogous structures?
The possible explanation may be that it is the similar habitat that has resulted in selection of similar adaptive features in different (distantly related) groups of organisms put toward the same function.
This phenomenon is termed adaptive convergence or convergent evolution which is the opposite of adaptive radiation as seen in the homologous structures.
(iii) Vestigial organs:
They are believed to be remnants of organs which were complete and functional in their ancestors.
The study of vestigial organs offer an evolutionary explanation of such rudimentary vestiges by stating that adaptations to new environment of the organism have made these structures redundant.
Such structures are called vestigial organs.
The rudiment of the reptilian jaw apparatus, the rudiment of the hind limbs of python and Greenland whales are some of the examples of vestigial organs.
In humans, many vestigial structures indicate a relationship to other mammals, including the primates.
For instance, muscles of the external ear and scalp are rudimentary and often non-functional.
But these are common to many mammals where they are functional.
The reduced tailbones and nictitating membrane of the eye, the appendix of the caecum, rudimentary body hair and wisdom teeth -all are examples of vestigial organs.
The appendix of man is thought to be a remnant of the large caecum -the storage organ for cellulose digestion in herbivorous mammals.
Similarly, the non-functional vestiges of the pelvic girdle in python and porpoise show, for instance, that the snake and the porpoise originally evolved from four footed ancestors.
Sudden reappearance of ancestral character is called atavism.
For example, tail in new born human baby.
The winged petiole of Citrus represents that the unifoliate condition is derived from the trifoliate leaf.
- Books Name
- A TEXT OF BIOLOGY - CLASS XII
- ACME SMART PUBLICATION
- CBSE Class 12
WHAT IS ADAPTIVE RADIATION?
Galapagos Islands are chain of 22 islands, present on the west coast of South America.
During his journey, Darwin went to Galapagos Islands.
There he observed an amazing diversity among creatures.
Of particular interest, small black birds, later called Darwin's finches, amazed him.
He realised that there were many varieties of finches in the same island.
All the varieties, he conjectured, evolved on the island itself.
From the original seedeating features, many other forms with altered beaks arose, enabling them to become insectivorous and vegetarian finches.
These birds in Galapagos island show differences in bills and feeding habits, but still resemble with the birds present on original mainland.
Hence, we have seen different species have evolved from single common ancestor.
And this process of evolution of differentm species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats) is called adaptive radiation of which the Darwin's finches represent one of the best example.
The evolution of Darwins finches on the Galapagos Islands is a classic example of species formation and illustrates how adaptation to local conditions produces the divergence, that is the heart of species formation, i.e., such an evolutionary process, giving rise to new species adapted to new habitats and ways of life, is called Adaptive Radiation or Divergent Evolution.
The clusters of species that have been formed on the Galapagos Islands are thus a Tasmanian wolf clear example of species formation arising by microevolutionary divergence from an ancestral form occupying different habitats of microevolution leading to macroevolution.
Another example is Australian marsupials.
A number of marsupials, each different from the others, evolved from an ancestral stock, but all within the Australian continent.
When more than one adaptive radiation appeared to have occurred in isolated geographical areas (representing different habitats), one can call this convergent evolution.
Placental mammals in Australia also exhibit adaptive radiation in evolving into varieties of such placental mammals each of which appears to be 'similar' to a corresponding marsupial (e.g., Placental wolf and Tasmanian wolf marsupial)
Placental mammals in Australia also exhibit adaptive radiation in evolving into varieties of such placental mammals each of which appears to be 'similar' to a corresponding marsupial (e.g., Placental wolf and Tasmanian wolf marsupial)
Picture showing convergent evolution of
Australian marsupials and placental mammals
3. EMBRYOLOGICAL EVIDENCES
The sequence of embryonic development in different vertebrates show striking similarities.
Gill clefts and notochord appear in the embryonic development of all vertebrates from fishes to mammals.
The notochord is replaced by the vertebral column in all adult vertebrates.
Similarly, gills are replaced by lungs in adult amphibians, reptiles and mammals.
Such similarities in embryonic development once again reinforce the idea of evolution from common ancestors.
Occasionally, embryonic features such as the tail and gill slits persist in adults.
According to Ernst Haeckel, ontogeny (development of embryo) is recapitulation of phylogeny (the ancestral sequence).
This view was summarised by his Biogenetic Law : Ontogeny Recapitulates Phylogeny.
Developmental evidence for evolution is also available from plants.
It is generally believed that mosses and ferns are more evolved than algae.
Protonema of mosses resembles certain green algae.
This provides a clue to their evolutionary relationship.
Both bryophytes and pteridophytes have ciliated sperms and require water for fertilisation.
Gymnosperms do not need water for fertilisation.
But Cycads and Gingko, the primitive gymnosperms, have ciliated sperms like the pteridophytes.
This suggests that gymnosperms have descended from pteridophyte-like ancestors.
The occurrence of ancestral traits in embryo is called Palaeogenesis.
4. PALAEONTOLOGICAL EVIDENCES
Fossils are the remains and/or impressions of organisms that lived in the past last few centuries and palaeontologists have painstakingly built up extensive collections of fossils from all over the world.
The fossil record has helped in building the broad historical sequence of biological evolution.
Phylogeny, the evolutionary history of the organism, can sometimes be reconstructed with the help of fossils.
Horse, elephant and man are good examples of relatively complete reconstructions of phylogeny.
Besides form and structure, the habits and behaviour of extinct species can be inferred from the well-preserved fossils.
It is also possible to reconstruct the entire habitat of an organism from fossils.
Fossils also indicate the connecting links between two groups of organisms.
Archaeopteryx shows features of both reptiles and birds.
Reptilian characters of Archaeopteryx.
(a) The body axis is more or less lizard-like
(b) A long tail is present.
(c) The bones are not pneumatic.
(d) The jaws are provided with similar teeth.
(e) Presence of a weak sternum.
(f) Presence of free caudal vertebrae as found in lizards.
(g) The hand bears typical reptilian plan and each finger terminates in a claw.
Avian characters of Archaeopteryx.
(a) Presence of feathers on the body.
(b) The two jaws are modified into a beak.
(c) The fore limbs are modified into wings.
(d) The hind-limbs are built on the typical avian plan.
(e) An intimate fusion of the skull bones as seen in the birds.
By careful analysis of the distribution of fossils in different strata of rocks, the time in history when different species were formed or became extinct can be inferred.
Time Line of Evolution
When scientists first began to study and date fossils, they had to find some way to organise the different time periods from which the fossils came.
They divided the earth's past into large blocks of time called eras.
Eras are further sub-divided into smaller blocks of time called periods, and some periods, in turn, are sub-divided into epochs.
The major geological eras, with their approximate dates in millions of years as given in the table.
"The geological time scale."
Table-I: THE GEOLOGICAL TIME SCALE
Types of Rocks –
(a) Sedimentary Rocks –
These are formed at the bottom of ancient oceans by deposition of sediments of sand, lime, coal and minerals which slowly change into hard layers.
Sedimentry rocks are also called as stratified rocks.
Eg. Lime Stone, Sand Stone.
Fossils are mostly found in sedimentary rocks.
(b) Igneous rocks –
Such rocks are formed by ancient volcanic deposits which slowly cooled down and hardened as rocks. Fossiles are absent in it. e.g., Granite rocks.
(c) Metamorphic rocks –
These are formed by change in chemical composition of sedimentary rock and igneous rocks or metamorphosis. Such metamorphosis may be caused by pressure heat and physical movements. e.g. Marble Slate rocks. Fossils are also absent in it – due to chemical changes in the fossils are destroyed.
Types of Fossils
(a) Unaltered Fossils :
In this type whole bodies of extinct organisms are found frozen in ice at the polar regions eg. Wooly mammoths (25000 yrs before extinct fossils were found from Siberian region)
(b) Petrified fossils – Most common type of fossil.
Replacement of organic part by mineral deposits is called petrification.
These fossils consists of only the hard parts e.g. bones, teeth, shells, wood etc. of extinct organisms.
In human body first fossilization occurs of teeth.
(c) Mould fossils –
Here no part of the original organism is present, only an impression of the external structure of body is preserved in wet soil.
(d) Cast fossils –
Sometimes minerals fills in the mould, resulting in cast fossils.
(e) Print Fossils –
Foot print or prints of wings, skin, leaves, stems etc made in soft mud which subsequently become fossilized are a common type of fossils.
(f) Coprolites –
These fossils include the fossil preservation of contents of the intestine or excreta of many ancient animals including particularly the reptiles or fishes.
By studying fossils following facts about organic evolution are evident –
1. Fossils found in older rocks are of simple type and those found in newer rocks are of complex types.
2. In the beginning unicellular protozoans were formed from which multicellular animals evolved.
3. Some fossils represents connecting links between two groups
4. Angiosperms among plants and mammals among animals are highly developed and modern organism
5. By fossils, we can study the evolutionary pedigree of an animal like stages in evolution of horse, elephant and man etc.
Our country has rich deposits of fossil plants spanning a gap of 3500 million years.
Twenty million years old fossil forests have been discovered and studied by the Birbal Sahni Institute of Palaeobotany, Lucknow. These forests need to be systematically studied and conserved for scientific understanding and enlightenment. Some of the excellent localities that can be raised to the status of national fossil parks are:
1. Fifty million years old fossil forests preserved in the sediments between the streaming lava flow that poured out into the Deccan country at Mandla district, Madhya Pradesh.
2. One hundred million years old fossil forest located in Rajmahal Hills, Bihar.
3. Two hundred and sixty million years old coal-forming forests in Orissa.
Microfossils and Fossil Fuel Exploration
Palaeobiological study helps in understanding and locating coal and hydrocarbon sources.
Palynofossils -tiny microscopic spores, pollen and other vegetal remains of the past -assist us in interpreting ancient environmental conditions favourable for organic matter accumulation and its conversion to fossil fuels by transformation and subsequent thermal alteration.
By quantitative analysis of microfossils, it is possible to determine the approximate location and configuration of near shore marine deposits, which are in turn responsible for formation and accumulation of hydrocarbons.
The main source of hydrocarbons are phytoplankton, marine and terrestrial algae as well as lipid-rich plant remains.
Thus, the study of fossil plants offers an effective tool in stratigraphical geology and can be exploited in tapping organic fuel resources.
Naturally occurring radioactive isotopes of certain elements are employed in this process.
Such isotopes are unstable and decay over the course of time at a steady rate, producing other isotopes.
One of the most widely employed methods of dating -the carbon -14 (14C) method uses estimates of the different isotopes present in samples of carbon.
Most carbon atoms have an atomic weight of 12.
But a fixed proportion of the atoms in a given sample of carbon consists of carbon with an atomic weight of 14 (14C).
14C is produced from 12C as a result of bombardment by particles from space.
But after an organism dies and is no longer incorporating carbon, its 14C gradually decays over time, by the loss of neutrons.
The common radioactive elements which lose their radioactivity and change into their non radioactive isotopes at a fixed rate are :
1. Potassium40 Argon40
2. Carbon14 Nitrogen14
3. Uranium238 Lead207
4. Rubidium87 Strontium87
6. Thorium232 Lead207
The half life of C-14 is 5730 ± 40 years which means in every 5730 ± 40 years, half of C14 will decay back to N14.
Radioactive carbon can be used to determine the age of fossils upto 70,000 years old.
Half life of potassium40 is 1.3 × 109 years.
Potassium -Argon method is useful because potassium is a common element found in all sorts of rocks.
Potassium decays into Argon extremely slowly.
Electron spin resonance method is relatively most accurate method for dating of fossils.
Evolution of Modern Horse
The evolution of modern horse began in the Eocene epoch.
The first fossil named Eohippus, 'dawn horse', was in North America.
This horse was about the size of a fox or terrier dog (a type of small dog for unearthing foxes), only 40 cm high at the shoulders.
It had short head and neck.
The fore limbs were with four complete fingers (2, 3, 4 and 5) and one splint of first finger and the hind limbs with three functional toes (2, 3 and 4) and one splint of fifth toe.
Splints are non-functional reduced fingers and toes of horse.
Teeth were with incomplete cement.
Molar teeth had no serrations.
Low-crowned molar teeth were adapted to browse soft lush vegetation.
Mesohippus, the intermediate horse, evolved from Hyracotherium about three crore years ago during Oligocene epoch.
It was of the size of modern sheep, about 60 cm high at the shoulders.
Fore feet had three fingers and one splint of fifth finger and hind feet possessed three toes, but the middle one was longer than others and supported most of the body weight.
Molar teeth had some serrations.
Merychippus, the ruminating horse, arose from Mesohippus in Miocene epoch about two crore years ago.
It was of the size of small pony, about 100 cm high at the shoulders.
It had a longer neck.
Its fore and hind limbs had three fingers and three toes, the middle finger and toe being longer than others and supported entire body weight.
There was no splint.
Teeth were longer with cement.
Molar teeth had well developed serrations.
Pliohippus, the Pliocene horse, evolved from Merychippus in Pliocene epoch about one crore years ago.
It was the size of modern pony, about 120 cm high at the shoulders.
Its each fore and hind limbs had one complete finger and one complete toe and two splints hidden beneath the skin.
Pliohippus is, therefore, referred to be the first one toed horse.
The, molar teeth were long with well developed cement and serrations.
Teeth were adapted for eating grass.
This is the modern horse which arose from Pliohippus in Pleistocene epoch about nine to ten lakh years ago in North America and later spread throughout the world except Australia.
It is about 150 cm high at the shoulders.
It has a long head and a long neck.
Each fore and hind limb of the modern horse has one finger and one toe and two splints.
The crowns of molar teeth are elongated with enameled ridges and are highly suitable for grinding.
During evolution of horse, there was:
(i) General increase (with occasional decrease) in size,
(ii) Progressive loss of toes,
(iii) Lengthening of toes that was retained,
(iv) Lengthening of limbs in general,
(v) Enlargement of brain, especially cerebral hemispheres,
(vi) Increase in height,
(vii) Increase in the complexity of molar teeth and an enlargement of the last three premolars until they came to resemble molars.
Connecting Links :
The organisms which possess the characters of two different groups. Examples
1. Proterospongia : A link between Protozoa and Porifera.
2. Neopilina : A connecting link between Annelida and Mollusca.
3. Peripatus : A connecting link between Annelida and Arthropoda.
4. Lungs fishes, e.g., Protopterus, Lepidosiren, Neoceratodus are considered the connecting links between the fishes and amphibians.
5. Egg laying mammals; example duck billed Platypus (Ornithorhynchus) and spiny ant eater (Echidna) are considered connecting links between reptiles and mammals.
Evolution of Vertebrates and Major Groups of Plants
The patterns of evolution of vertebrates and major groups of plants are conspicously different.
The major groups of vascular plants have left relatively small number of fossils which even show gaps (fossilless dark periods).
There are relatively few major lineages, and all the lineages are very distinct from one another.
Instead of showing gradual and continuous change through time, the major lineages appear suddenly in the fossil record.
After that, they persisted with little fundamental change for hundreds of millions of years.
The existence of many of the major subdivisions of the vascular plants living today can be recognised about 345 million years ago on the basis of their distinctive reproductive structure.
All primitive land plants reproduce via tiny spores contained in the sporangia. The major taxonomic groups are distinguished by the position of sporangia on the plant.
The sporangia are terminal, located at the tip of the plant in the most primitive Psilopsida.
These are placed at the base of the leaves in the Lycopsida (represented in the modern flora by Lycopodium and Selaginella).
The sporangia are arranged in whorls at the top of the plant in Sphenopsida (horsetails).
Fossil evidences document that these basic patterns have been maintained for more than 350 million years.
Few, if any, intermediates are known between these patterns.
The origin of seeds in the land plants was achieved about 345 million years ago in lineages recognised as ancestral to all more advanced vascular plants.
The last major evolutionary advancement among the vascular plants was the emergence of flowering plants (the angiosperms) about 140 million years ago.
But the fossils left no clue as to their ancestors.
The fossil records also indicate that nearly all the living orders of angiosperms and most of the characters of their modern-day representatives evolved from them.
The continuous change of a character within an evolving lineage is termed as evolutionary trend.
A lineage is an evolutionary sequence, arranged in linear order from an ancestral group to a descendant group.
The number of trends in any lineage is, therefore, same as the number of characters evolving.
A trend may be progressive (a general increase in size of organs) or retrogressive (a general degeneration and loss of organs).
- Books Name
- A TEXT OF BIOLOGY - CLASS XII
- ACME SMART PUBLICATION
- CBSE Class 12
The study of patterns of distribution of animals and plants in different parts of the earth is called Biogeography.
Alfred Russel Wallace (1823 -1913) divided the whole world into six major biogeographical regions or realms.
1. Palaearctic : Europe and Asia north of the tropics, north-western corner of Africa, including the Atlas Mountains.
2. Nearctic : North America exclusive of the tropics, Alaska, Canada, United States and Mexico.
3. Neotropical : Central America including low lands of Mexico, islands of the Caribbean and all of South America.
4. Ethiopian Africa (with exception of the Atlas Mountains), Madagascar and adjacent islands.
5. Oriental Tropical part of Asia (including India) south of the Himalaya Mountains and eastward through Sumatra, Java, Borneo and the Philippines.
6. Australian Australia, Tasmania, New Guinea and all islands of the Indonesian archipelago that lie east of Borneo, beginning with Celebes.
Biogeographic map of the world is that in which the six major biogeographic realms are present.
Geologists believe that millions of years ago, all the continents we demarcate today, were in the form of a single land mass.
On account of geological changes, especially movements of crustal plates below the surface of the earth, huge land masses broke off and drifted apart from one another.
As these land masses (continents) moved away, the seas separated them and acted as barriers to the free movement of organisms among the continents.
Because of variable environmental conditions prevailing on the different continents, over centuries, plants and animals evolved independently in each biogeographical region.
India falls under Oriental realm. Two geographical regions separated by a high mountain ranges are Palaearctic and Oriental.
Consider, for example, two instances which show similarities in the pattern of distribution of plants and animals between two land masses which were once part of a larger land mass.
1. The flora and fauna on each of the Galapagos Island -a chain of 22 islands in the Pacific ocean on the west coast of South America resemble those of the South American mainland with which the Galapagos Islands were once connected.
2. Magnolias, Tulips and Sassafras are found naturally growing in the eastern USA and in China. Hence, these show disjunct distribution which means that these flora have different groups that are related but widely separated geographically.
The distribution pattern of the present-day animals and plants as well as the distribution of fossils are best explained on the basis of the theory of evolution.
The birds in Galapagos Islands show differences in bills and feeding habits.
The bills of several of these species resemble those of different, distinct families of birds on the mainland.
All these birds are thought to have evolved from a single common ancestor.
- Books Name
- A TEXT OF BIOLOGY - CLASS XII
- ACME SMART PUBLICATION
- CBSE Class 12
Mechanism of Evolution
Populations evolve, but individual organisms do not. A population is an interbreeding group of individuals of one species in a given geographic area at the same time. A population evolves because the population contains the collection of genes called the gene pool. As changes in the gene pool occur, a population evolves.
- Hugo de Vries worked on evening primrose. He gave the idea of mutations.
- Mutation is the difference arising suddenly in a population. Hugo de Vries theory of mutation differs from Darwin
- He stated that mutation causes evolution and not the minor variations that as suggested by Darwin.
- Mutations are sudden, random and directionless while Darwinian variations are small and directional.
- Evolution according to Darwin was slow and gradual whereas Hugo de Vries believed that mutation caused large changes that led to speciation. He therefore called it is saltation (single step large mutation).
- Saltation: It is a large and an abrupt evolutionary change that has been brought about due to sudden large-scale mutation.
- Books Name
- A TEXT OF BIOLOGY - CLASS XII
- ACME SMART PUBLICATION
- CBSE Class 12
HARDY -WEINBERG PRINCIPLE
Five basic processes affect the Hardy Weinberg equilibrium and cause variations at genetic level. These are:
(ii) Gene migration
(iii) Genetic drift
(v) Natural selection
The Hardy-Weinberg principle states that the proportions of different alleles will stay the same in a large population if mating occurs at random and the above mentioned forces are absent.
In algebraic terms, the Hardy-Weinberg principle is written as an equation.
Its form is what is known as a binomial expansion.
For a gene with two alternative alleles, called A and a, the frequency of allele A can be expressed as p and that of alternative allele a as q, because these are only two alleles, p + q must always be equal to one. The equation looks like this
For example, if q is the frequency of the allele a, then the Hardy-Weinberg equation states that q2 = percentage individuals homozygous for allele a say 16%.
q2 = 0.16, q = 0.4
Factors of Evolutionary Change
(i) Mutation (ii) Migration (iii) Genetic Drift
(iv) Recombination (v) Natural Selection
Replica Plate Experiment of Lederberg and Lederberg
1. Mutations are random (indiscriminate) with respect to the adaptive needs of organisms.
2. Most mutations are harmful or with no effect (neutral) on their bearer.
3. Mutation rates are very slow.
The Lederberg Replica Plating Experiment, a beautiful example of the genetic basis of a particular adaptation was demonstrated in bacteria by an ingeneous method devised by Joshua Lederberg and Esther Lederberg.
E.coli bacteria are usually grown in the laboratory by plating dilute suspensions of bacterial cells on semi-solid agar plates.
After a period of growth, discrete colonies appear on the agar plates.
Each of these colonies originates from a single bacterium through a large number of cell divisions.
The Lederbergs inoculated bacteria on an agar plate and obtained a 'master plate' containing several bacterial colonies.
They, then created several replicas of this master plate by a simple procedure.
A sterile velvet disc, mounted on a wooden block, was gently pressed on the master plate.
Some bacteria from each colony adhered to the velvet.
By pressing this velvet on to new agar plates, they obtained exact replicas of the master plate, because the few bacteria transferred by the velvet formed colonies on the new agar plates.
However, when they attempted to make replicas using plates containing an antibiotic such as penicillin, most colonies found on the master plate did not grow on the replica plates.
The few colonies that did grow were obviously resistant to penicillin.
How did the bacteria acquire the ability to grow in a new environment (here, agar medium, containing penicillin)? In other words, what was the origin of this adaptation?
A Lamarckian interpretation of this adaptation would have been that penicillin somehow induced a change in one or more bacteria, enabling them to grow in the presence of penicillin.
A Darwinian view is that there were, in the original suspension of bacteria from which the master plates were prepared, a few bacteria carrying mutant genes which conferred on them the ability to survive the action of penicillin and form colonies.
These mutations, which had arisen by chance, and not induced by penicillin, were present only is small numbers in the original suspension.
Lederberg's experiment provided evidence that mutations are actually preadaptive.
These kinds of mutations are regarded as advantageous mutations.
They appear without exposure to the environment.
Actually, the preadaptive mutations express themselves only after exposure to the new environment to which the organisms are to adapt themselves.
The new environment does not induce the formation, it only selects the preadaptive mutations that occurred earlier.
Migration, defined in genetic terms as the movement of individuals from one population into another, can be a powerful force in upsetting the genetic stability of natural populations.
If the characteristics of the newly arrived animal differ from those already there, the genetic composition of the receiving population may be altered, if the newly arrived individual or individuals can adapt to survive in the new area and mate successfully.
Gene pool : A total collection of all genes and its allele in a population is called gene pool. Thus, gene pool will have all genotypes i.e., genes of the organisms.
Gene flow: If genes are exchanged between two different populations of a species, it is gene flow.
(III) GENETIC DRIFT I SEWALL WRIGHT EFFECT I NON-DIRECTIONAL FACTOR
Natural selection is not the only force responsible to bring about changes in gene frequencies. There is the role of chance or Genetic Drift also.
Genetic Drift causes the change in gene frequency by chance in a small population.
In a small population, the one individual alleles of a gene are represented by a few individuals in a population.
These alleles will be lost if these the individuals fail to reproduce.
Allele frequencies appear to change randomly, as if the frequencies were drifting, Janes thus, a random loss of alleles in small population is Genetic Drift.
A series of small populations that are isolated from one another may come to differ strongly as a result of Genetic Drift.
Genetic Drift has two ramifications are described below.
1. Bottle neck effect:
It is the decrease in genetic variability in a population, e.g., cheetah population in Africa decreased due to hunting.
Their decreased numbers have limited cheetahs genetic variability, with serious consequences.
The present cheetah population is susceptible to a number of fatal diseases.
If any of these diseases attacks the cheetah population, the path of extinction of cheetah cannot be reversed.
2. Founder's effect:
When one or a few individuals are dispersed and become the founders of a new, isolated population at some distance from their place of origin, the alleles that they carry are of special significance.
Even if these alleles are rare in the source population, they will be a significant fraction of the new population's genetic endowment.
This effect by which rare alleles and combinations of alleles may be enhanced in new populations -is called the founder's effect.
The founders effect is particularly important in the evolution of organisms on islands, such as Galapagos Islands which Darwin visited.
Most of the kinds of organisms that occur in such areas were probably derived from one or a few initial founders.
Fixation of new mutations:
Genetic drift fixes new alleles, genes that arise by mutation, from time to time and eliminate the original gene, thereby changing the genetic make up of small population.
Gene recombination is also an important source of variations.
It occurs during crossing over at the time of meiosis, free assortment (selection) of genes at the time of gamete formation, random union of gametes at the time of fertilization and even chromosomal aberrations.
They cause reshuffling of gene recombinations which provide new combinations of existing genes and alleles.
This is the entity of gene recombination.
Gene recombination can occur not only between genes but also within genes resulting in the formation of a new allele.
Since it adds new alleles and combination of alleles to the gene pool, it is an important process during evolution which causes variations.
(V) NATURAL SELECTION
It causes allele frequencies of a population to change. Depending upon which traits are favoured, natural selection can produce different results.
Forms of Selection: There are three kinds of natural selections
Stabilizing Selection (Normalizing selection) :
When selection acts to eliminate both extremes from an array of phenotypes, the frequency of the intermediate type which is already the most common, is increased.
Directional Selection (Progressive selection) :
When selection acts to eliminate one extreme from an array of phenotypes, the genes determining this extreme become less frequent in the population. The industrial melanism is peppered moth, Biston betularia provides good example of directional selection from nature.
Disruptive Selection (Diversifying selection) :
In some situations, selection acts to eliminate, rather than favour, the intermediate type. The individuals at both the extremes are favoured.
Examples of Natural Selection-Industrial Melanism
First studied by R.A. Fisher and E.B. Ford and in recent time by H.B.D. Kettlewell.
One of the most striking examples, which demonstrates the action of natural selection, is the industrial melanism in England.
The peppered moth Biston betularia, with a dull grey colour or white was abundant in England before the Industrial Revolution.
A black coloured form of the same moth (melanic, a dominant mutant differing in a single gene), carbonaria, was very rare.
Within a couple of hundred years, however, the proportion of carbona ria increased to almost 90 per cent.
The moths rest on tree trunks.
Before the Industrial Revolution, the tree trunks used to be covered with grey coloured lichen.
The dull grey moth easily blended with this background, while the black moth stood out conspicuously, and was therefore more susceptible to predation by birds.
With the advent of the Industrial Revolution, large-scale burning of coal became common.
The enormous amount of smoke produced resulted in the deposition of particulate matter on tree trunks, turning them black.
As a result, the grey moths now became more conspicuous than the black variety, and hence more susceptible to predation.
The frequency of black coloured moths in the population therefore increased.
Gradual replacement of coal by oil and electricity, as well as the improved methods of controlling soot production, reduced the soot desposition on the trees.
Conditions then became more suitable for the survival of grey moths, consequently their frequency once again increased.
Thus, reduction in pollution is now correlated with reverse evolution.
Industrial melanism, as this phenomenon is called, is thus a particularly interesting example which clearly brings out the action of natural selection.
This has been observed in about 70 different species of moths, and in several other European countries as well.
This understanding is supported by the fact that in areas where industrialisation did not occur e.g., in rural areas, the count of melanic moths was low.
This showed that in a mixed population, those that can better-adapt, survive and increase in population size.
Remember that no variant is completely wiped out.
Similarly, excess use of herbicides, pesticides, etc., has only resulted in selection of resistant varieties in a much lesser time scale.
This is also true for microbes against which we employ antibiotics or drugs against eukaryotic organisms/cell.
Hence, resistant organisms/cells are appearing in a time scale of months or years and not centuries.
These are examples of evolution by anthropogenic action.
This also tells us that evolution is not a direct process in the sense of determinism.
It is a stochastic process based on chance events in nature and chance mutation in the organisms.
Change In Genotypic Frequencies
If the alleles for grey and black colours are denoted by G and B, the genotypes of the moths would be GG, GB and BB.
Since B is dominant, GB and BB will be black.
Due to greater predation by birds on the black (melanic) phenotype the proportion of B in the population was maintained at a much lower value than G.
Resistance of Mosquitoes to Pesticides
Mosquitoes have always been a major health hazard, especially as they are responsible for the spread of diseases such as malaria and filaria.
When DDT was first introduced to control mosquitoes, it was tremendously successful; most mosquitoes were sensitive to DDT and were therefore killed.
However, DDT has now become ineffective against mosquitoes.
This is explained as follows:
In the original population of mosquitoes, some individuals were resistant to DDT.
However, in the absence of DDT, such resistant individuals were few because they had no advantage over the DDT-sensitive mosquitoes.
However, when DDT was used on a large-scale, only the resistant genotypes were able to survive and reproduce.
As a result, over a period of time, almost the entire population came to consist of the resistant type, which made DDT quite ineffective.
Evolution is thus a change in gene frequencies in the population in response to changes in the environment-in this case the introduction of DDT.
The principle of natural selection thus helps us to understand, why such chemical insecticides would remain useful only for a limited time.
Plants Growing Around Mines
A few plants are now known to grow on the tailings or refuse around mines. Professor A.D. Bradshaw studied one such grass, the bent grass Agrostis tenuis growing on tailings of lead mines in Wales, U.K.
He took some of these and planted them in soil from a pasture nearby.
Similarly, he transplanted live Agrostis plants from the pasture to the lead-rich soil.
The bent grass from the mine soil grew very slowly on normal pasture soil.
The one from the pasture, on the other hand, could not survive in the lead-rich soil.
A very small percentage (three out of sixty), however, could grow in the soil rich in lead.
These were undoubtedly the kind, from which the race of bent grass capable of growing in lead-rich mine soil evolved originally.
Plants tolerant to selenium such as Astragalus and Haplopappus have been reported from the U.S.A.
These plants are not only capable of growing in seleniferous soils, but require selenium as an essential element.
In our country, Professor Y.D. Tyagi discovered populations of Impatiens balsamina growing around Zwar zinc mines in Udaipur, Rajasthan.
The presence of such plants, which have evolved metal tolerance, can indicate the occurrence of specific metal deposits.
Such plants are called bioindicator plants.
Sickle Cell Anaemia Is an example of balancing selection.
(i) In few RBCs, 1-2% became sickle shaped during lack of oxygen.
(ii) The heterozygotes (HbA / HbS), who have one copy of sickle cell allele, coupled with one normal allele are better survivors in the areas where malaria is endemic; because the malarial parasite spends a part of the life cycle in the RBC; if they enter into the RBC which are sickle shaped, they will die.
(iii) The women who are heterozygote have higher fertility; that's why natural selection has not eliminated the allele.
(iv) The loss of deleterious recessive genes through deaths of homozygotes (HbS / HbS) is being balanced by the gain resulting from successful reproduction by heterozygotes in malaria prone areas. For this reason, the selection is called balancing selection.
(v) Heterozygotes enjoy some resistance to malaria, so they survive the malarial parasite more successfully than either normal or sickle cell homozygotes.
Some genetic variability is always present in a population.
Some alleles make organisms better adapted to the environment, and thus make them more successful in survival and reproduction.
As a result, the frequency of such alleles in a population gradually increases.
This is called selection; these alleles are thus 'selected' over the other alleles.
This process operating in natural populations is therefore called 'Natural Selection'.
The process of natural selection, acting on variability inherent in the population, over millions of years, has given rise to the great diversity we see in the biological world.
Ancestry of different breeds can be traced to wild rock pigeon. (Artificial Selection)
Man has been using a similar process for improving the qualities of domesticated plants and animals for centuries.
Plant-breeding and animal-breeding are very similar to the action of natural selection, the difference being that the role of nature is played by man.
The criteria for selection are based on human interests.
To obtain cows with high milk yield, the dairy scientists monitor milk production of a large number of cows.
Only the calves produced by cows which are high-yielders, are chosen to breed and form the next generation.
When this process is repeated (i.e., artificial selection is applied) for many generations, a population of cows with high milk yield is obtained.
Here, the work of selection is done by man.
Mimicry: It is a kind of adaptation. The term mimicry was introduced in Biology by Bates (1862). It is defined as "the resemblance of one organism to another or to any natural object for the purpose of concealment, protection or for some other advantages. The organism which exhibits mimicry is called a mimic. The organism or object which is mimicked or imitated is called a model.
Bateslan Mimicry : It is a form of mimicry in which an edible species resembles an inedible one.
Mullerian Mimicry : When two or more inedible or unpalatable species resemble each other the mimicry is termed Mullerian mimicry.
Both Batesian and Mullerian mimicries are two forms of protective mimicries.
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- A TEXT OF BIOLOGY - CLASS XII
- ACME SMART PUBLICATION
- CBSE Class 12
SPECIATION AND ISOLATION
Speciation is the formation of one or more new species from an existing species.
The crucial episode in the origin of species occurs when the gene pool of a population is severed from other populations of the parent species and gene flow no longer occurs.
Speciation can take place in two modes based on the geographical relationship of a new species to its ancestral species.
1. When a population, formerly continuous in range, splits into two or more geographically isolated populations and form new species, the mode of speciation is called allopatric speciation.
This can happen by subdivision of the original population, when a geographical barrier, such as a creeping glacier, a land bridge (e.g., Isthmus of Panama) or ocean or mountain, cuts across a species range.
Alternatively, a small number of individuals may colonise a new habitat which is geographically separated from the original range.
Darwin's finches that formed separate species in the Galapagos islands and the Australian marsupials that radiated to form new species are its examples.
2. In the second speciation mode, a subpopulation becomes reproductively isolated in the midst of its parent population; this is sympatric speciation.
So, sympatric speciation is the formation of species within a single population without geographical isolation.
The usually quoted example of sympatric speciation comes from polyploidy, which is the multiplication of the normal chromosome number.
This can happen when chromosomes fail to segregate at meiosis or replicate without undergoing mitosis.
(I) Multiplicative speciation (Cladogenesis) : It is the formation of two or more species from a single species, example, allopatric speciation, sympatric speciation. This can be gradual or abrupt.
(II) Fusion species : It is an allogenous transformation. Isolation mechanism may break down due to a mutation. The two species will interbreed and merge to form a single species.
(III) Phyletic speciation (Anagenesis) : It is autogenous transformation of a species with passage of time due to piling up of variations.
3. Species concept:
Species is the basic unit of classification.
The term was coined by John Ray (1693).
Most taxonomists define species as morphologically distinct and reproductively isolated natural population or group of populations where individuals resemble one another more closely than with members of other species, have a similar anatomy, karyotype and biochemicals, interbreed freely and form a genetically closed system. There are three basic concepts about the species.
4. Morphospecies concept:
It is the earliest concept of species.
Davis and Heywood (1963) have defined it as "assemblage of individuals with morphological features in common and separable from other such assemblages by correlated morphological discontinuities in a number of features."
However, the number of morphological characters chosen for separating species varies from taxonomist to taxonomist.
"Lumpers" will combine all the populations with broadly similar traits into a single species while "Splitters" will separate various populations with even minor morphological differences into distinct species.
5. Biological species concept:
Though first proposed by Buffon (1753), biological species concept was formulated by Mayr(1942).
According to it, a biospecies (biological species, Biological Species Concept) is sexually interbreeding or potentially interbreeding group of individuals which is reproductively isolated from other species and is therefore, separated from others by absence of genetic exchange.
Normally species are distinct from one another by both morphological traits and reproductive isolation.
However, sibling species are those distinct species which are almost identical morphologically but are distinct from each other due to absence of interbreeding, e.g., Drosophila pseudoobscura and D. persimilis.
Biological species concept is, therefore, mainly based on absence of cross fertilisation between members of two species.
Cross fertilisation tests carried out by taxonomists between individuals of morphological and geographically separated populations have, resulted in revision of species and grouping of many of them into single species, e.g., several species of North American sparrows as subspecies and races of a single song sparrow, Passarella melodia.
The only problem of using reproductive isolation is the absence of sexual reproduction in several organisms -prokaryotes, some protists, some fungi, some plants (e.g., commercial Banana) and animals.
Further, cross fertilisation experiments cannot be performed on such a large number of species that occur in varied geographical areas.
Reproductive isolation cannot be used as a criterion in case of fossils.
The living organisms and fossils can be grouped only on the basis of their morphology and biochemistry.
Mayr (1987) has named morphologically grouped asexual species as paraspecies while Ghiselin (1987) has named them pseudospecies.
6. Evolutionary species concept:
All evolutionary taxonomist have been in search of a proper definition of species which is basic unit of classification.
One such definition has been given by Simpson.
According to Simpson (1961) "an evolutionary species is a lineage (an ancestor-descendent sequence of population) evolving separately from others and with its own unitary evolutionary role and tendencies."
The concept stresses on evolutionary isolation with sexual isolation being its one aspect.
It is more dependent on differences which can be morphological, genetical, behavioural and ecological, to know evolutionary distance.
However, evolution does not occur simultaneously in all the traits.
Neither its rate nor direction (in which it is occurring) are the same.
7. Reproductive isolation may be defined as the existence of intrinsic barrier to the interbreeding in natural populations. Each of these intrinsic barriers is called a reproductive isolating mechanism. According to Mayr (1942), reproductive isolating mechanisms are the biological properties of individuals which prevent the interbreeding of naturally sympatric populations.
8. Reproductive isolation in the form of hybrid sterility is known since long. In the laboratory or in zoos, hybrids can be produced between species that do not interbreed in nature. Horses and donkeys are two different species; a hybrid, mule, is produced from the mating of a male donkey and a mare (female horse).
9. Similarly, mating between stallion (male horse) and female donkey results in a hybrid called hinny. Both mule and hinny are sterile.
10. There are examples of species, which can produce fertile hybrids in captivity. You might have heard about the famous 'tigons', a hybrid of African lioness (Panthera leo) and Asian tigers (Panthera tigris), which is fertile. No barrier to hybridisation between these species has evolved during their long isolation from each other Natural selection has not favoured a reduction in hybridisation for the simple reason that no hybridisation has been possible. Other examples of species that breed in captivity and produce fertile hybrids are mallard (a duck) and the pintail duck, the polar bear and the Alaskan brown bear and the platy and swordtail fishes. But these species do not interbreed at all in natural condition.
BARRIERS TO HYBRIDISATION
Prezygotic Mechanisms: (Prevent mating or formation of zygote)
1. Ecological isolation : Two species live in different habitats and do not meet. (One may be living in fresh water and the other in the sea).
2. Temporal isolation : Breeding seasons or flowering time may be different in the two species.
3. Behavioural isolation: The males of one animal species are unable to recognise the females of another species as potential mates.
4. Mechanical isolation: The structural differences in genitalia of individuals belonging to different animal species interfere with mating.
5. Gametic isolation : The sperms and ova of different species of animals are unable to fuse. In plants, the pollen coming from a different species may be rejected by the stigma.
Postzygotic Mechanisms : A hybrid zygote is formed but it may not develop into a viable fertile adult.
1. Hybrid Inviability : Hybrid zygotes fail to develop. In plants, embryos arising from interspecific crosses abort.
2. Hybrid sterility : Hybrid adults do not produce functional gametes. (Mules and hinny are common examples in mammals. Several hybrid ornamental plants are sterile.)
3. Hybrid breakdown : The offspring of hybrids are inviable or infertile.
MODERN SYNTHETIC THEORY OF EVOLUTION
1. Evolution on the grand scale of geological time is called macroevolution.
2. Evolution at genetic level is called microevolution.
3. Studies of how individual traits evolve within natural populations provide powerful evidence that natural selection can be a powerful agent of microevolutionary change within species. The progressive change in allele frequencies within the population is micro-evolution.
4. Units of evolution is population.
5. Unit of natural selection is individual.
The modern synthetic theory of evolution is the result of the work of a number of scientists as T. Dobzhansky, R.A. Fisher, J.B.S. Haldane, Sewall Wright, Stebbins.
The synthetic theory includes the following factors.
(i) Gene mutations
(ii) Changes in chromosome structure and number
(iii) Genetic recombination
(iv) Natural selection
(v) Reproductive isolation
(vi) Migration lied
NEUTRAL THEORY OF EVOLUTION
According to Kimura most of the mutations are neutral, and are not eliminated from the population.
This is against natural selection.
Kimura proposed that speciation is not due to selection of advantageous genotypes no but elimination of deleterious alleles and random selection of neutral alleles.
It emphasized that most mutations are of neutral value and genetic drift is responsible for divergence.
It means that all mutations are alike in adaptive value.
It is only chance or random drift which delineates a novel collection of mutants into a group divergent from the parental population.
Place of Humans in the Animal Kingdom
Today human evolution is being studied by:
(i) Homology in the chromosomes of man and great apes. The banding pattern of human chromosome number 3 and 6 are compared with those of particular autosomes in the chimpanzee. It shows a common origin for man and chimpanzee.
(ii) Today, besides the autosomal chromosomes, V-chromosomes and mitochondrial DNA are being studied, as they are uniparental in origin and do not take part in recombination.
(iii) Evidence from blood proteins -It has been proved by the blood protein tests that man is most closely related to great apes (chimpanzee and gorilla).
(iv) Evidence from blood groups -The blood groups A and B are found in apes and not in monkeys.
(v) Evidence from haemoglobin -There is 99 percent homology in haemoglobin of man and gorilla.
Human beings are vertebrates and belong to the class mammalia.
Mammals evolved from primitive reptiles in early Triassic period, about 210 million years ago.
But for nearly 150 million years, mammals existed as relatively inconspicuous group of small rat-like creatures, completely dominated by the large number of gigantic reptiles of the Mesozoic age.
It is only after the great extinction of dinosaurs and other large reptiles, that mammals diversified and began to occupy the earth's many different habitats.
Within the class mammalia, human beings belong to the order primates, a group that originated about 65 million years ago and includes not only monkeys and apes but also the lorises, lemurs and tarsiers.
The Anthropoid apes or the ancestors of monkeys, apes and humans evolved about 36 million years ago and the hominids or the ancestors of apes and humans evolved about 24 million years ago.
Today, the apes are represented by two families, namely, Pongidae which include chimpanzees, gorillas and orangutans and Hylobatidae which includes gibbons.
The chimpanzee and gorilla are restricted to Africa, whereas the orangutans and gibbons are found only in Asia.
Humans belong to the family Hominidae in which Homo sapiens is the only living species.
Early Human Ancestors
Tracing the evolution of human beings, both by fossil hunting and molecular methods, is one of the most exciting and active areas of research in biology.
The fossil evidence clearly indicated that such genera as Ramapithecus and Sivapithecus were the forerunners of Hominids.
A genus called Australopithecus appeared in Africa about five million years ago and ultimately gave rise to Homo about two million years ago.
But even Australopithecus had a brain measuring only about 350-450 cm3.
The most important change that must have occurred during the three million years or so, between the appearance of Australopithecus and that of our genus, must therefore have been a phenomenal increase in brain size, all the way up to 1400 to 1450 cm3, that is characteristic of our species.
A combination of molecular data and a modern interpretation of the fossil record suggests that the gibbons probably diverged from the main line of Hominoid evolution about ten million years ago, that the orangutan did so about eight million years ago and that the ancestors of gorilla and chimpanzee, about four million years ago.
The gorilla and chimpanzee have become separated from each other only 2.3 million years ago.
Place and Sequence of Human Evolution
There is evidence that almost all of Hominid evolution occurred in Africa, and Asia and that the evolution of the human species took place in Africa.
Several species belonging to the genus Homo can be recognised from the fossil record. For example, Homo habilis lived in Africa, about two million years ago and was characterised by having a larger brain than Australopithecus, used tools and was bipedal.
Another species, Homo erectus appeared about 1.7 million years ago, used fire and is believed to have migrated to Asia and Europe.
Fossils of the socalled 'Java man' and 'Peking man', belong to Homo erectus. Homo erectus was replaced by Homo sapiens.
A primitive form of Homo sapiens, called Neanderthal man (Homo sapiens neanderthalensis), was common in Europe and Asia.
The Neanderthal men resembled us, though they were relatively short and stocky and more powerfully built.
The Neanderthals made tools and used animal hides as clothing.
They built hut-like structures for dwelling and buried their dead.
There is evidence that an abrupt transition occurred all over Europe whereby the Neanderthal man was wiped out and gave way to the more efficient cousin, the cro-Magnon, about 34,000 years ago.
The Cro-Magnon people left behind very elaborate cave paintings showing the attainment of a form of culture not unlike our own.
After the last glacial period (about 10,000 years ago), modern Homo sapiens sapiens began to spread all over the globe, cultivated plants, domesticated animals and reached enormous population sizes.
Homo sapiens appeared in Africa about 500,000 years and probably replaced Homo erectus there.
But in Asia, Homo erectus appears to have survived for another 250,000 years when it was finally replaced by Homo sapiens migrating from Africa.
- Books Name
- A TEXT OF BIOLOGY - CLASS XII
- ACME SMART PUBLICATION
- CBSE Class 12
BRIEF ACCOUNT OF EVOLUTION
About 2000 million years ago (mya) the first cellular forms of life appeared on earth.
The mechanism of how non-cellular aggregates of giant macromolecules could evolve into cells with membranous envelop is not known.
Some of these cells had the ability to release O2.
The reaction could have been similar to the light reaction in photosynthesis where water is split with the help of solar energy captured and channelised by appropriate light harvesting pigments.
Slowly, single-celled organisms became multi-cellular life forms.
By the time of 500 mya, invertebrates were formed and active.
Jawless fishes probably evolved around 350 mya.
Sea weeds and few plants existed probably around 320 mya.
We are told that the first organisms that invaded land were plants.
They were widespread on land when animals invaded land.
Fish with stout and strong fins could move on land and go back to water.
There are no specimens of these left with us.
However, these were ancestors of modern day frogs and salamanders.
The amphibians evolved into reptiles.
They lay thickshelled eggs which do not dry up in sun unlike those of amphibians.
Again we only see their modern day descendents, the turtles, tortoises and crocodiles.
Representative evolutionary history of vertebrates through geological period
This was about 350 mya.
In 1938, a fish caught in South Africa happened to be a Coelacanth which was earlier thought to be extinct.
These animals called lobefins evolved into the first amphibians that lived on both land and water.
In the next 200 millions years or so, reptiles of different shapes and sizes dominated on earth.
Giant ferns (pteridophytes) were present but they all fell to form coal deposits slowly.
Some of these land reptiles went back into water to evolve into fish like reptiles probably 200 my a (e.g. Ichthyosaurs).
The land reptiles were, of course, the dinosaurs.
The biggest of them, i.e., Tyrannosaurus rex was about 20 feet in height and had huge fearsome dagger like teeth.
About 65 mya, the dinosaurs suddenly disappeared from the earth.
We, do not know the true reason. Some say climatic changes killed them.
Some say most of them evolved into birds.
A family tree of dinosaurs and their living
modern day counterpart organisms like crocodiles and birds
The truth may live in between.
Small sized reptiles of that era still exist today.
The first mammals were like shrews.
Their fossils are small sized.
Mammals were viviparous and protected their unborn young inside the mother's body.
Mammals were more intelligent in sensing and avoiding danger at least.
When reptiles came down mammals took over this earth.
There were in South America mammals resembling horse, hippopotamus, bear, rabbit, etc.
Due to continental drift, when South America joined North America, these animals were over ridden by North American fauna.
Due to the same continental drift, pouched mammals of Australia survived because of lack of competition from any other mammal.
Lets we forget, some mammals live wholly in water.
Whales, dolphins, seals and sea cows are some examples.
Evolution of horse, elephant, dog, etc., are special stories of evolution.
You will learn about these in higher classes.
The most successful story is the evolution of man with language skills and self-consciousness.
THEORIES OF EVOLUTION
1. LAMARCK'S THEORY OF EVOLUTION
His theory is often called as the Theory of Inheritance of Acquired Characters or the Theory of Use and Disuse of Organ.
The first attempt to explain origin of species and their adaptation to the environment was done by Jean Baptiste Lamarck (1744-1829).
He was the greatest French naturalist.
Lamarck's theory was published in 1809 (year of Darwin 's birth) in his book 'Philosophie Zoologique'.
According to this theory the organisms undergo changes to adapt themselves to the environment.
The changes acquired by the organisms during their life time are passed on to the next generation.
He took the example of long neck of Giraffe, they continuously stretched their neck to reach to the vegetation on trees.
This acquired change was passed to the next generation.
He also gave the principle of Use and Disuse.
Use of an organ leads to strengthening of the organ, and disuse will lead to weakening of the organ.
Lamarck arranged his theory in the form of four postulates.
(i) Internal forces tend to increase size of the body.
(ii) Formation of new organs is the result of the need or want continuously felt by organisms Doctrine of Appetency/Desires.
(iii) Development and power of action of an organ is directly proportional to its use.
(iv) All changes acquired by the organism during its life are transmitted to the offsprings by the process of inheritance.
Lamarck's theory was discarded by A. Weismann who gave the Theory of Germplasm.
He cut the tail of new born mice generation after generation but could not get tailless mice nor the mice developed shorter and shorter tail.
Today, again the faith in Lamarck's theory has been revived as it is said that if the environment influences the genes of the organisms, the acquired change will be transmitted to the next generation.
2. DARWIN'S THEORY OF EVOLUTION
Charles Robert Darwin put forward the concept of natural selection as the mechanism of evolution.
The theory was put forward along with Alfred Russell Wallace.
Darwin had written the book 'Origin of Species'.
Darwin was greatly influenced by 'An Essay on Population' written by Thomas Rev Malthus and he was also influenced by Charles Lyell's essays on "Principles of Geology".
Darwin was a British naturalist.
In 1831, at the age of 22, he was appointed upon a world survey ship of British government, H.M.S. Beagle.
For five years on his ship, Darwin explored the fauna and flora of continents and islands.
Branching descent and Natural Selection are the two key concepts of Darwinian Theory of Evolution.
According to Wallace's Chart, the main points of Darwin's theory of Natural Selection were as follows:
(i) High rate of reproduction
(ii) Total number almost constant
(iii) Struggle for existence
(v) Survival of fittest
(vi) Natural selection
All the successful organisms have a high Biotic Potential or Reproductive Rate.
The organisms produce a large number of offsprings that can possibly survive, example a mice produces a dozen of mice at one time.
A rabbit produces 6 young ones in a litter and there are four litter in a year.
A rabbit starts reproducing at the age of six months.
1. Not all but only some individuals which survive, reach adulthood, and those which reach adulthood, reproduce at different rates, this is called 'Differential Reproduction'.
2. The success in survival and reproduction depends upon the characteristic traits of an organism, example only those rabbits will survive which are fastest. There is 'Struggle for Existence' and in this there will be 'Survival of Fittest'. The Phrase 'Survival of Fittest' was first used by Herbert Spencer. The same context was asserted by Darwin as 'Natural Selection'.
So, evolution is the change in the genetic composition of the population which is brought about by natural selection which acts upon the variability in population.
Causes of Variations
1. Mutation is the ultimate source of variations.
2. At the next level is recombination .
3. Intermingling of two widely separated populations.
Weakness of Darwinism
He was not able to explain the cause of discontinuous variations observed by himself in nature and the mode of transmission of variants to the next generation.
In 1868, Darwin put forward the Theory of Pangenesis.
According to this theory, every organ of the body produces minute hereditary particles, called Pangenes or Gemmules and they are carried through the blood into the gametes.
Weismann's 'Theory of Germplasm' (1892) rejected Darwin's theory of pangenesis.
He established that the germ (sex), cells are set apart from other body (somatic) cells early in the embryonic development, so, only the changes in the germplasm affect the characteristics of future generations.
Alfred Wallace had written the book 'On the Tendencies of Varieties to Depart Indefinitely from the Original Type'.
Alfred Wallace (1823-1923), a naturalist from Dutch East Indies, was working on Malay Archipelago (present Indonesia).
Erasmus Darwin :
Charles Darwin's grandfather, Erasmus Darwin, wrote about evolution more than 60 years before his grandson's theory was presented.
Erasmus Darwin cited things such as the metamorphosis of insects, the new varieties produced by selective breeding, the variations among similar organisms in different climates, and the similarities of vertebrate structure as evidence that all life was "produced from a similar living filament".
3. MUTATION THEORY
In 1901, Hugo de Vries proposed the Mutation Theory on the basis of his observation on the wild variety of evening primrose Oenothera lamarckiana.
According to mutation theory, new species originate as a result of large, discontinuous variations which appear suddenly.
The main features of mutation theory are as follows :
1. Mutations arise from time to time amongst the individuals of a naturally breeding population.
2. Mutations are heritable and establish new forms or species.
3. Mutations are large and sudden and are totally different from fluctuating variations of Darwin, which are small and directional.
4. Mutations may occur in any direction.
- Books Name
- A TEXT OF BIOLOGY - CLASS XII
- ACME SMART PUBLICATION
- CBSE Class 12
Place or origin of man :
It has been established that Dryopithecus is one of the oldest fossil which in turn evolved into apes and men.
The origin and evolution of man can be studied in the following three major headings : prior to ape men, ape men including prehistoric man and true men including the living modern man.
A. Prior to Ape Men
1. Dryopithecus. Discovery:
The fossil of Dryopithecus africanus was discovered from Miocene rocks of Africa and Europe.
It lived about 15 million years ago.
Dryopithecus and Ramapithecus were hairy and walked like gorillas and chimpanzees.
Ramaplthecus was more man like while Dryoplthecus was more ape like.
Dryopithecus is the direct ancestor of modern day apes.
It was ape-like, but had arms and legs of the same length.
Heels in its feet indicate its semierect posture.
It had large brain, a large muzzle and large canines.
It was without brow ridges.
It was arboreal, knuckle-walker and ate soft fruits and leaves.
Dryopithecus africanus is regarded a common ancestor of man and apes (gibbons, orangutan, chimpanzee and gorilla).
It is also called proconsul.
2. Proconsul. Discovery:
Proconsul africanus or o. africanus was discovered by Louis S.B. Leakey in 1948 from the rocks around lake Victoria of Kenya, Africa.
It lived in early Miocene epoch.
It was morphologically intermediate between apes and man in many features.
It had rounded man-like forehead and long, pointed ape like canines.
It moved upon land on all the four limbs and hence is not considered amongst the direct ancestors of man.
Proconsul gave rise to the ancestors of chimpanzee and gorilla in the Pliocene, about 4 million years ago.
The chimpanzee and gorilla diverged from each other only about 2.3 million years ago, in the Pleistocene epoch.
3. Sivapithecus. Discovery :
This fossil was discovered from middle and late Pliocene rocks of Shivalik Hills of India, hence it is named Shivapithecus.
It was like Dryopithecus. Its fore limbs, skull and brain were like those of monkeys, while the face, jaws and teeth resembled those of apes.
4. Ramapithecus. Discovery:
It has been established that in late Miocene epoch, Dryopithecus gave rise to Ramapithecus ('Rama' =The hero of Indian legend, Pithecus =Ape) which was on the direct line of human evolution.
Ramapithecus survived from late Miocene to Pliocene.
Thus, he appeared about 14-15 million years ago.
Fossil of Ramapithecus was discovered by Edward Lewis (1932) from Pliocene rocks of Shivalik Hills of India.
Kenyapithecus wickeri was discovered by L.S.B. Leakey (1962) from Pliocene rocks of Kenya (Africa).
It was similar to Ramapithecus. But Ramapithecus was older than Kenyapithecus.
B. Ape-men Including Prehistoric Men
1. Australopithecus (First Ape-man) :
The early human stock gave rise to Australopithecus.
It is the connecting link between apes and man.
Discovery. Raymond Dart (1924), South African anthropologist, discovered the fossil of Australopithecus africanus (African Ape-man) from Pliocene rocks near Tuang in Africa.
They appeared about 5 million years ago.
Actually skull discovered by Dart was of 5-6 year old baby so it is also called "Tuang baby".
Some fossils of A. africanuswere also discovered from Pleistocene epoch.
Two mya, Australopithecines probably lived in east African grasslands.
Evidence shows they hunted with stone weapons but essentially ate fruits.
Australopithecus africanus was about 1.5 metres high and had human as well as ape characters.
It was with bipedal locomotion, omnivorous diet and had erect posture.
It had human like teeth, but it had more of an ape brain than a human brain.
Its brain capacity was about 500 cc., similar to that of an ape.
He lived in caves.
Brow ridges projected over the eyes.
It did not have chin.
There was lumbar curve in the vertebral column.
The pelvis was broad. Australopithecus africanus existed until about 1.5 million years ago and gave rise to Homo habilis, about two million years ago.
Australopithecus africanus also gave rise to man-like apes called Australopithecus robustus and Australopithecus boisei along a separate line that ended blindly (They did not give rise to any other creatures).
In 1981 Donald Johanson, found a 3.2 million years old skeleton of a female human ancestor.
He nicknamed it Lucy.
Lucy's scientific name is Australopithecus afarensis.
Six species of Australopithecus are known.
These are A. africanus (African Ape man, Southern Ape or Tuang baby), A. afarensis, (Lucy), A. ramidus, A. aethiopicus, A. robustus and A. boisei. So we can say that Australopithecus had two main types.
(i) Gracile type: Australopithecus afarensis (Johanson) represented by fossil Lucy with small brain, small molar teeth pelvic girdles and short fingers like human.
(ii) Robust type: A. robustus had heavier body structure and massive check tooth (also called originally Paranthropus) cranial capacity -600cm2.
(Other examples -Zinjanthropus / A. boisei of R. Leakey -Africa, Meganthropus from -Java)
2. Homo habilis (Able or Skillful man, The tool maker, or 'Handy man'). Discovery:
Louis S.B. Leakey and his wife Mary Leakey (1960) discovered the fossils of Homo habifis from Pleistocene rocks of Olduvai Gorge in East Africa.
He lived in Africa, about 2 million years ago.
The first human like being the hominid was Homo habilis. They probably did not eat meat.
He was about 1.2 to 1.5 metres tall.
He had bipedal locomotion and moved erect.
It had about 650-800 cc. cranial capacity.
The teeth were like that of modern man.
Homo habilis (habilis = mentally able or skillful) was the first tool maker and used tools of chipped stones extensively.
It is also called handy man because heaps of tools found with these fossils included sharpened stones which indicate that Homo habilis was capable of 'making tools'.
He also led community life in caves and greatly cared for the young ones.
3. Homo erectus (Erect man) :
Homo erectus appeared about 1.5 million years ago, in middle Pleistocene.
Homo erectus probably ate meat.
He is called middle pleistocene man. H. erectus evolved from Homo habilis.
He was about 1.5-1.8 metres tall.
Homo erectus males were probably larger than females. He had erect posture.
His skull was flatter than that of modern man.
He had protruding jaws, projecting brow ridges and small canines and large molar teeth.
The cranial capacity was 900 cc.
Cranium was domed to accommodate the large brain.
He was omnivorous.
He made more elaborate tools of stones and bones, hunted big game and perhaps knew the use of fire.
Homo erectus includes three fossils : Java Ape-man, Peking man and Heidelberg man.
(a) Java Ape-man. Discovery:
In 1891, Eugene Dubois discovered a fossil from Pleistocene rocks in central Java (Island of Indonesia).
Eugene Dubois named it as Pithecanthropus erectus.
Pithecanthropus means 'ape man' Mayer in 1950 assigned it as Homo erectus erectus.
Body 1.65 to 1.75 metres tall and weight about 70 kg.
Legs long and erect, but body slightly bent when moving.
Inconspicuous chin and somewhat broader nose.
Forehead low and receding, but brow ridges high, like those of apes.
Skull cap thick and heavy, flattened infront.
Cranial capacity 800 to 1000 ce. (average 950 cc).
Lower jaw large and heavy.
Teeth large, but quite like those of modem man, except larger canines of the lower jaw.
Lips thick and protruding.
He was omnivorous and cannibal.
Perhaps, he was the first prehistoric man to make use of fire for hunting, defence and cooking.
(b) Peking Man. Discovery:
W.C. Pei (1924) discovered the fossils of Peking man from the lime stone caves of Choukoutein near Peking (Beijing -capital of China was formerly known as Peking) and named them Sinanthropus.
Davidson Black (1927) named it Sinanthropus pekinensis.
Mayer (1950) renamed it as Homo erectus pekinensis (a subspecies).
The Pleistocene rocks from which fossils of Peking man were excavated are about 6 lakh years old.
Placing Java ape man and Peking man as subspecies of Homo erectus has a sound basis, because of close similarities between these.
The body structure was quite similar in both.
Being about 1.55 to 1.60 metres tall, Peking man was slightly shorter and a little lighter and weaker.
The only noticeable difference of Peking man from Java ape man was its large cranial capacity, which ranged from 850 to 1100 cc.
Like Java ape man, the Peking man was omnivorous and cannibal.
There is a clear evidence of use of fire by it.
It has been confirmed that both Java and Peking men used to live in caves in small groups or tribes.
The tools of Peking man were relatively more sophisticated.
(c) Heidelberg man. Discovery :
In 1908 one of the most perfect fossil jaws belonging to middle Pleistocene was found by workmen working near Heidelberg, Germany.
It was shown to Otto Schoetensack, who gets the credit for its discovery. It was named Homo erectus heidelbergensis.
It had lower jaw with all the teeth.
The teeth were human like.
The massive jaw was ape-like.
He used the tools and fire.
The cranial capacity is believed to be about 1300 cc; which is intermediate between those of erect man (H. erectus) and Neanderthal man (H. sapiens neanderthalensis).
Thus, it is regarded as intermediate between pithecanthropines and Neanderthals.
C. True Men Including the Living Modern Man
1. Neanderthal Man (Homo sapiens neanderthalensis). Discovery:
Fossils of Neanderthal man were first obtained from Neanderthal Valley in Germany from the late Pleistocene epoch by C. Fuhlrott (1856).
Later, many other fossils were excavated in various countries by different palaeontologists.
He had slightly prognathous face.
Neanderthal man walked upright, as we do, and had low brows, receding jaws, and high domed heads.
If there was anything truly different about them, it was that they were much stockier than we are.
The cranial capacity was 1,300 to 1,600 cubic centimetres.
Neanderthal man existed half a million years ago, but was most numerous from about 100,000 years ago.
Became extinct 30,000 years ago.
Neanderthal man was the legendary cave dweller, having heavy brow ridge and humped back.
He was adapted to a cold environment, who encountered a succession of glaciers that passed over most of the northern temperate regions of the world.
He was not only skilled hunter but true predator, a specialization that did not occur among hominids before or after them.
The Neanderthal man was cannibal and fashioned the skin into clothing to protect himself against the harsh climate.
Natural caves became campsites that were illuminated and heated by fire.
It is believed that he buried his dead with flowers and tools. He may have had a religion.
It is usually considered that Homo sapiens neanderthalensis did not evolve into Homo sapiens.
2. Cro-magnon man (Homo sapiens tossilis). Discovery:
It has been known as Cro-Magnon man, because its fossils were first discovered in 1868 from Cro-Magnon rocks of France by MacGregor.
Cro-Magnon man emerged about 34,000 years ago in Holocene epoch.
Thus, he is regarded as most recent ancestor of today's man.
The Cro-Magnon man had, like us, about 1.8 metres tall, well-built body.
His face was perfectly orthognathous with an arrow, elevated nose, braad and arched forehead, moderate brow-ridges, strong jaws with man-like dentition, and a well developed chin.
His cranial capacity was, however, somewhat more than ours, being about 1650 cc.
It is, therefore, believed that Cro-Magnon man was somewhat more intelligent and cultured than the man of today.
It could walk and run faster and lived with families in caves.
It made excellent tools and even ornaments, not only of stones and bones, but also of elephant tusks.
Its tools included spears, bows and arrows, as he was omnivorous.
Use of the skin clothes by this man is also confirmed.
A number of cave paintings done by Cro-Magnon man have been discovered.
The Cro-Magnon man was the direct ancestor of the living modern man.
Prehistoric cave art developed about 18,000 years ago.
3. The Living Modern Man (Homo sapiens sapiens). Discovery :
Further evolution of man after Cro-Magnon involves the evolution of culture rather than that of anatomy.
Homo sapiens sapiens appeared about 25,000 years ago in Holocene epoch and started spreading all over the world about 10,000 years ago.
Agriculture came around 10,000 years back and human settlements started.
The evolutionary journey to modern humans ends with the appearance, about five hundred thousand years ago, of Homo sapiens ("wise man"), our own species.
We are newcomers to the human family -H. sapiens has not been around nearly as long as H. erectus was.
Still, humans have changed quite a bit, since those first days.
Out of Africa -Again
The origin of human races is a much-debated point among scientists studying human evolution.
Many argue that the different races evolved from H. erectus independently, and that each adapted to a different place -Orientals in Asia, Caucasians in Europe, Aborigines in Australia, and so on.
Others believe that the same species would be unlikely to evolve more than once and argue that human races appeared after H. sapiens evolved from H. erectus.
Recently, scientists studying mitochondrial DNA from living humans all over the world have argued that their research shows that all human races originated from one H. sapiens ancestor in Africa.
Scientists looked at mitochondrial DNA to study evolution, because the DNA within mitochondria is transmitted only by females.
Female's eggs carry many mitochondria within them that become part of a new baby, while sperms contribute no mitochondria to the new baby.
Sperms carry their mitochondria wrapped around their tails and so do not inject them into the egg during fertilization.
For that reason, particular versions of a mitochondrial gene can be traced back through a family tree, from mother to grandmother to great-grandmother.
Human races evolved only recently in the evolutionary scale of things, and there has not been enough time for many mitochondrial DNA differences to accumulate, so the exact human tree cannot be reliably traced using this approach.
So far, however, the greatest number of different mitochondrial DNA sequences occur among modern Africans.
Since DNA accumulates mutations over time, the oldest DNA should show the largest number of mutations.
This result thus argues, that humans have been living in Africa longer than on any other continent.
While researchers are not in complete consensus, this line of investigation appears to suggest that H. sapiens evolved in Africa and that the human races evolved after that, and not independently from separate species of H. erectus.
If this Is correct, then H. sapiens was born in Africa and from there spread to all parts of the world, retracing the path taken by H. erectus half a million years before.
Homology In Chromosomes of Man and Great Apes
The somatic cells of humans contain 46 chromosomes (44 autosomes and 2 sex- chromosomes).
Human chromosomes are usually obtained by culturing certain types of white blood cells from the peripheral blood.
They can then be treated with specific stains to produce characteristic bands along the length of each chromosome.
The pattern of banding so obtained is unique for each pair of chromosomes.
Banding techniques enable the identification of individual chromosomes and their parts.
The diploid number of chromosomes in gorilla, chimpanzee and orangutan is 48.
Comparisons have been made between banded chromosomes of man and those of the great apes.
The total amount of DNA in human diploid cells and that of the great apes are not dissimilar.
But what is most interesting from an evolutionary viewpoint, is that the banding pattern of individual human chromosomes is very similar and in some instances, identical to the banding pattern of apparently homologous chromosomes in the great apes.
Diagrammatic representations of the banding pattern of human chromosome numbers 3 and 6 are compared with those of particular autosomes in the chimpanzee.
This remarkable similarity in the fine structural organisation of the chromosomes is understandable only in terms of a common origin for man and chimpanzee.
Cope's Law: It states that there is a tendency for animals to increase in size during the long course of evolution.
Bergman's Law : It states that warm blooded animals become larger in the northern and colder parts of their range.
Allen's Law : It states, that in animals which live in very cold climates, their extremities such as ears, tails etc. become progressively smaller.
Gause's Law: (Gause, 1934) or the Competitive exclusion Principle (Hardin, 1960). It states that two species having the same ecological requirements cannot continue to occupy indefinitely the same habitat.
Gloger's rule: It states that among warm blooded animals, those living in warm and moist climate develop more melanin pigment (are darker than animals in cold, dry climates) whereas forms in dry, hot climates have more yellow and red pigment.
Jordan's rule: Temperature also influences the morphology of certain fishes and is found to have some relation with the number of vertebrae. Fishes inhabiting water of low temperature tend to have more vertebrae than those of warmer water.
Some Important Points
1. Mars has CO2, water vapours and supposed to have life. CO2 is present in traces. It has no green house effect, hence it is very cold and does not support life. Mercury and moon do not have any sign of life due to the absence of water vapours. This extra terrestrial origin of life was visualised by Hoyle and Wickremsinghe.
2. Darwin used the term 'warm little pond' for early hot sea, rich in biomolecules. These primitive seas were alkaline.
3. K. Bahadur exposed ammonia, formaldehyde, and ferrous chloride to strong sunlight and obtained a mixture of amino acids called Jivam.
4. Variation In behaviour -Cicada insect has a life span of 17 years and it emerges from soil, remains alive for 5 weeks and then dies after mating and laying eggs. Dolphins can imitate and laugh. Bat can detect small insects of size (o.oa mm). Male baya (weaver's bird) of India builds its elaborate nest and decorates it with colourful petals to attract female.
5. Multiformity among organisms -Internal differentiation increases with the progress in evolution. Human beings are one of the most recently evolved animals. They show the following features :
(a) Total length of blood vessels in our body is 96,000 km.
(b) Fastest nerve impulse travels at the rate of 532 km per hour.
(c) Internal area of our lungs is 93-100 m2 which is 40 times than the external surface area of our body.
(d) Human brains have 10,000 million nerve cells.
(e) We have more body hair than apes but shorter and softer.
(f) O2 disappears from atmosphere at 16 km height.
(g) We remain blind for 30 minutes per day by blinking our eyes.
(h) Bones are as strong as concrete and as hard as granite but far lighter than both.
(i) We retain only 18% of total that we learnt yesterday.
6. Synapsid reptiles were mammal like reptiles that gave rise to mammals. They had a single temporal fossa on the lateral side of skull and heterodont teeth. They are extinct. They originated in Permian period.
7. In 1858 Dr. P.L. Sclater divided for first time the earth into six regions (realms) according to the distribution of birds. Later on Alfred Russel Wallace (1876) classified the earth into six regions (realms) for all animals and plants.
8. In all animals, early development is similar i.e., passes through morula blastula gastrula stages, showing their common origin.
9. Early embryos of all the vertebrates show basic similarity in having somites, tail, gill clefts, notochord etc. These traits can be explained as a character of evolution.
10. Any vertebrate organ also passes through different stages during development. e.g., mammalian heart is initially two chambered, then becomes three chambered and then becomes four chambered.
Development of all triploblastic animals starts from zygote, undergoes similar changes to form gastrula having 3 primary germ layers (ectoderm, mesoderm and endoderm) which have same fate in organogenesis. Early embryos of different vertebrates resemble in possessing similar structures like gill slits, notochord, tail etc. Not only this, in the course of development, at different stages, an embryo looks like the embryo of different phyla form, which the given organism has evolved.
It can be explained on the basis of Recapitulation theory (Von Baer) / Biogenetic law (Haeckel) which states that "ontogeny (developmental history of an individual) repeats phylogeny" (developmental history of race).
11. Types of fossils
(i) Macrofossils: Larger than 1 cm in size.
(ii) Unusual fossils: Sudden preservation of entire organism e.g., Solenhofen Limestone quarry of Southern Germany -containing fossils of Archaeopteryx.
(iii) Bioclast: Fossils or fragments of fossils enclosed in sediments. The term is usually applied to thin sections of fossils under microscope.
(iv) Gastroliths: These are found in abundance in the body cavities of certain reptiles.
(v) Moulds and casts: The material surrounding the fossil hardens and preserves the outer details. The actual bodies diSintegrate and are removed by slippage of the ground leaving the harden cavities called moulds. When moulds are filled with natural deposits, they are·called as casts e.g., fossils of Pompeii city buried in volcanic ash of mount Vesuvious in A.D. 79.
(vi) Tracks and trails : The footprints or tracks left in the soft moist mud gets hardened up e.g., tracks of amphibians discovered near Pittsburg, Germany from Pennsylvanian period.
12. Preservation in ice : Woolly mammoths from Siberia. The flesh is so well preserved that it can be fed to dogs. Discovered from Lena delta in 1790 and Siberia in 1901.
13. Fossils in petroleum springs and asphalts : Rancho La Brea now in Los Angeles.
14. Fossils in resins and ambers: Fossil fly in amber from Baltic forests of Europe during Oligocene period.
15. The process of fossilization to preserve the finer details is known as histometabasis.
16. Mummies: Bodies of dead animals or plants become dehydrated in the deserts and are preserved as mummies.
17. T. Dobzhansky wrote the book 'Genetics and origin of species'.
18. Darlington wrote the book 'The evolution of genetic systems'.
19. Darwin wrote 'Descent of Man and Selection in relation of sex' in which he put forward his theory of evolution of man from ape like ancestors.
20. Law of Superposition : The lower strata of a geological formation was first to be deposited and is the oldest.
21. Willston's Rule : During evolution of lineage, serially homologue parts tend to reduce in number but get more and more differentiated e.g., prawn's leg.
22. Allometry: The study of differential growth rate was called allometry.
23. Missing links : The fossils which act as transition between two present day groups of organisms ego Archaeopteryx -a fossil of crow sized toothed bird act as a link between reptiles and birds.
24. Empedocales (493 -435 B.C.) -regarded as "Father of the Concept of Evolution".
25. Seymouria (Extinct Reptile) is a connecting link between amphibia and reptilia.
26. Lycaenops (Extinct Reptile) is a connecting link between reptiles and mammals.
27. Wallace's Line: In 1863 A.R. Wallace drew an imaginary dividing line on the map between the Oriental and Australian realms (regions). This line is known as Wallace's line.
28. Sibling species: Species which are morphologically looking similar but are reproductively isolated, are called sibling species.
29. Living Fossils: A living fossil is a living animal of ancient origin with many primitive characters. A living fossil has been living as such from the time of origin without many changes.
30. Eugenics: It is the branch of science which deals with improvement of human race genetically. It can also be divided into two types : Negative eugenics and Positive eugenics. Under negative eugenics, people with inferior and undesirable (dysgenic) traits are prevented from reproducing.
32. Homo sapiens or modern man is a member of the order primate, sub-order Anthropoidea. Primates have supposed to have evolved from primitive, tiny, insect eating quadruped, similar to modern tree shrews, which lived between 75 and 60 MYA during Eocene period. These belong to order Insectivora. Two evolutionary lines diverged leading to present day prosimians (treeshrews, lemurs, loris and tarsier) and the Anthropoidea (including old world and new world monkey, Ape and man)
33. The first (Ape + Man) ancestor originated in Oligocene period 30-35 MY A under the name Propliopithecus. (Fayiim deposits of Egypt). It is represented by fossil jaws and teeth. Aegyptopithecus is contemporary of Prppliopithecus (Kahira).
34. Dryopithecus -The Oligocene ancestor gave rise to Miocene group of (Apes + Man) called Dryopithecus (formerly known as proconsul). It lived in Africa and Asia. It had semi-erect posture with hind limbs and fore limbs of the same size. Hands and skull were monkey like, fore head like humans, jaw and dentition like apes. Shivapithecus discovered by Chopra Simon team is another fossil ape from Shivalik hills in India (derived from Dryopithecian stock).
An aberrant branch from Oryopithecus evolved in late Miocene early Pliocene, Oreopithecus which later on became extinct. Ramapithecus (= kenyapithecus) 14-15 MYA few teeth and fragments of jaw. Believed to have evolved from Dryopithecus in Shivalik hills in India during late Miocene and early Pliocene.
35. Gremaldi -From caves in a village Grimaldi on the Mediterranean coast, cranial capacity 1655 cc., believed to have given rise to Negroid stock.
Chancelade -Rock shelter near chancelede in Dordogne France, Cranial capacity -1450 cc., gave rise to Modern Eskimos.
36. Modern Man -H.sapiens sapiens evolved about 25,000 years ago but spread to various parts of world about 10000 -11000 years ago. There is thining of skull bones, slight reduction in cranial capacity (1400 -1450 cm3), four flexors in vertebral column and slight raising of skull cap. Modern man underwent cultural evolution. Paleolithic (age of tools of stones, bones, cave paintings). Mesolithic (age of animal husbandry, development of language reading and writings). Neolithic (development of agriculture, manufacture of pottery and clothes). Bronze age followed by iron age.
Forgery of Piltdown Man (Eoanthropus dawsoni) Charles Dawson in 1921 reconstructed the skull from the cranium of modern man and lower jaw of ape. The fossil skull known as Piltdown, the English hamlet where it was found.
38. Transitional forms connecting Home erectus with Homo sapiens have been uncovered from Europe -Steinheim skull (Germany) Swanscombe skull (2nd Interglacial period) Fontechvade skulls (France 3rd Interglacial period) Ehringsdore skull. The above all are called as early H.sapiens.
The course of evolution of man started in Africa.