Origin of life
Many theories of “Origin of life” have been propounded by several researchers which are as follows—
(1) Special creation theory : God created universe in six days. This theory was published in the ‘Bible’ in 16th century.
First day Earth and heaven were created
Second day Sky was created
Third day Dry earth and plants were made
Fourth day Sun, moon and stars were created
Fifth day Fishes and birds were created
Sixth day Man and all other animals were created.
This theory was given by Father Saurej and supported by Milton .
(2) Theory of spontaneous creation (Abiotic genesis) : It is believed that all living forms were generated spontaneously from the inanimate matter.
Nile and Von Helmont supported this theory by performing certain classic experiments.
(3) Biogenesis : According to this theory life can only be originated from life. Francesco Redi (1763) was the propounder of this theory and Lazzaro Spallanzani (1765) confirmed it.
(4) Modern concept of origin of life: The life originated in a gradual manner and not in a sudden manner. Haeckel proposed this theory and said that life originated from inorganic substances which were affected by the external physical active forces.
This theory was elaborated by Oparin (1922).
Chemical evolution
1. Changes in the atmosphere : Formation of C, H, N, P, S, Ni, Al, Fe, Cu, Si, etc. on earth.
2. Formation of inorganic substances : Formation of CO2, NH3, hydrogen water and water vapours, CN etc.
3. Formation of organic substances : Formation of CH4, C2H2, carbohydrates, oxidation of aldehydes and ketones resulted in fat formation, formation of amino acids and proteins.
Complex substanes like carbohydrates, fats and proteins were created in the bottom of sea. J.B.S. Haldane referred these as hot soup. Then formation of nucleoproteins took place.
Organic evolution
Biogeny : Hot soup was a colloid. After a combination of these substances the bigger drops were formed and then the formation of free genes or protovirus took place.
These protovirus were having the capacity of self synthesis of substances and thus the origin of prokaryotic cell took place, DNA was covered by a nuclear membrane and called as nucleus and rest of the substance was known as cytoplasm. Such cell was known as protistan cell.
Miller’s and Urey’s experiments : These two scientists did the following experiment. They mixed ammonia, methane and hydrogen gas in the ratio
2 : 2 :1 in a 5 litre flask. A small flask of ½ litre was connected with bigger one. Small flask contained water. This flask was managed to keep on the water boiling. Bigger flask was provided with two tungsten electrodes. A voltage of 60,000 volts was applied for 7-8 days. The gases thus produced were condensed by the condensor and the connecting ‘U’ tube was filled with red liquid. This fluid contained glycine, alanine and spartic acid along with certain complex organic substances.
This experiment proved that C, H, O and N form the different biologically significant complex compounds.
Theories of organic Evolution
Charles Darwin (1809-1882) was a well-known naturalist who published his views on the concept of evolution. He gave following two theories :
(i) Theory of natural selection.
(ii) Theory of pangenesis.
Theory of natural selection : This theory is based on six fundamental propositions :
(i) Enormous fertility in the organisms : Every living organism has an innate desire of producing offspring and has an enormous power of fertility e.g. rabbit produced 6 young ones in one litter and there are about 4 litters in a year. Thus they produce a million of young ones in a very limited time. The slowest breeder is elephant. During the whole life time it produces 6 young ones. If progeny survive, then within 750 years a single elephant produces 1,90,00,000 young ones. Housefly lays eggs about 120 in number. In 6th generation 80,000 cubic metre space is required for all house flies.
(ii) Struggle for existence : Every living being struggles for existence. Struggle can be of three types.
(a) Intraspecific;
(b) Interspecific;
(c) Environmental.
(iii) Variations and their inheritance : Variations are defined as the differences from the parents or from the same species. Without this the evolution is not possible. Variations give rise to new characters.
Three types of variation are recognised:
(a) Meristic and substantive variation
(b) Continuous and discontinuous variation
(c) Somatogenic and blastogenic variation
(iv) Natural selection or “Survival of the fittest” : Due to the struggle for existence, huge number of animals compete with each but animals with favourable variations only survive and some of them transmit these variations to their offspring. The others with no favourable variations die out by the nature. Thus the natural selection goes on and the fittest and best suited for the conditions only survive but unfit and ill-suited perish. This was called as ‘Natural Selection’ By Herbert Spencer.
(v) Variation in the organisms due to changing environment: Environment is never constant and has a changing unfovourable environment. Thus for the existence in the changing environment, a living being must also be capable to change and adapt according to the new environment. e.g. the great reptiles called dinosaurs of 12 metres long and 6 metres height arose in mesozoic era. They dominated the earth, sea and air but in later era the environment changed and these giant reptiles (dinosaurs) could not adapt to the changing environment and soon became extinct.
(vi) Origin of new species : Due to the changes in environment, the inheritable variations were caused. Each variation after many generations became so different that a new species was formed. This process continued for millions of years. The force of variation has given rise to the modern world animals.
Criticism of Darwin’s theory
(1) Natural selection is not the only cause but it is also the mutation which causes the variation in the animals.
(2) Darwin did not explain the heritable and non-heritable variations and also the causes of variation which were present before selection.
(3) The natural selection is a guiding factor but it was called creative factor by Darwin.
For removing the above objections, Neo-Darwin theory of origin of new species was proposed. The mutation alongwith the natural selection was taken into consideration to remove all the objections of Darwin’s theory.
Theory of pangenesis : Darwin was aware of the experiment performed by Mendel but he realised that heredity factor is most essential for the evolution. He proposed this theory as follows :
(i) All animals produce very small, exact and invisible copies. Darwin called it as pangenes.
(ii) These pangenes after circulation through the blood reach the gametes. So the pangenes (gemmules) of male and female will have the different organs.
(iii) The pangenes of opposite sex will come together in a zygote after fertilization.
(iv) These pangenes were separated and form the different parts containing the maternal and paternal characters during development.
(v) Excess use and disuse alter the pangenes which cause the variation in the offspring.
Evidences of evolution
Morphological evidences : This evidence is concerned with the structural similarities. This is the main evidence to support the idea of organic evolution. The structural similarities have not appeared randomly or suddenly or by chance but it indicates that these similarities have come from a common ancestor. These can be discussed as follows :
Homology and analogy : The organs which have the same embryological origin but may have different functions are known as homologous organs and the phenomenon is known as homology. But the organs which develop from different embryonic structures but may have the same function are known as analogous organs and the phenomenon is known as analogy. The homology indicates the evolutionary chains of the animals.
Examples : (i) Forearm of man, foreleg of horse, wings of bat, wings of bird and flippers of seal. These structures perform the different functions but their origin (embryological) is the same. They are built on one fundamental plan.
(ii) Dorsal vertebral column from fishes to man. These are formed in many vertebrae.
Wings of birds or bats and wings of insects have the same function e.g. flying but their origin is different. So these are analogous structures. This is due to the similarity of environment and similarity in function. This shows the superficial similarities of analogous structures.
Vestigial organs : Some structures were useful in ancestors but these structures are now no more required by the body or they have stopped the function but are present in reduced form e.g.
(1) Second finger in the bird’s wings.
(2) Non-functional eyes of some burrowing animals.
(3) Splint bones in the horse feet.
(4) Rudimentary limbs of snakes.
(5) Reduced wings of kiwi.
(6) Nictitating membrane of plica semilunaris, vermiform appendix, coccyx or tail vertebrae and tail muscles, pinna muscles are some vestigial organs in man.
Embryological evidences : There is a greater resemblance between embryos of different species of a group. But the embryos of higher groups resemble the adults of the lower groups also, e.g. embryos of fish, frog, turtle, bird and man resemble their embryos of the same stages, even it is difficult to differentiate them.
This provides a proof of evolution and also indicates that these animals have evolved from a common ancestor.
The tadpole of a frog has many similarities with the fishes such as compressed tail, external and internal gill slits and lateral line sense organs. The aortic arches are 6 pairs in the embryos of fishes, frogs, reptiles, aves and mammals.
Haeckel’s recapitulation theory or Biogenetic law : An organism in its development repeats the embryonic development of its ancestors or its ancestral history thus ontogeny recapitulates phylogeny.
This theory was put forward by the Von Baer and Haeckel. The embryonic development from zygote to the adult stage is known as ontogeny. The evolutionary history of an animal starting from its ancestors to the present form is known as phylogeny.
Human evolution
The earliest and primitive ape lived about 40 million years ago (oligocene) and is known as Parapithecus. It is thought to represent an ancestral stock from which all Hominoidea (including man) were derived. Another fossil Propopithecus was found in Egypt and was of oligocene times. One of the smaller miocene apes Limnopithecus resembled the gibbon. Proconsul is considered to be an ancestor of the modern chimpanzee. These resembled humans in appearance. This also showed remarkable similarity in the mandibular condyle of the jaw, the incisors and the frontal bone of the skull. Thus it may be considered ancestor to Homo.
In Africa, Asia and Europe certain fossils were found which were of miocene and early pliocene times. These indicate that anthropoid apes were flourished which were of generalized type. Some of them became modified and evolved the modern apes and some other provided the basis of evolution for the ‘Hominidae’.
Dryopithecus : In miocene times, a group of ape-like mammals called Dryopithecus developed. It was known as Proconsul. This resembled chimpanzee very much. It is supposed to be ancestor of man and apes. It had long canines and rounded forehead.
Ramapithecus (Ramapithecus punjabicus) : Fossil of miocene age which existed about one crore years ago were found from Shivalik hills in India by G.E. Lewis.
Australopithecus
(1) About one million years ago, fossil were found in South Africa.
(2) Cranial capacity about 350-600 c.c.
(3) Jaws and teeth like man.
(4) The pelvic girdle structure showed the erect walking.
(5) They did not make any tools.
Java ape man (Homo erectus)
(1) In early and middle Pleistocene about 6,00,000 years ago.
(2) Fossil was discovered at Trinil in Java.
(3) Cranial capacity was about 750-1000 c.c.
(4) Body height 170 cm and weight 70 kg.
(5) Long straight legs, slopping forehead, high eyebrow ridges and flat skull.
(6) They were cave dwellers in small groups.
(7) They made tools of stones and bones and used fire to cook meat.
Peking man (Homo erectus pekinensis)
(1) About 5,00,000 years ago a fossil was found near Peking in China in
1920-24.
(2) Cranial capacity was about 1075 c.c.
(3) Forehead was low and receding chin was not present.
(4) Canine teeth were large.
(5) They used sharp chisel shaped tools of stones for cutting and killing the animals.
(6) They were also cannibal.
Neanderthal man (Homo neanderthalensis)
(1) Existed about 1,50,000 years ago. Fossil was found in the Neanderthal valley in Germany.
(2) Cranial capacity was about 25,000 years ago.
(3) Cranial capacity was about 1450 c.c (=man).
(4) Face was long, jaw projecting, large eye socket and rounded.
(5) Slightly stooped body, slightly bent knee, height 54′ (approximately).
(6) True man with a culture and knew the manufacturing of tools and weapons. Followed burial customs of dead.
(7) These are supposed to be descended from the Pithecanthropus as a side line.
(8) They were found in Europe, Far East and Africa.
Cro-Magnon man (Homo sapiens fossilis)
(1) About 50,000 years ago and became extinct about 20,000 years ago, Their fossil was discovered in a rock shelter at Cro-Magnon in France in 1968.
(2) Cranial capacity was about 1660 c.c. much more than the modern man.
(3) This race succeeded Neanderthals and were direct ancestors of modern man.
(4) Skull was narrow and large with a broad face.
(5) Rounded forehead, narrow nose, thick and strong jaws, and a narrow prominent chin.
(6) They were swift footed, cave dwelling hunters.
(7) Their races were distributed in the Middle East, Africa and Europe.
(8) They were hard working, intelligent and had an art to draw good pictures and paintings.
Modern Man (Homo Sapiens )
(1) They were living for at least 25,000 years and included all the existing men of whatever races such as negroid, caucasiod or mongoloid.
(2) Entirely erect posture with an upright head and eyes facing infront.
(3) Limbs were straight and arms shorter than legs.
(4) Cranial capacity about 1350-1500 c.c.
(5) Much enlarged cerebrum.
(6) Steep forehead eyebrow ridges much reduced, prominent chin and snout disappeared.
Future man : According to H.L. Shapiro of America the future man will be taller, baldy, more developed body with little more cranial capacity without phallanges in legs. Shapiro has given the name to this as Homo futuris.
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