BIOLOGICAL EVOLUTION
The first living organisms Originated among organic molecules and in an oxygen free atmosphere (reducing atmosphere) they presumably obtained energy by the fermentation of some of these organic molecules. They were anaerobes capable of respiration 1in the absence of oxygen. They depended on the existing organic molecules for their nutrition and thus they were heterotrophs.
When the supply of existing organic molecules was exhausted, some of the heterotrophs might have evolved into autotrophs. These organisms were capable of producing their own organic molecules by chemosynthesis or photosynthesis
The organisms performing chemosynthesis are called chemoautotrophs. They were anaerobes. Chemoautotrophs developed the ability to synthesis organic molecules from inorganic raw materials. The photosynthetic organisms, the photoautotrophs, developed the pigment chlorophyll by combination of simple chemicals. They prepared organic food by using solar energy captured with the help of chlorophyll. They lacked the biochemical pathways to produce oxygen. They Were still anaerobes and utilized hydrogen from sources other than water.
At a later stage, oxygen releasing photosynthetic organisms developed. These were. similar to the existing blue green algae (cyanobacteria). They used water to get hydrogen and released oxygen Aerobic respiration evolved sufficient oxygen in the primitive atmosphere. The prokaryotes gradually modified and adapted themselves according to new conditions. They developed a true nucleus and other specialized cell organelles. Thus free-living eukaryotic cell-like organisms
Origin of the ancient ocean, presumably about 1.5 years ago. Primitive eukaryotes led to the evolution Of Protists, plants, fungi and animals. Life was present on earth about 3.9 billion years ago However, the oldest microfossils discovered so far are that of photosynthetic cyanobacteria that appeared 3.3 to 35 billion years ago.
EVIDENCES OF EVOLUTION
Evolution or organic evolution or bio evolution is 'descent with modification', (Darwin) or 'continuity of life with constant modifications.
The convincing evidences supporting this are: comparative morphology, anatomy and embryology, paleontology, geographical distribution, taxonomy, connecting links,cytology, biochemistry, physiology and genetics etc.
Morphological and anatomical evidence
They include related organ systems, homology, analogy adaptive radiation, adaptive convergence, vestigial organs and atavism etc.
Respiratory system of all terrestrial vertebrates has two lungs, a trachea, a larynx, nasal chambers and nostrils. is the similarity between organs of different animals based on common structural organization or common embryonic origin.
• Homologous organs have common origin and are built on the same fundamental pattern, but perform varied functions and have varied appearances. E.g, Scale flipper, bird wing, bat wing, horse fore limb, and human arm look different, perform different functions but exhibit same structural plan Legs and mouth parts of various insect groups, thorns and tendrils of some plants, are other examples of homology. Homology confirms divergent evolution or adaptive radiation.
• Adaptive radiation represents evolution of new storms in several directions from the common ancestral type (divergence). e.g the limbs in mammals are variously adapted for climbing. flying, running, swimming or burrowing etc.
Homologous bones of Forlimbs
• Analogous organs are totally different in basic structure and developmental origin, but perform the same function, eg, wings of a butterfly (chitinous), bird (feathered), pterodactyl and bat (skin fold) serve the same purpose of uplifting the body in air but differ in structure Other examples are fins of fishes and flippers of whale, cephalopod and vertebrate eyes, etc. In plants, leaves and cladodes, tuberous root and potato etc. are analogous organs Study of analogous organs illustrates the occurrence of convergent evolution.
• Vestigial or rudimentary organs are the useless remnants of structures or organs which might have been large and functional in the ancestors. Vermiform appendix in man, nictitating membrane, auricular muscles, vestigial tail in vertebrates, wisdom teeth, mammary glands in males and body hair are some of its examples.
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| Analogous wing |
• Atavism or reversion is the reappearance of a certain ancestral, not parental structure which has either completely disappeared or greatly reduced. Occurrence of short tai in some babies, additional mammale, larger canine teeth, power of moving pinna, very long and dense hair etc. are some of the examples of atavism in humans.
Embryological evidence
Comparative embryology also provides evolutionary evidence. Homology in early embryonic development and fate of three germ layers establish a close relationship among the animals and suggest that all have evolved in a series from a common ancestor. Homology in the embryos of closely related vertebrates indicates the evolutionary relationship of the adult vertebrates. Haeckel formulated the "Recapitulation theory or Biogenetic Law regarding this. This theory says that "ontogeny recapitulates phylogeny" i.e, life history recapitulates evolutionary history, This means an organism repeats its ancestral history during its development. For example in the development of frog a fish like tailed larva is formed, which swims with the tail and respires by gills This indicates that the frog has been evolved from a fish like ancestor Von Baer's (Father of Modern Embryology) principles of embryonic differentiation constitute a better guide embryological evidences for evolution During metamorphosis, ascidian tadpole loses all the chordate features like notochord, nerve cord and myotomes. This is called retrogressive metamorphosis. Larva has helped in determining its chordate nature. Retention of larval features (neoteny) provides evidence in favour of evolution through natural selection. Axolotl larva is the classical example of neoteny in which an adult retains the embryonic traits. In some animals (salamanders) gonadal development occurs in immature larval or preadult animals (precocious development).
Paleontological evidences
Palaeontology is the study of past life based on the fossil records. It furnishes the most direct and most reliable evidence for evolution. The term fossil (Latin, fossil something to dug out) reters to the petrified remains or impressions of organisms that lived in the past preserved in sedimentary rocks. Bones, teeth, shells, impressions or imprints in the sof mud, or the moulds and casts of entire organisms are some examples of fossils. The study of fossil plants is called palaeobotany while the study of fossil animals is called palaeozoology. From the fossil records it has been concluded that evolution has taken place from simple to complex in a gradual manner. The different methods of fossilization are intact preservation, petrification, moulds and casts, impressions, mummies, tracks and trails etc. The media in which fossils occur include sedimentary rocks, amber, asphalt, volcanic ash, ice, peat bogs, sand and mud. Age of a fossil is determined by dating of rocks in which it was found. The dating of rocks is called geochronology and the dating system is called "clock of rocks" Geological time is a chronological order or history of evolution based upon the study of fossils. It has been divided into eras, periods and epochs. The fossil studies have given evidence of several mass extinctions. Mass extinction is death and disappearance (extinction) of large groups of plants and animals over a short span of time t might be due to the crash of a meteor or comet with earn or due to the drifting, coalescing and breaking apart of continents. This shapes the overall pattern of macroevolution. The largest mass extinction came at the end of the Permian, about 250 million years ago. The most well-known extinction Occurred at the boundary between the cretaceous and tertiary period. this called the K/T boundary, is dated around 65 million years ago. This extinction eradicated the dinosaurs.
Connecting links
Living organisms possessing characters of two different groups of organisms are know as the connecting links. Examples Viruses: between non living and living. Euglena: between plants and animals, Peripatus: between annelida and arthropoda, Balanoglossus: between non-chordates and chordates, Chimaera: between cartilaginous and bony fishes. They give a hint about the evolution of one group from the other. On the other hand fossils that show combined features of two groups are termed missing links, e.g., Archaeopteryx
Molecular and physiological evidences
Similarities in the biochemical composition, reactions and physiological activities are the most convincing evidences of common ancestry Aspects of biochemistry that indicate biochemical affinity are metabolic processes, enzymes, hormones, blood and lymph, blood proteins, blood groups and molecular homology. Blood proteins tests have shown that the man is nearest to the great apes (gorilla and chimpanzee), and next nearest, in order, are the old world monkeys, the new world monkeys and the tarsiers.
Genetics or "science of heredity'' shows the final line of evidence for evolution. Genetic mutations provide raw material for evolution and give rise to new species. Hereditary variations also occur due to genetic recombination. The latter causes Examples of hybridization and mutations are available which show that evolution has taken place. Pattern of transmission of characters is also similar in various organisms. It shows interrelationship among different living beings. Man has developed many varieties of useful animals (e.g. high milk yielding cows, ancon sheep, horses, hornless cattle etc.) and crops (eg., wheat, rice, cotton, etc.). No doubt, the varieties of each of the animals and plants still belong to the same species, but it is possible that if the development of such variations is continued, there would emerge forms which would be regarded as new species. These animals and plants have, thus, undergone change and this is evolution