'Nothing in biology makes sense except in the light of evolution.' said Theodosius Dobzhansky (1900-1975) to emphasise the fact that evolution is one of the unifying ideas in biology. The evidence of evolution is found in all fields of biology. Darwin himself pointed out evidences from taxonomy, embryology, comparative anatomy, geographical distribution of species and from fossil records. Since Darwin many more fields of study, such as biochemistry, genetics and molecular biology, have come into existence and each of these fields show evidences of evolution.
Here are some of the evidences found in some of these fields of biological study:
The fossil records: Fossils are the remains or traces of previously existing animals or plants preserved in the earth’s crust. There are two conditions necessary under which a fossil might generally form from a living organism. The first is that the animal should have hard parts and the second is that it has to be buried under some protecting medium. Quick burial tends to retard decomposition of the animal by oxidation or bacterial action.
Usually the fossil has undergone some changes, with the original hard parts having gradually been replaced by some mineral substances such as calcium carbonate, silica or iron pyrite. This particle by particle replacement is so slow that the microscopic structure of the hard parts is preserved, and the cell walls of wood, for example, can still be studied even though the organic matter is completely gone. If the original hard parts are dissolved, a mould of the shape may then be left in the surrounding rock.
The greatest accomplishment of the palaeontologists has been their reconstruction of the sequences of past events. Water borne sediments are deposited in layers or strata that are then, through pressure, converted to rock. Undisturbed deposition over a long period of time has thus given rise to an accumulation of sediments many feet thick, with the oldest deposits at the bottom and the most recent at the top. The fossils in the bottom layers must, therefore, represent the oldest species and the fossils in the surface layers must be the latest species. Thus the earth’s history has been constructed with subdivisions of the geological time.
In order to get some idea of what type of information is available in the fossil records, the history of the vertebrates could be outlined. The first vertebrate fossil appears in the Ordovician period of the Palaeozoic era. These fishlike animals were small, armoured, bottom dwellers, and lacked jaws and paired fins. They belonged to the class Agnatha. The Agnatha remained common throughout the Silurian and Devonian periods. The first vertebrates to have jaws and paired appendages, the Placodermi, appeared among the late Silurian fossils, were very common in the Devonian, and had virtually disappeared from the Carboniferous record. The Chondrichthyes, a group to which the present-day sharks and rays belong, first appeared in the middle and late Devonian, became abundant in the Carboniferous and has remained common up to the present day. At about the same time the bony fishes, the Osteichthyes, appeared in the fossil record and have flourished ever since. The first land vertebrates, with legs and lungs, did not appear as fossils until the late Devonian. These first tetra pods were amphibians, a group that peaked during the Carboniferous. The first known reptiles were found in the rocks of Carboniferous origin. They increased in numbers during the Permian and were the dominant land vertebrates during the Mesozoic era. The first birds appeared in the fossil records in the Jurassic period of the Mesozoic. Though mammal like reptiles existed in the late Palaeozoic, the first true mammals did not appear as fossils until the Triassic and they did not form an important part of the fauna until the Cenozoic.
These fossils constitute an actual record of the organisms that lived on the earth at different times in the past. An examination of this record shows that animals and plants changed gradually with time. Thus, species adjacent in time are more alike than species separated by vast time spans, and the more recent the fossils, the more they tend to resemble living species. The theory of evolution, of decent with modification, provides the most logical explanation for the fossil records.
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