The earliest jawless vertebrates appeared in early Paleozoic about 300 million years ago (Bodmer, 1976). The fishes, amphibians and reptiles also arose, in that order, during the Paleozoic. The reptiles ruled the earth for many millions of years. During the Mesozoic giant like reptiles, the dinosaurs were dominating the earth for over a million years, and then became extinct. From the reptiles, the birds and mammals seem to have diverged along two separate lines, the mammals culminating in man. In this paper we would be discussing the evolution of man.
Literature Review The earliest organism resembling human beings was Australopithecus, an erect biped walking tool making creature discovered from cave deposits of Pleistocene in South Africa more than 5 million years ago. It had skull, brain and face like an ape while dentition was similar to humans. After existing for a long time, australopithecines or ape man evolved into Homo erectus with double the size of brain better bipedal and skills. In the late 50 to 55 thousand years, Homo erectus seems to have given way to Homo sapiens, the present day man (Richards, 1992).
The evolutionary development of man took place mainly along the following lines: a) Growth of Brain, b) Erect posture, c) Slow rate of development after birth, d) Complexities of social life and behavior, e) Growth in human population and f) Technical advancement. The size of brain in the modern ape is about 500-600 gm, which is about the same as the cranial capacity of Australopithecus. In the primitive men of the Pleistocene, this capacity increased to 900 gm. In modern man, the brain size ranges from 1200 gm in the minimum to 2000 gm in the maximum (Fisher & Bennett, 2000).
It seems the size of the brain increased in relation to mental development. In the evolution of man, the brain case has been getting bigger in size compared with the rest of the skull and the jaws have become proportionately small. The development of erect posture has allowed man to use two free hands for other skills and has changed the shape of the spinal column. Due to slow rate of growth, the young one of man matures in about two decades after birth while a gorilla matures in less than ten years.
Lastly, with increase in mental powers man has gained ability for perfection in social, behavioral and technical skills. A few thousand kinds of proteins are required for the functioning of an E. coli cells and about a million kinds are required by human beings. Proteins offer a better field for evolutionary study than other macro molecules like nucleic acids because they are more heterogeneous both structurally and functionally, and are easy to isolate for analysis. Recently a number of proteins have been characterized by the method of sequence analysis.
Sequence data are now available from diverse biological groups ranging from micro-organisms to mammals. Comparison of sequences allows study of interrelationship between structure and function, and to deduce how proteins have evolved. The laws governing evolution of proteins are similar to those for heritable traits, and interrelatedness of different organisms points toward their descent from a common ancestor. The high degree of stability of nucleotides in nucleic acids, and because the changes in nucleotide sequences are slow, they provide valuable information about their origin and evolution.
The divergence of prokaryotes and eukaryotes can be traced through the development of the genetic code. Transfer RNA determines specificity of the genetic code as it transports specific amino acids for the protein change. When nucleotide sequences of t-RNAs carrying different amino acids are compared they are found to be closely similar. It is suggested that the different t-RNAs originated from a common ancestral t-RNA, a proto-t-RNA which acted as a non-specific catalyst and polymerized amino acids by a mechanism similar to that used in present day species (Lynch, 2007).
As the genetic code is common to all living species, it is thought to have arisen in a common ancestral line before the divergence of prokaryotes and eukaryotes. During his voyage round the world aboard the Beagle, Darwin made a halt at Galapagos Archipelago on the coast of Equador in the Pacific Ocean. Here he studied the flora and fauna on a group of islands isolated from the main continent of South America (Lynch, 2007). He noticed the variation in the finches and giant tortoises, each island possessing a distinct variety.
The variations made the forms distinct enough to be regarded as separate species. Yet Darwin was impressed that the finches for instance, formed a homogenous group and he thought that they may have descended from a common ancestor. Darwin also listed a large number of plant species on the four islands. He noticed variation in them and could determine that many of them were not present anywhere else in the world; the majority of plants were endemic to a single island in Galapagos.
From his detailed study, Darwin concluded that the evolutionary process was producing new varieties of plants and animals. When finches and tortoises came to Galapagos Islands they evolved in a different direction to other similar populations (Lynch, 2007). The increase in population of a species is due to reproductive capacity, survival of young ones, low rate of mortality, and absence of competition. Darwin held the view that mortality took place so that the less adapted would not survive. Darwin accounted for evolutionary change through this system.
In natural selection, the environment determines the efficiency of a species by eliminating the less fit individuals. The best adapted individuals survive and their offspring inherit these capabilities. Thus when a new population reaches a habitat, it will gradually become adapted to its new environment. This is the basic idea of evolution by natural selection. The rediscovery of Mendels work has revised Darwins theory of evolution by natural selection (Neo-Darwinism) but without altering the basic underlying principles.
Bodmer, W F. 1976. Genetics, Evolution and Man. W H Freeman & Co. pp. 102-105. Fisher, R A & Bennett, J H. 2000. The Genetical Theory of Evolution. Oxford University Press, USA; 1st edition. Lynch, John M. 2007. Darwins Theory of Natural Selection. Thoemmes Continuum, pp. 881-890. Richards, Robert J. 1992. The Meaning of Evolution: The Morphological Construction and Ideological Reconstruction of Darwins Theory. Chicago: University of Chicago Press.