Monday, September 12, 2005

The evolution of brain

Dr. K. Rohiniprasad

When we look at the photographs of monkeys, apes, early and modern humans, we notice marked change in the shape and size of the face relative to the head. There is a gradual reduction in the size of the face and jaws. In early humans, the face was large and positioned in front of the braincase. As the teeth became smaller and the brain expanded, the relatively small face of modern humans is located below, rather than in front of, the large, expanded braincase.

The earliest of human ancestors belonging to the australopithus category lived in eastern Africa and started walking on two legs. With a brain size a little larger than those of chimpanzees (about 400 to 500cc) they did not make any tools. While some of them progressed into evolutionary ‘dead ends’ others evolved into species that made stone tools 2.5 million years ago. Later (1.5 to 1.6 million years ago) the species Homo erectus evolved with a large-brained, small-toothed form, showing a greater sophistication in tool making. Spreading into Asia about 700,000 years ago they learnt the use of fire and their behaviour was becoming more complex and efficient. Fossil records show that during this period their brain sizes grew from 750 to 800cc to 1100 to 1300cc (comparable to modern humans). Human habitats expanded from Africa to parts of Asia and early men faced several challenges posed by climate and geography. All these factors made adaptation inevitable and only an evolving brain could cope with difficulties.

The ratio of brain to body mass also increased gradually and human evolution progressed into new directions compared to other mammals. The size of the brain has more than tripled as part of a complex interrelationship that included the elaboration of tool use and tool making, as well as other learned skills, which permitted our ancestors to be increasingly able to live in a variety of environments. Running legs, working hands and speaking tongues brought about marked changes in the functioning of many areas the cortex in the human brain and this has set humans apart from other mammals.

Anthropological studies indicate that Neanderthals and Cro-Magnons, who preceded modern man, had brains larger than modern men but seem to have become extinct nevertheless. It is being surmised that in spite of the size certain areas of the brain related to forethought, analytical abilities and other modern attributes did not develop sufficiently in the case of these pre-modern men.

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