Out of Africa

THE question of the origin of modern human beings - Homo sapiens - has been the cause for considerable debate in the past. One hypothesis goes by the name of 'Multi-regionat theory of human evolution'. As the name suggests, it contends that our ancestral species, the Homo erecrus, spread to different parts of the world over one million years ago, and that the Homo sapiens evolved gradually and in parallel in these places. Regular genetic exchange ensured that the same species evolved in different locations. At,the same time, local differences remained to a fair degree; these, so the assertion goes, are still to be found in the human races of today.

The second hypothesis is called the 'Out of Africa' theory. According to it, Homo sapiens originated in Africa between 100,000-200,000 years ago and then migrated worldwide. The two points of view agreed on one thing - that all humans descended from African ancestors. There is an essential distinction, however. The first theory says our last common African relative lived a million years ago and belonged to another species; the second says that our African Adams and Eves were around much more recently, and besides, were one of us'.

To a great extent, the debate has been fuelled by findings on variations in human DNA. One assumes a certain rate of change in the letters that constitute the same DNA molecule in the two groups of humans that we wish to compare. Then one estimates the extent of change by calculating the most likely genetic distance covered between the last common ancestor and the two representative samples. Knowing the Tate and the distance immediately gives us a measure of the time since divergence.

A Fictitious example might help. It is known that when the same language is used in widely separated areas, spellings tend gradually to drift. Say, the average rate of drift works out one letter in 100 years. Suppose there are two given spellings for the'same word, such as 'lapbit' and 'robbet'. it would be a reasonable, therefore, to draw an inference which would point to the fact that the two must have shared a common ancestor and that the ancestral word was, probably, 'rabbit'.

D B Goldstein, L L Cavalli-Sforza and colleagues at Stanford University, us, have taken a fresh took at the problem by collecting information on what are known as DNA microsatellites. These are very short segments of DNA which tend to be repeated one after another a large number of times. It appears that they have no significant functional role to play, because their sequences - the 'letters in the word' - drift randomly but with the clocklike regularity expected of anything which evolves as fast as it mutates.

By applying a new measure of molecular distance, Goldstein and colleagues studied 30 microsatellites in 14 human populations and constructed a tree of relationships. The toot of their constructed tree separates Africans from all non-Africans, suggesting first of all that the primeval sequence was closest to that found in modern-day Africans.

Next, using available information on the rate of mutation in the microsatellites that they looked at, they came up with a figure of 156,000 years as the best estimate for the time since African and non-African populations diverged. This offers yet another piece ofsupport to the proponen ts of the 'Out of Africa' theory (Proceedings of the National Academy of Sciences of the USA, Vol 92, 1995).

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