Steve Jones - The Language of the Genes

The decrease in diversity outside Africa, humankind's native continent, may be because genes were lost as small bands of people moved, split and founded new colonies in the trek across the globe. Just as for Afrikaner surnames the number of variants dropped each time a new colony was founded. The high levels of diversity among Africans is evidence that Homo sapiens has been in that continent for longer than anywhere else. Its decrease at the tips of the evolutionary branches in South America and Polynesia shows how human evolution was driven by chance as the migrants passed through a succession of bottlenecks.

A comparison of the genes of Africans with those of their descendants elsewhere in the world makes it possible to guess at the numbers involved in those early colonisations. The order of bases along a short length of DNA is in some ways a 'genetic surname', a set of inherited letters which pass together as a group down the generations. The name written in nucleic acids around one of the haemoglobin genes has been looked at in detail worldwide. The results are quite unexpected.

All populations outside Africa, from Britain to Tahiti, share a few common sequences. Africa itself has a different pattern of distribution. Just like the names in the Johannesburg telephone book compared to that of Amsterdam, the shift in pattern from the ancestral continent to its descendants may be a relic of a population bottleneck at the time of migration — this time from, rather than to, Africa. We can do some statistics (and make quite a lot of guesses) to work out the size of this hundred-thousand-year-old group of emigrants. They show that the whole of the world's population outside Africa may descend from fewer than a hundred people. If this is true, non-Africans were once an endangered species.

Science has two cultures: one (to which most scientists belong) uses mathematics and the other understands it. Such guesses about ancient population bottlenecks demand statistical acrobatics. They also depend on one crucial, and perhaps quite mistaken, assumption; that the genes involved do not alter the chances or survival or of sex. Molecular biologists tend to assume that small changes in the structure of DNA are unimportant. It is just as possible that they do have an effect on fitness. If, for example, Africans have more variation on the surfaces of their cells because it helps to combat disease, then to claim that a reduction elsewhere is due to an ancient bottleneck is simply wrong.

Any attempt to reconstruct the distant past is bound to suffer from ambiguities such as these. Genetics has not yet revealed just how many Adams and Eves there may have been, but shows that much of the human condition has been shaped by accident: an observation that might at least instil a certain humility into those whose genes have defeated the i iws of chance by surviving to the present day.

Renaissance painters on religious themes hat! a problem: when they showed Adam and Eve, should they have- navels? If they did, then surely it was blasphemous as it implied that they must have had a mother. If they did not, then it looked silly. Although some compromised with a strategic piece of shrubbery, that did not resolve matters. And where was the Garden of Eden? Various theories had it in Israel, Africa and even the United States. When it existed seemed obvious because to add up the ages of the descendants of the primal couple as given in the Bible set the start of history as 4 October 4004 BC.

The reason for leaving Eden was also clear. Its inhabitants had, with the help of an apple, learned forbidden truths, and as a punishment were forced out into the world. No longer could they depend on a god-given supply of food falling into their hands. Instead, they had to make a living. The first economy was born.

The escape from Eden — the colonisation of the Earth — showed how genetic change is linked to economic development. Economics is often seen as a kind of enlightened self-interest. The desire to increase one's own wealth may, as Adam Smith has it, be the invisible hand which is at the foundation of all social progress. The same argument is used by some evolutionists. Genes are seen as anxious to promote their own interests, even at the expense of their carriers. In its most naive form, this view of life is used to explain (or at least to excuse) spite, sexism, nationalism, racism and the economic and political systems that grow from them.

Theories of economics and of evolution have obvious ties. Darwin was much influenced by the works of Malthus, who had been disturbed by the new slums of the English cities of the eighteenth century. In his Essay on the Principles of Population Malthus argued that populations will always outgrow resources. That notion led Darwin to the idea of natural selection.

Karl Marx, himself a denizen of one of the most congested of London districts, was just as impressed by the dismal conditions of the new proletariat. He sent Darwin a copy of Das Kapital (which was found unread after his death). Marx, in a letter to Engels three years after The Origin of Species, wentso faras to say that 'It is remarkable how Darwin recognises among beasts and plants his English society, with its division of labour, competition, opening up of new markets, inventions, and the Malthusian struggle for existence. 1Engels took it further. In his essay The Part Played by Labour in the Transitioft from Ape to Man he argued that an economic change, the use of hands to make things, was crucial to the origin of humans. If one substitutes the term 'tools' for 'labour' his views sound rather like those of modern students of fossils.

Genetics shows that much of evolution is, as Engels said, linked to social advance. However, far from society being impelled by its genes, social and economic changes have produced many of the genetic patterns in the world today. Every technical development, from stone tools on, has led to an evolutionary shift and to biological consequences that persist for thousands of years. Society — and most of all the economic pressures that cause people to move — drives genes, rather than genes driving society. Relentless expansion is at the centre of human evolution: in Pascal's pessimistic words, 'AH human troubles arise from an unwillingness to stay where we were born.'

Fossils show that almost as soon as they evolved, humans began to migrate. Why our ancestors were so restless, nobody knows. Technological progress may have been involved, as the emergence of modern humans coincided with improvements in stone axes and the* like (although tools had been made for at least two million years before the great diaspora).

Perhaps climatic change was as important. The Sahara Desert was once a grassy plain and Lake Chad a sea bigger than the present Caspian. Both dried up about a hundred thousand years ago, so that food shortage may have driven man out of Africa. A microcosm of that process is taking place at the southern edge of the Sahara. As the rains fail, the desert has spread into the Sahel and migrants are on the move.

The earliest economies had a simple foundation. People used what nature provided, until it ran out. The world is filled with fossils of large and tasty animals that were driven to extinction soon after humans arrived. In Siberia, so many mammoths were killed that the hunters made villages from their bones. In Australia, too, there was a shift from forests to grasslands as the immigrants burned their way across the continent. The record of destruction is preserved in the Greenland ice-sheet. The snows which fell tens of thousands of years ago retain the soot and ash from gigantic forest fires set by our ancestors.

New Zealand was not colonised until the time of William the Conqueror. For a few years there flourished a culture based on the exploitation of a dozen species of moas, giant flightless birds. The ritual slaughtering grounds where the birds were killed (and where half a million skeletons have been found) are still around. The birds themselves are not. In Europe, too, whole faunas went not long ago. Humans did not reach Crete, Cyprus and Corsica until around ten thousand years before the present. Before then they had some extraordinary inhabitants; pygmy hippos, deer and elephants, and giant dormice, owls and tortoises. Soon after the arrival of the first tourists, all were gone, and the burnt bones of barbecued hippos are scattered among the remnants of the earliest Cypriots.