sequence corresponds almost exactly to our own. They are highly intelligent, feel empathy for others, fabricate a variety of tools, and lead a complex social life. But by chance and circumstance, chimpanzees took one path through evolutionary space, the human lineage took another. Perhaps the chimp path required rather little change, whereas the human lineage, seeking a way of life beyond the trees, became so different because it was constantly forced to innovate.
The relentless search for new solutions produced not one but a whole clutch of hominid species. At least three—the Neanderthals, Homo erectus, and Homo floresiensis—survived until modern humans made their exit from Africa. Had these archaic peoples endured till the present day, our own species would surely seem less special, being evidently just one of many ways in which evolution could spin variations out of the basic ape lineage.
But if evolution generates new species by mechanisms that are in part random, should human existence be ascribed just to a long sequence of chance events? In the passage quoted above, that concludes his Descent of Man, Darwin gives a typically careful answer, a yes with a reservation. Man has risen to the summit of the organic scale, he says, "though not through his own exertions." Yet "some pride" in the result would be excusable. Why so, if no human exertion was involved? The reference a few lines later to man's powers of sympathy, benevolence and intellect is presumably Darwin's answer, and we can perhaps begin at last to see what he meant. Though evolution through natural selection depends on random processes, it is shaped by the environment in which each species struggles to survive. And for social species the most important feature of the environment is their own society. So to the extent that people have shaped their own society, they have determined the conditions of their own evolution.
The nature of this interaction between culture and evolution is not yet clear, because it has only just come to light. It has long been assumed by historians, archaeologists and social scientists that human evolution was completed in the distant past, probably before any kind of culture had begun, and that there has been no evolutionary change, or only a negligible amount, within the last 50,000 years or so. Even evolutionary psychologists, who are committed to explaining the mind in terms of what evolution shaped it to do, assume that evolution's work was completed in a preagricultural past more than 10,000 335
But the evidence now accumulating from the genome establishes that human evolution has continued throughout the last 50,000 years. The recent past, especially since the first settlements 15,000 years ago, is a time when human society has undergone extraordinary developments in complexity, creating many new environments and evolutionary pressures. Hitherto it has been assumed the human genome was fixed and could not respond to those pressures. It now appears the opposite is the case. The human genome has been in full flux all the time. Therefore it could and doubtless did adapt to changes in human society. And this may mean that people have adapted in various ways, both good and bad, to the kinds of society they lived in. Following is a review of the evidence that evolution is an active and vigorous force in the human population, a brief look at some of the implications, and a discussion of where human evolution might be headed in the future.
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