The 10,000 Year Explosion
Henry Harpending
 The book's main thesis is that human civilisation greatly accelerated increases in the rates of evolution. The authors begin their discussion by quoting the conventional wisdom: :Something must have happened to weaken the selective pressure drastically. We cannot escape the conclusion that man’s evolution towards manness suddenly came to a halt. --Ernst Mayr, 1963. :There’s been no biological change in humans in 40,000 or 50,000 years. Everything we call culture and civilization we’ve built with the same body and brain --Stephen J. Gould, 2000. This had become the established viewpoint - when modern humans appeared, evolution was essentially over. The received wisdom is based on the doctrine that human minds are the same, everywhere: Bastian's Psychic Unity of Mankind. Unfortunately, the authors continue, this is no more than wishful thinking. Were it true, human bodies would also be the same worldwide, which clearly they are not. Finns cannot be mistaken for Zulus, nor Zulus for Finns. Not only are there strong reasons to believe that significant human evolution is theoretically possible, or even likely; it is completely obvious that it has taken place, assert the authors, since people look different from one another. The first four of the book’s seven chapters serve as a preamble to the final three. First, Cochran and Harpending present evidence for recent, accelerated human evolution after the invention of agriculture. In itself, this argument represents a paradigm shift, albeit one that now has clear data to back it up. tie the advent of agriculture — and the selection pressures resulting from the new diets, new modes of habitation, new animal neighbors, and new modes of living that agriculture made possible — to this accelerating evolution. Wolpoff writes that Cochran and Harpending continue to refute conventional wisdom in their discussion of the Neandertals. For natural selection to have a chance, they argue, there need to be favourable mutations, or favourable combinations of existing alleles such as genes for blue eyes or pale skin. Cochran and Harpending concentrate on the Neolithic farming revolution as the beginning of major population expansions that provided enough mutations to accelerate genetic change. Infectious diseases were another consequence of the early urban populations and soon became a new source of selection pressures. The origins of many recently adapted genes have now been traced to this period, creating effects such as regional differences in skin colour and skeletal gracility. Adaptations may have sacrificed muscle strength for higher intelligence and less aggressive human behaviours. By 5000 years ago, the authors estimate that adaptive alleles were coming into existence at a rate about 100 times faster than during the Pleistocene. This is the ‘‘explosion’’ of the book’s title. Research cited by Cochran and Harpending provides evidence of genetic mixing between modern humans and an ancient Homo lineage such as the Neanderthals. It supports the idea that modern humans could have benefited by acquiring adaptive alleles evolved by our Neanderthal relatives - in this case, microcephalin, an adaptive allele associated with brain development. Microcephalin (MCPH1) regulates brain size, and has evolved under strong positive selection in the human evolutionary lineage. One genetic variant of Microcephalin, which arose about 37,000 years ago, increased its frequency in modern humans too rapidly to be compatible with neutral genetic drift. As anatomically modern humans emerged from Africa and spread across the globe, the "indigenous" Homo populations they encountered had already inhabited their respective regions for long periods of time and might have been better adapted to the local environments than the colonizers. It follows that modern humans, although probably superior in their own way, could have benefited from adaptive alleles gained by interbreeding with the populations they replaced, as appears to be the case for the brain size-determining gene microcephalin. Farming, which, the authors note, produces 10 to 100 times more calories per acre than foraging, carried this trend further. Over the period from 10,000 BC to AD 1, the world population increased about a hundredfold - estimates range from 40 to 170 times. An accelerated rate of evolution is a direct result of the larger human population. More people will have more mutations, thereby increasing opportunity for evolutionary change under natural selection. The spread of rapidly expanding populations eventually outpaced the spread of favourable mutations under selection in those populations, so for the ﬁrst time in human history favourable mutations could not fully disperse throughout the human species. In addition, of course, selection pressures changed once farming was adopted, favouring distinctive adaptations in different geographic areas. Farming, rather than just reduced sunlight, may have helped trigger pale skin in Europeans. In a 2007 study, almost all Africans and East Asians have one allele of the SLC24A5 gene, whereas 98% of the Europeans studied had the other. These data suggest that a selective sweep occurred as recently as 5,300 to 6,000 years ago, replacing darker skins with light skins at astonishing speed. It implies that Europeans had been dark-skinned for tens of thousands of years. Several decades ago, Stanford's Cavalli-Sforza had argued that European hunter-gatherers, herders and fishers could have survived from the vitamin D content of their diet, alone. Only when farming took hold did Europeans - replacing meat and fish with grain - need to absorb more sunlight to produce the vitamin in their skin. Other writers, including Darwin, Miller, and Dawkins, have proposed that skin colour changes were driven by sexual selection. Cochran and Harpending reject the sexual selection idea when used to imply that race is no more than skin deep ("perhaps little more than a fad"), pointing out that experts can easily determine race from skeletal evidence alone. About halfway through the book, Cochran and Harpending pause to consider two different ways of looking at the information found in gene variants. Researchers commonly see them merely as markers of human migration, ignoring their functions. The authors support such research, but argue for a more complete understanding of the geographic distributions of genes. Where the usual geographical analysis treats the distribution of genes as an effect of history, in the authors' view, the genes themselves are a major cause: Two variants in the same gene do not necessarily have the same effect, and their relative, selective benefits will  control the spread of genes through populations in both space and time. From that platform the authors discuss ideas that range from the possible origins of the Arthurian legend in Britain to the Spanish colonization of the Americas. Others have attempted this, for example in Jared Diamond’s Guns, Germs, and Steel. But, according to Kelleher, Cochran and Harpending go one better than Diamond. Where he was content with environmental determinism, opposing the roles of human biology and population differences, Cochran and Harpending embrace them both. Their discussions of gene flow becomes the core of an argument for biology as central to history, and the backdrop for the book’s two major hypotheses. The first seeks to resolve a longstanding debate in historical linguistics by making a case for the Kurgan hypothesis on the origins of the Indo-European language group. The Kurgan theory holds that Indo-European speakers came from lands between the Black and Caspian seas before spreading their language by conquest. The authors suggest that dairy farming and a complementary adaptation - the ability to digest lactose in adulthood - lie behind their conquests. With a walking food source, the milk-drinking warriors defeated their plant-growing neighbours. Drinking milk, from cows, horses, or camels, is a behavior shared by many of history’s greatest conquering peoples, whether Kurgans, Scythians, Arabs, or Mongols. Without continuing evolution, the ability to digest milk could never have arisen. In fact, it has done so several times, in different ways, in various places, and it has helped shape human history. Kelleher comments that the authors’ argument makes it difficult to imagine the language in which their book would have been written, were it not for the ability to digest milk. The second major argument, which takes up the final chapter, sets out to explain why Ashkenazi Jews have a mean IQ so much higher than that of the population in general. This argument had been published previously in an earlier paper that attracted wide media coverage, generating extensive criticism and praise. This final chapter prior to the book's conclusion has been described as a consistent, thorough, biological history — or perhaps, better, a consistent biological hypothesis of a specific history, and a falsifiable one to boot.