The 10,000 Year Explosion

The 10,000 Year Explosion: How Civilization Accelerated Human Evolution

Author	Gregory Cochran, Henry Harpending
Publisher	Basic Books
Publication date	2009
Pages	304
ISBN	0-465-00221-8
Dewey Decimal	599.93'8-dc22
LC Class	GN281.4.c632 2009

The 10,000 Year Explosion: How Civilization Accelerated Human Evolution is a 2009 book by anthropologists Gregory Cochran and Henry Harpending. Starting with their own take on the conventional wisdom that the evolutionary process stopped when modern humans appeared, the authors explain the genetic basis of their view that human evolution is accelerating, illustrating it with some examples.^[1]

Opinions in book

Cochran and Harpending put forward the idea that the development of agriculture has caused an enormous increase in the rate of human evolution, including numerous evolutionary adaptations to the different challenges and lifestyles that resulted. Moreover, they argue that these adaptations have varied across different human populations, depending on factors such as when the various groups developed agriculture, and the extent to which they mixed genetically with other population groups.^[2]

Such changes, they argue, include not just well-known physical and biological adaptations such as skin colour, disease resistance, and lactose tolerance, but also personality and cognitive adaptations that are starting to emerge from genetic research. These may include tendencies towards (for example) reduced physical endurance, enhanced long-term planning, or increased docility, all of which may have been counter-productive in hunter-gatherer societies, but become favoured adaptations in a world of agriculture and its resulting trade, governments and urbanization. These adaptations are even more important in the modern world, and have helped shape today's nation states. The authors speculate that the scientific and Industrial Revolutions came about in part due to genetic changes in Europe over the past millennium, the absence of which had limited the progress of science in Ancient Greece. The authors suggest we would expect to see fewer adaptive changes among the Amerindians and sub-Saharan Africans, who have farmed for the shortest times and were genetically isolated from older civilizations by geographical barriers. In groups that had remained foragers, such as the Australian Aborigines, there would presumably be no such adaptations at all. This may explain why Indigenous Australians and many Native Americans have characteristic health problems when exposed to modern Western diets. Similarly, Amerindians, Aboriginals, and Polynesians, for example, had experienced very little infectious disease. They had not evolved immunities as did many Old World dwellers, and were decimated upon contact with the wider world.^[2]

Summary

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 find, 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.^[2] 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, since people are 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.^[3] The International HapMap Project and other studies have shown that selection is ongoing and has accelerated over time. This has been a key discovery in human biology, and Cochran and Harpending, building on their own work and that of others such as John Hawks of the University of Wisconsin-Madison,^[4] 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.

Neanderthals

Wolpoff writes that Cochran and Harpending continue to refute conventional wisdom in their discussion of the Neanderthals. 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^[2] 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.^[1]

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.^[5] 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.^[6]

Agriculture

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.^[2]

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.^[7] 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 vitamin D in their skin.^[8] Other writers, including Darwin, Miller,^[9] and Dawkins,^[10] 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"),^[11] pointing out that experts can easily determine race from skeletal evidence alone.

Gene flow

About halfway through the book, Cochran and Harpending pause to consider two different ways of looking at the information found in gene variants.^[3] 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.

Expansions

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.^[12] But, according to Kelleher,^[3] Cochran and Harpending go on better than Diamond. He goes on to state that where Diamond was content with environmental determinism, at times opposing the roles of human biology and population differences, Cochran and Harpending embrace them both. Their discussion 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.^[3]

Ashkenazi Jews

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^[13] 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".^[3]

Reviews

Wolpoff^[1] believes that Cochran and Harpending ‘‘get it’’ when so many paleo-anthropologist specialists don’t seem to: the significance of Neanderthal genetic contributions to the modern gene pool is found in the importance of the genes that persisted, not in their quantity.

In Seed, T.J. Kelleher positively described this book as "a manifesto for and example of a new kind of history, a biological history, and not just of the prehistoric era. Covering broad ground over human history and prehistory, the authors argued successfully for the singular importance of genes in human history, not just as markers of it, but also as its makers."^[3]

In respect of Ashkenazi Jews, however, Kelleher feels that "Cochran and Harpending's impatience to get to the core of their argument drives them to quickly dismiss most of their critics  –  too quickly, given the long and rancorous debate in the United States about the genetic heritability of intelligence. It would have benefited the book to consider those criticisms at greater length."^[3]

See also

• Before the Dawn: Recovering the Lost History of Our Ancestors
• Human evolution

References

External links

• The 10,000 Year Explosion Official website for the book
• Human Cultural Diversity ("a chapter of [the] book [...] that the editor discarded as 'too academic'" )
• West Hunter Cochran and Harpending's blog.