The 10,000 Year Explosion

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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]

Some reviewers considered that the book raised valuable questions but relied on discredited views. Others criticized it for oversimplifying history and reifying the concept of race.

Opinions in book

Cochran and Harpending put forward the idea that the

The book's main thesis is that human

This had become the established viewpoint—when modern humans appeared, evolution was essentially over. The received wisdom is based on the

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. According to Cochran and Harpending, 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, they argue, 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, it is argued, 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 was proposed to be the case for the brain-size-determining gene microcephalin.^[6] However, analysis of the genomes of neanderthals did not find the microcephalin gene variant in question to be present,^[7][8][9] and later studies have not found the gene variant to be associated with mental ability.^[10][11][12]

Agriculture

Farming, which, the authors say, 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]

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.^[13] But, according to Kelleher,^[3] Cochran and Harpending go one 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

In New Scientist, Christopher Willis wrote that the "evidence the authors present an overwhelming case that natural selection has recently acted strongly on us". However, Willis criticizes the authors for not discussing what the "recent and continuing evolution means for our species as a whole". Willis concludes by saying that "the book offers a limited and biased interpretation of some very exciting research".^[18]

In Evolutionary Psychology, Gregory Gorelik and Todd K. Shackelford wrote, "Although many of their arguments need more fleshing out and some may not withstand the assault of further scientific analysis, the authors are stunningly creative when considering human history. If even a handful of their arguments survive the onslaught of rigorous scientific scrutiny, Cochran and Harpending will have offered a valuable and novel approach to addressing questions of recent human evolution."^[19]

In Evolution and Human Behavior, anthropologist Edward Hagen wrote that the book makes "many unsupported and often questionable assertions", but it is nevertheless valuable in raising "bold questions about major historical encounters between populations — Neanderthal and modern humans, German tribes and Romans, Europeans and Native Americans — in light of formidable (but not unassailable) arguments from population genetics". Hagen considered that it "should also be on the summer reading list of all evolutionary social scientists".^[20]

Anthropologist Cadell Last wrote that by using race as a natural fact, the book "undermines the attempt to find a legitimate scientific approach to understanding recent human evolution and conceptualizing human genetic diversity" and that it was "unfortunate" that it had received "praise from prominent, influential well-established biological anthropologists" such as John D. Hawks.^[21]

Evolutionary anthropologist Keith Hunley, writing for the