In essence, Cochran and Harpending challenge the notion that human evolution stopped around 50,000 years ago.
In Chapter 1 of The 10,000 Year Explosion, Cochran and Harpending (2009: 1) argue that
(1) human evolution has actually been accelerated by various pressures and historical developments over the past 10,000 years, as the environments and niches occupied by human beings radically changed, andThe view that Cochran and Harpending oppose is as follows: the idea that the last stage of significant human evolution occurred between about 50,000 to 40,000 years ago and then ceased. That is to say, from 50,000 to 40,000 years ago during the Upper Palaeolithic humans went through a flowering of culture and material culture (such as weapons, tools, art and clothing), but then human evolution of the mind and body, in significant ways, ended around this time, and modern humans are essentially the same as humans of about c. 40,000 years ago (Cochran and Harpending 2009: 2).
(2) that evolution in human beings has been about 100 times faster in the past 10,000 years than the long-run, average rate during all 6 million years of human and hominid evolution (Cochran and Harpending 2009: 23, citing Hawks et al. 2007).
The assumption lying behind this is that the environment occupied by humans became basically static about 50,000–40,000 years ago, and so no great new selective pressures caused by new environments continued to modify the human genome and phenotypes (Cochran and Harpending 2009: 2). Such a scenario is not impossible if a species occupies an environment that is stable: e.g., horseshoe crabs today are probably genetically and phenotypically much the same as horseshoe crabs 100 million years ago, because these organisms have occupied the same stable, static environment.
But Cochran and Harpending (2009: 3) contend that this assumption about the environments occupied by humans within the past 40,000 years – and especially the last 10,000 years – cannot possibly be taken seriously.
Instead, the evidence suggests that, while many species may well exist for long periods in stasis in stable environments, they can then easily be subject to rapid evolution in response to rapid environmental and selective pressures from natural selection (Cochran and Harpending 2009: 5, 19). For example, modern breeds of dogs have been created very recently in the space of about 15,000 years by human beings through artificial selection and breeding: to take one example, we have been able to change wolves into chihuahuas.
Changes can also happen rapidly in cognition or behaviour, e.g., domesticated dogs are significantly different in their cognitive and behavioural characteristics from wolves. And the Russian scientist Dmitri Belyaev was able to breed domesticated foxes from wild foxes in about 10 years of selective breeding (Cochran and Harpending 2009: 7), as described in this video:
Cochran and Harpending point to the following reasons for rejecting the stable environment hypothesis with respect to humans over the past 40,000 years:
(1) even after 40,000 years ago, humans continued to migrate around the surface of the planet, into southeast Asia, Australia, Europe, and also into northern Eurasia, Japan, and the Americas, where they experienced different environments and different evolutionary pressures for the following 10,000s of years, and, above all, some experienced the extremely harsh environment of the last Ice Age in the area of northern Eurasia (the last phase of which was the Last Glacial Maximum when the ice sheets were at their greatest extension from 27,000–18,000 BC).In short, Cochran and Harpending contend that these different environments have continued to shape human beings and even accelerate human evolution well after 50,000 years ago, and that even minor changes in allele frequencies in different human populations driven by selective pressures have caused phenotypic differences in external appearance, morphology, metabolism, defence against infectious diseases, and even cognitive and behavioural traits (Cochran and Harpending 2009: 19, 22).
(2) humans outside of Africa encountered and competed with other archaic humans such as the Neanderthals and Denisovans, as well as new animals and pathogens in these environments. Humans outside of Africa also interbred with Neanderthals and acquired a small amount of Neanderthal DNA.
(3) differential cultural and technological development occurred in these different regions, which in turn caused new selective pressures on the people in various areas, e.g., spears and arrows drove selection for faster, lighter humans who could hunt more successfully with these weapons (Cochran and Harpending 2009: 3).
(4) the agricultural revolution from c. 10,000 BC and the emergence of cities also created radically new environments from those inhabited by hunter gatherers before this time.
(5) as human populations rose with farming, mutations and beneficial individual traits caused by genetic mixing in sexual reproduction were more likely to occur (and then spread in these populations) than in much smaller populations of hunter-gatherers. And, importantly, even comparatively minor genotypic changes in alleles or gene variants, but occurring more frequently, can cause very profound and deep phenotypic changes quite rapidly in a species.
This of course means not only that human beings of around 100,000 years ago were different from human beings c. 40,000 years ago, but also that humans c. 40,000 years ago or even 10,000 years ago were phenotypically different – in significant ways – from human beings alive today (Cochran and Harpending 2009: 18–19).
Cochran and Harpending (2009: 18–19) also contend that accelerated human evolution means that even humans in historical times from around 1,000 BC should be regarded as different – both genotypically and phenotypically – from us today. This has profound implications for our understanding of human history, and our understanding of why and how humans historically developed in terms of their cultures, technologies, economies, and social organisation.
To end with some concrete examples: most Europeans today are lactose-tolerant into adulthood. Europeans are generally lactose-tolerant because they have a mutation that allows the synthesis of lactase – an enzyme that digests milk sugar. But this evolutionary trait is quite recent: it only spread amongst Europeans from 3,000 to 2,000 BC as Indo-European-speaking Yamnaya-culture people from what is now southern Russia migrated into Europe and spread this mutation that they had evolved (Allentoft et al. 2015: 171). Before about 3,000 BC, Europeans were not lactose-tolerant into adulthood.
And if we went back in time to Europe of about 40,000 BC, we’d discover that Europeans of that era looked quite different from their modern descendants: e.g., they would have had heavy brow ridges, prognathism from the much larger teeth that humans had before the Neolithic farming revolution, and probably much darker skin.
Allentoft, Morten E. et al. 2015. “Population Genomics of Bronze Age Eurasia,” Nature 522 (11 June): 167–172.
Arden, Rosalind. 2009. Review of The 10,000 Year Explosion: How Civilization Accelerated Human Evolution by Gregory Cochran and Henry Harpending, Twin Research and Human Genetics 12.4: 409–410.
Cochran, Gregory and Henry Harpending. 2009. The 10,000 Year Explosion: How Civilization Accelerated Human Evolution. Basic Books, New York.
Gorelik, G. and T. K. Shackelford. 2010. Review of The 10,000 Year Explosion: How Civilization Accelerated Human Evolution by Gregory Cochran and Henry Harpending, Evolutionary Psychology 1: 113–118.
Hawks, John, Wang, Eric T., Cochran, Gregory M., Harpending, Henry C. and Robert K. Moyzis. 2007. “Recent Acceleration of Human Adaptive Evolution,” Proceedings of the National Academy of Sciences of the United States of America 104.52 (December 26): 20753–20758.
Wills, Christopher. 2009. Review of The 10,000 Year Explosion: How Civilization Accelerated Human Evolution by Gregory Cochran and Henry Harpending, New Scientist 201.2695: 46–47.