Sunday, July 23, 2017

The 10,000 Year Explosion, Chapter 4: A Summary

Chapter 4 of Gregory Cochran and Henry Harpending’s The 10,000 Year Explosion: How Civilization Accelerated Human Evolution (2009) is called the “Consequences of Agriculture.”

The effects of agriculture accelerated human evolution and selective pressures in the following ways:
(1) Infectious disease
Farming caused the creation of sedentary settlements, cities and overcrowding, as well as greater contact with animals, and the bacteria, viruses and parasites that animals bear. Therefore disease vectors increased. In this environment, new and more virulent infectious diseases (like measles and typhus) spread and became much more dangerous (Cochran and Harpending 2009: 86). Human farmers were therefore subject to evolutionary changes induced by selective pressures brought by new diseases. The primary outcome was the evolution of much greater immunity and defences against these diseases, the kind of biologically-evolved defences that hunter gatherers simply did not have (Cochran and Harpending 2009: 87). In different regions, different diseases from different pathogens also caused different types of evolution too. For example, in tropical regions, malaria has induced evolution amongst the people of the tropics to give them a much greater immunity to the disease. By contrast, Europeans – who evolved in a different region not subject to malaria to the same extent – are much more vulnerable to malaria. And European evolution conferred on European farmers and their modern descendants immunity and defences against a set of diseases that other human populations do not have. This differential evolution caused a catastrophe after Europeans colonised the New World and other regions: e.g., Amerindians, Australian Aborigines, and Polynesians were biologically different from Europeans, because of a separate evolutionary history, and many millions of these native peoples died because they simply did not have the same level of immunity to diseases introduced by Europeans (Cochran and Harpending 2009: 90–91).

(2) Light skin
The farmers of northern Eurasia (namely, Europeans and East Asians) have both evolved much lighter skin colour compared to people in the tropics, because of the lower levels of UV radiation in the north and the vitamin-deficient diet of early farmers. In Europeans and Caucasians generally, light skin seems partly driven by the SLC24A5 gene variant. Genetic evidence suggests that evolution of white skin in Europeans is shockingly recent: it probably happened between 10,000 and 4,000 BC, and perhaps even at the more recent end of that range (Cochran and Harpending 2009: 91–92). Cochran and Harpending (2009: 92–94) contend that the evolution of light skin provides other selective advantages not yet properly understood, but in addition to vitamin D synthesis.

(3) Skeletal structure
Certain human populations have evolved a much more gracile bone and skeletal structure in differential evolution. For example, bones have become lighter and less thick, jaws have shrunk, and prominent brow ridges largely disappeared. Cochran and Harpending (2009: 95) point to archaeological evidence that indicates that many Europeans – even as late as c. 1,000 BC – had prominent brow ridges, which means that evolution since 1,000 BC has largely removed this phenotypic trait in Europe.

Archaeology seems to show that even in England within the past 1,000 years (that is, from 1,000 AD to today) skull structure has evolved: skull size has actually noticeably increased within 1,000 years (Cochran and Harpending 2009: 95, citing Rock et al. 2006; see also Jantz and Jantz 2016 on changes in cranial shape and size in European Americans over the past 190 years).

(4) Driving genes
Driving genes are a special type of gene which arise much more frequently in large human populations than in smaller ones. The population explosion allowed by agriculture has caused driving gene alleles to be generated at a rate of about two orders of magnitude higher than in Stone Age populations (Cochran and Harpending 2009: 97). Genetic evidence shows that we now have a large number of these driving genes, which either have already reached the point of fixation, or are well on their way to fixation in gene sweeps. However, many of these gene sweeps occur in specific population groups, mainly in Europeans and East Asians. This indicates recent differential regional evolution. But such recent evolution comes with harmful side effects too: human beings have a strangely high miscarriage rate compared with other animals, and this is probably the consequence of such rapid accelerated evolution (Cochran and Harpending 2009: 98).

(5) Personality and cognition
Some of the recent gene alleles seem to regulate neurotransmitters, central nervous system structure, and even levels of serotonin. Serotonin metabolism has a powerful affect on mood and emotion – and hence on personality and behaviour. Other alleles seem to influence brain development (such as axon growth, formation of synapses, cerebral cortex structure, and general brain growth). However, these types of new genes are regional and not found in all populations (Cochran and Harpending 2009: 98–99).

A specific example of this is the set of genes that regulate muscle fibres and brain function associated with dystrophin. Genetic evidence suggests that human evolution has seen the loss of muscle strength but a compensatory increase in brain complexity and size. Changes in dystrophin regulation may have induced this.

Yet another cognitive trait that may have been selected for in farmers is the ability to defer gratification (Cochran and Harpending 2009: 114). This was an extremely important type of behaviour on which farming is based, and needed for sowing of crops or breeding of animals, when those plants or animals can be eaten in the present. Farmers with personality traits such as delayed gratification, patience, a work ethic, self-control, and long-term planning would have survived to produce more offspring (Cochran and Harpending 2009: 114). Curiously, this would also have bred more selfish people in contrast to hunter gatherers (Cochran and Harpending 2009: 115).

(6) Malthusianism and differential survival rates
Farming increased human populations, but brought with it a new Malthusian population trap. People experienced mass death from different causes, e.g., war, disease, general plagues, and famine and starvation. However, the specific general manner in which people die can have genetic and evolutionary effects. When societies had social and socio-economic stratification which gave more wealth and privileges to elites or higher status groups, these people were more likely to survive than lower status people, especially during certain types of disaster. That is to say, internal differential survival rates of children would have brought genetic change to a society. This has consequences for evolution if traits of the elite tended to propagate in the general population over long periods (Cochran and Harpending 2009: 102–105). That is, elite reproductive advantage has genetic effects.

(7) State Societies and Evolution
Farming allowed the creation of more advanced state-based societies that developed systems of law and order and punishments. Many such societies have imposed the death penalty for socially-harmful behaviour, as in crimes like murder, violence, and so on. In a stable society over time, this would likely kill off more aggressive individuals (usually men) and leave that society with a gene pool favouring less aggressive and less violent individuals (Cochran and Harpending 2009: 111–112). Some have argued that the high levels of social conformity in East Asian societies are not just a cultural phenomenon, but the result of long-run genetic changes influencing personality arising from the thousands of years of how these state-based societies in East Asia have operated. This raises the interesting possibility that highly developed state societies have “tamed” human beings in certain ways, not just culturally but also genetically (Cochran and Harpending 2009: 112–113), and that in modern agricultural societies (which have had agricultural and state systems for thousands of years), the average man today might be less aggressive and less violent than the average man 2,000 years ago, or 10,000 or 20,000 years ago. People from state-based, agricultural societies – with thousands of years of history – probably have different cognitive traits, on average, as compared with people in hunter-gatherer societies not subject to the same kind of long-term evolutionary change.
Cochran and Harpending (2009: 121) conclude their chapter by suggesting that the long-run social, economic and even scientific development of Old Agricultural societies is likely to be based – at least in part – on the biological evolutionary traits of the people which these societies have produced over thousands of years of distinctive, regional Darwinian evolution.

Cochran, Gregory and Henry Harpending. 2009. The 10,000 Year Explosion: How Civilization Accelerated Human Evolution. Basic Books, New York.

Jantz, Richard L. and Lee Meadows Jantz. 2016. “The Remarkable Change in Euro-American Cranial Shape and Size,” Human Biology 88.1 (15 January): 56–64.

Rock, W. P. et al. 2006. “A Cephalometric Comparison of Skulls from the Fourteenth, Sixteenth and Twentieth Centuries,” British Dental Journal 200: 33–37.

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