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.

Saturday, July 22, 2017

The 10,000 Year Explosion, Chapter 3: A Summary

Chapter 3 of Gregory Cochran and Henry Harpending’s The 10,000 Year Explosion: How Civilization Accelerated Human Evolution (2009) is called the “Agriculture: The Big Change,” and examines the evolutionary impact of the agricultural revolution and urban life.

When human populations were low during the Stone Age, the smaller number of people entailed that new favourable traits from random sexual reproduction and mutations occurred at a low rate: that is to say, low-population Stone Age people had trouble generating sufficient genetic change in the first place (Cochran and Harpending 2009: 65).

Cochran and Harpending (2009: 65) argue that, when the human population of earth had hit about 60 million people in 1,000 BC, new, positive mutations only took about 400 years to appear, whereas previously they might have taken place every 100,000 years.

The emergence of agriculture after 10,000 BC was truly revolutionary: it allowed a population explosion. The larger populations accelerated both genetic and cultural evolution: with more people, there were increased numbers of people capable of innovation and invention.

While Jared M. Diamond’s book Guns, Germs and Steel: The Fates of Human Societies (2005) emphasised the cultural side of higher population and density in greater rates of innovation, Cochran and Harpending (2009: 66) stress that the other side of this phenomenon is greater rates of genetic innovation.

It follows directly that peoples in various parts of the world who failed to develop agriculture – and so failed to experience the accelerated genetic and cultural evolution agriculture caused – were not subject to the same evolutionary development as people in agricultural societies (Cochran and Harpending 2009: 67). It also follows that people who adopted agriculture at different times were subject to different evolutionary histories as well, on the basis of how much time they were subject to the accelerated evolution agricultural societies caused.

From c. 27,000–18,000 BC, the earth experienced the Last Glacial Maximum when the ice sheets were at their greatest extent. But from c. 18,000 to 17,000 BC deglaciation began in the Northern Hemisphere and the Ice Age ended. Rapid warming occurred down to 10,000 BC.

In the Levant, from 12,500–9,500 BC people of the Natufian culture began the harvesting of wild plants in sedentary or semi-sedentary settlements. See this video:

But when a cold period called the Younger Dryas from 10,900–9,700 BC caused a mini ice age, this induced a drought in the Near East and the Natufians were driven to invent farming and herding:

With the end of the Younger Dryas around 9,700 BC, the climate became perfect for farming. By 9,500 BC farming had become common in the Fertile Crescent and spread to Egypt and western India by 7,000 BC, and then into Europe from c. 6,500 BC–4,000 BC as Anatolian farmers from northern Greece and north-western Turkey migrated into central Europe.

In China, agriculture was invented by 7,000 BC, and it was invented in the Americas too (Cochran and Harpending 2009: 67–70).

Agriculture caused radical changes in human diet and nutrition, diseases, and social structure, and, above all, a population boom. Malthusian checks to population growth also became more severe. Since food could be produced and stored, so wealth could be accumulated. Towns and cities created specialised classes of people devoted to new trades or activities, and non-productive elites, and the ability to produce material culture radically increased.

The genetic consequences of agriculture were as follows:
(1) with the creation of permanent settlements came higher population density, and exposure to and association with domesticated animals. This significantly increased the prevalence of infectious diseases and brought new diseases as well. Although it is possible that deaths from interpersonal violence decreased, a higher percentage of the population would have died of infectious disease or starvation (Cochran and Harpending 2009: 70).

(2) agricultural communities, then, were a new environment in which human beings were subjected to selective pressures and more evolution (Cochran and Harpending 2009: 70). Adaptation, and the spread of new traits, was made easier by the higher population and greater rate of mutation, not just the selection of desirable traits from pre-existing individual genetic variation (Cochran and Harpending 2009: 74). Cochran and Harpending (2009: 74) suggest that by 3,000 BC new adaptive mutations occurred about roughly 100 times more rapidly than in the Stone Age Pleistocene era.

(3) human natural selection and evolution in the period after agriculture was invented has involved about 400 generations, and has allowed favourable alleles to increase in frequency or “sweep” across the genomes of a given population group over time, where those alleles cause advantageous phenotypic traits: Cochran and Harpending (2009: 75) argue that even amongst modern humans we see hundreds of ongoing sweeps, which were begun 1,000s of years ago, and are in the process of going to the point of “fixation” (where in a given population the frequency of the allele is at 100%).

(4) many of these sweeps are in specific population groups, and not in others. That is to say, differential human evolution has occurred in different areas within populations with recent common descent. Genetic research indicates that many of these ongoing allele “sweeps” originated after 10,000 BC and determine aspects of human traits like metabolism, digestion, immunity from infectious disease, reproduction, DNA repair, and the functioning or structure of the central nervous system (Cochran and Harpending 2009: 75–76).

(5) a concrete example of how humans experienced phenotypic change after 10,000 BC in agricultural societies is as follows: while agriculture increased the scale of food available, it actually seems to have decreased the nutritional value of food: early farmers may well have been subject to health problems because their new diet was one of low-protein, vitamin-deficiency, and high-carbohydrates from plants (Cochran and Harpending 2009: 76). This was so serious that the average height of farmers compared to earlier humans decreased: the new farmers saw their average height fall by about five inches (or 12 centimetres; Cochran and Harpending 2009: 76; see also Diamond 1987).

(6) the increased rate of diseases and nutritional problems amongst farmers selected those human beings who were better adapted to this new environment: those who were better able to process nutrients from an agricultural diet and who were able to digest lactose into adulthood were more successful and survived to pass these traits onto their children (Cochran and Harpending 2009: 77). For example, the Indo-European-speaking farmers and herders north of the Black Sea evolved lactose tolerance perhaps by c. 5,000 BC and they passed this on to modern Europeans when they invaded and settled Europe c. 3,000–2,000 BC.

(7) another evolutionary adaptation is light skin. Amongst Europeans, the Stone Age hunter-gatherers in Europe obtained sufficient Vitamin D from their meat-rich diet and probably had considerably darker skin than modern Europeans (see here). But when farming spread to Europe, the loss of a Vitamin D-rich meat diet caused selection for lighter skin: Europeans therefore evolved white skin because light-skinned people survived to reproduce more often since they could produce more Vitamin D internally through easier exposure to ultraviolet radiation through the skin (Cochran and Harpending 2009: 78).

(8) the high-carbohydrate diet of farmers also seems to have induced metabolic changes: farmers evolved greater protection from rapidly-spiking blood sugar levels in the form of new alleles involved in insulin regulation, so that they had reduced risk of diabetes (Cochran and Harpending 2009: 79).
This is by means an exhaustive list of the evolutionary changes that human farmers and their progeny experienced, but there is much evidence that modern human beings who never invented farming, or who invented it much later but lived in small communities, have not undergone this type of evolution (Cochran and Harpending 2009: 79–84).

Here is a chronology of events in history relevant to the issues in Chapter 3 of The 10,000 Year Explosion:
12,500–9,500 BC – the Natufian culture in the Levant; harvesting of wild plants allows more free time

c. 12,100–c. 11,700 – the Older Dryas, a cold period

12,000 BC
12,000 BC onwards – Europeans are Western hunter gatherers

c. 12,000 BC – beginning of possible migration from the Near East or the Balkans of the Villabruna Cluster people into Europe

12,000–8,000 BC – most mammoths die out; small population of 500–1000 woolly mammoths lived on Wrangel Island until 1,650 BC

11,000 BC
c. 11,000–8,000 BC – the Late Glacial or Tardiglacial, the beginning of the warm period when the Northern Hemisphere warmed substantially with significant accelerated deglaciation after the Last Glacial Maximum (c. 23,000–11,000 years ago). Human beings in refuge areas started to repopulate northern Europe and Eurasia. See the map here

10,900–9,700 BC – mini ice age called the Younger Dryas causes sharp decline in temperatures over much of the northern hemisphere. Younger Dryas was triggered by vast meltwater probably from Lake Agassiz flowing into the North Atlantic, which caused disruption to thermohaline circulation

c. 10,900–9,700 BC – the Younger Dryas causes severe problems in Natufian culture from drought; Natufians abandoned settlements and became nomadic; on the shores of disappearing lake Galilee, Natufians began farming; others began herding

c. 10,700 BC – extinction of the North American megafauna

10,000 BC – possible human population at 4 million

c. 10,000 BC – Jericho is a settlement, and before that a camping ground for Natufian hunter-gatherer groups

after 9,700 BC – after the end of Younger Dryas, climate in Near East perfect for farming, which then spreads with combination of farming and herding

c. 8,000 BC – wall of Jericho constructed; domestication of goats in the Near East; domestication of dogs from wolves in Asia

c. 7,200 BC – Çayönü, a Neolithic settlement in southeastern Turkey, is the site where emmer wheat is first cultivated, and where the first domestic cattle and pigs are domesticated

c. 7,000 BC – farming spreads into Elam

c. 6,500 BC–4,000 BC – Neolithic Anatolian farmers from northern Greece and north-western Turkey started migrating into central Europe through the Balkan route and then by the Mediterranean route to the Iberian Peninsula (see here)

c. 6,500–3,800 BC – Ubaid period, a prehistoric period of Mesopotamia; in North Mesopotamia, from c. 5,300 and 4,300 BC

c. 6,500 BC – first known settlement in southern Mesopotamia established at Eridu by farmers with the Hadji Muhammed culture

6,250–5,050 BC – in China, domesticated millet is farmed in northern China at Xinglonggou, Yuezhang, Dadiwan, Cishan, and several Peiligang sites

6,200 BC – Bond climatic event 5 ends Middle Eastern Neolithic B culture (see Bond event), a sudden cold period lasting 200 to 400 years causing problems to humans worldwide and migrations in search of food and water

c. 5,500 BC – agriculture spreads throughout ancient Egypt

c. 5,000 BC – speakers of pre-proto-Indo-European migrate into the regions north of the Black Sea from central Asia

3,500–2,300 BC – Yamna (or Pit Grave Culture) culture of Indo-European-speakers in the Pontic-Caspian

3,000–2,000 BC – Indo-European-speaking Yamnaya-culture people swept into Europe from the Russian steppe
The blog of Gregory Cochran and Henry Harpending:
West Hunter
Cochran, Gregory and Henry Harpending. 2009. The 10,000 Year Explosion: How Civilization Accelerated Human Evolution. Basic Books, New York.

Diamond, Jared M. 1987. “The Worst Mistake in the History of the Human Race,” Discover 8.5: 64–66.

Diamond, Jared M. 2005. Guns, Germs and Steel: The Fates of Human Societies. Vintage, London.