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).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).
(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).
Chronology
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 timeLinks
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 HunterBIBLIOGRAPHY
https://westhunt.wordpress.com/
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.
http://discovermagazine.com/1987/may/02-the-worst-mistake-in-the-history-of-the-human-race
Diamond, Jared M. 2005. Guns, Germs and Steel: The Fates of Human Societies. Vintage, London.
No comments:
Post a Comment