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The dominant belief on this site seems to be in the "psychological unity of mankind". In other words, all of humanity shares the same underlying psychological machinery. Furthermore, that machinery has not had the time to significantly change in the 50,000 or so years that have passed after we started moving out of our ancestral environment.
In The 10,000 Year Explosion, Gregory Cochran and Henry Harpending dispute part of this claim. While they freely admit that we have probably not had enough time to develop new complex adaptations, they emphasize the speed at which minor adaptations can spread throughout populations and have powerful effects. Their basic thesis is that the notion of a psychological unity is most likely false. Different human populations are likely for biological reasons to have slightly different minds, shaped by selection pressures in the specific regions the populations happened to live in. They build support for their claim by:
- Discussing known cases where selection has led to rapid physiological and psychological changes among animals
- Discussing known cases where selection has led to physiological changes among humans in the last few thousand years, as well as presenting some less certain hypotheses of this.
- Postulating selection pressures that would have led to some cognitive abilities to be favored among humans.
In what follows, I will present their case by briefly summarizing the contents of the book. Do note that I've picked the points that I found the most interesting, leaving a lot out.
- Dogs were domesticated from wolves around 15,000 years ago: by now, there exists a huge variety of different dog breeds. Dogs are good at reading human voice and gestures, while wolves can't understand us at all. Male wolves pair-bond with females and put a lot of effort into helping raise their pups, but male dogs generally do not. Most of the dog breeds we know today are no more than a couple of centuries old. There is considerable psychological variance between dog breeds: in 1982-2006, there were 1,110 dog attacks in the US that were attributable to pit bull terriers, but only one attributable to Border collies. Border collies, on average, learn a new command after 5 repetitions and respond correctly 95 percent of the time, while a basset hound needs 80-100 repetitions for a 25 percent accuracy rate.
- A Russian scientist needed only forty years to successfully breed a domesticated fox. His foxes were friendly and enjoyed human contact, very unlike wild foxes. Their coat color also lightened, their skulls became rounder, and some of them were born with floppy ears.
- While 50,000 years may not be enough for new complex adaptations to develop, it is enough time for them to disappear. A useless but costly adaptation will vanish in a quick period: fish in lightless caves lose their sight over a few thousand years at most.
- An often-repeated claim is that there's much more within-group human genetic variation than between-group (85 and 15 percent, to be exact). While this is true, the frequently drawn conclusion, that phenotype differences between individuals would be larger than the average difference between groups, does not follow. Most (70 percent) of dog genetic variation is also within-breed. One important point is that the direction of the genetic differences tends to be correlated: a particular Great Dane may have a low-growth version of a certain gene while a particular Chihuahua has a high-growth version, but on the whole the Great Dane will still have more high-growth versions. Also, not all mutations have the same impact: some have practically no effect, while others have a huge one. Since the common ancestry of humans (or dogs) is so short, observable differences between populations must have evolved rapidly, which is only possible if the mutations had a strong selective advantage.
- There are gene variants causing observable differences in appearance between human populations, such as the ones causing light skin color or blue eyes. For such systematic differences to appear, there must have been big effects on fitness, anything up from a 2 or 3 percent increase. From the rate at which new alleles have spread, this must be the case at least for genes that determine skin color, eye color, lactose tolerance, and dry earwax.
- Molecular genetics has found hundreds of cases of mutations that indicate recent selection. Many of them are very recent. A significant number of Europeans and Chinese bear mutations that originated at about 5,500 years ago. The rate at which new mutations have been popping up and spreading over the past few thousand years is on the order of 100 times greater than the long-term rate over the past few million years.
The second chapter of the book is devoted to a discussion about the "big bang" in cultural evolution that occured about 30,000 to 40,000 years ago. During that time, people began coming up with technological and social innovations at an unprecedented rate. Cave paintings, sculpture and jewelry starting showing up. Tools made during this period were manufactured using materials hundreds of miles away, when previously they had been manufactured with local materials - implying that some sort of trade or exchange developed. Humans are claimed to have been maybe 100 times as inventive than in earlier times.
The authors argue that this was caused by a biological change: that genetic changes allowed for a cultural development in 40,000 BC that hadn't been possible in 100,000 BC. More specifically, they suggest that this could have been caused by interbreeding between "modern" humans and Neanderthals. Even though Neanderthals are viewed as cognitively less developed than modern humans, archeological evidence suggests that at least up to 100,000 years ago, they weren't seriously behind the modern humans of the time. Neanderthals also had a different way of life, being high-risk, highly cooperative hunters while the anatomically modern humans probably had a mixed diet and were more like modern hunter-gatherers. It is known that ongoing natural selection in two populations can allow for simultaenous exploration of divergent development paths. It would have been entirely possible that the anatomically modern humans interbred with Neanderthals to some degree, the Neanderthals being a source of additional genetic variance that the modern humans could have benefited from.
How would this have happened? In effect, the modern humans would have had their own highly beneficial alleles, in addition to which they'd have picked up the best alleles the Neanderthals had. Out of some 20,000 Neanderthal genes, it's highly likely that at least some of them were worth having. There wasn't much interbreeding, so Neanderthal genes with a neutral or negative effect would have disappeared from the modern human population pretty quickly. On the other hand, a beneficial gene's chance of spreading in the population is two times its fitness advantage. If beneficial genes are every now and then injected to the modern human population, chances are that eventually they will end up spreading to fixation. And indeed, both skeletal and genetic evidence shows signs of Neanderthal genes. There are at least two genes, one regulating brain size that appeared about 37,000 years ago and one playing role in speech that appeared about 42,000 years ago, that could plausibly have contributed to the cultural explosion and which may have come from the Neanderthals.
The third chapter discusses the effect of agriculture, which first appeared 10,000 or so years ago. 60,000 years ago, there were something like a quarter of a million modern humans. 3,000 years ago, thanks to the higher food yields allowed by agriculture, there were 60 million humans. A larger population means there's more genetic variance: mutations that had previously occurred every 10,000 years or so were now showing up every 400 years. The changed living conditions also began to select for different genes. A "gene sweep" is a process where beneficial alleles increase in frequency, "sweeping through" the population until everyone has them. Hundreds of these are still ongoing today. For European and Chinese samples, the sweeps' rate of origination peaked at about 5,000 years ago and at 8,500 years ago for one African sample. While the full functions of these alleles are still not known, it is known that most involve changes in metabolism and digestion, defenses against infectious disease, reproduction, DNA repair, or in the central nervous system.
The development of agriculture led, among other things, to a different mix of foods, frequently less healthy than the one enjoyed by hunter-gatherers. For instance, vitamin D was poorly available in the new diet. However, it is also created by ultraviolet radiation from the sun interacting with our skin. After the development of agriculture, several new mutations showed up that led to people in the areas more distant from the equator having lighter skins. There is also evidence of genes that reduce the negative effects associated with e.g. carbohydrates and alcohol. Today, people descending from populations that haven't farmed as long, like Australian Aborigines and many Amerindians, have a distinctive track record of health problems when exposed to Western diets. DNA retrieved from skeletons indicates that 7,000 to 8,000 years ago, no-one in central and northern Europe had the gene for lactose tolerance. 3,000 years, about 25 percent of people in central Europe had it. Today, about 80 percent of the central and northern European population carries the gene.
The fourth chapter continues to discuss mutations that have spread during the last 10,000 or so years. People in certain areas have more mutations giving them a resistance to malaria than people in others. The human skeleton has become more lightly built, more so in some populations. Skull volume has decreased apparently in all populations: in Europeans it is down 10 percent from the hight point about 20,000 years ago. For some reason, Europeans also have a lot of variety in eye and hair color, whereas most of the rest of the world has dark eyes and dark hair, implying some Europe-specific selective pressure that happened to also affect those.
As for cognitive changes: there are new versions of neurotransmitter receptors and transporters. Several of the alleles have effects on serotonin. There are new, mostly regional, versions of genes that affect brain development: axon growth, synapse formation, formation of the layers of the cerebral cortex, and overall brain growth. Evidence from genes affecting both brain development and muscular strength, as well as our knowledge of the fact that humans in 100,000 BC had stronger muscles than we do have today, suggests that we may have traded off muscle strength for higher intelligence. There are also new versions of genes affecting the inner ear, implying that our hearing may still be adapting to the development of language - or that specific human populations might even be adapting to characteristics of their local languages or language families.
Ruling elites have been known to have far more offspring than those of the lower classes, implying selective pressures may also have been work there. 8 percent of Ireland's male population carries an Y chromosome descending from Niall of the Nine Hostages, a high king of Ireland around AD 400. 16 million men in central Asia are direct descendants of Genghis Khan. Most interestingly, people descended from farmers and the lower classes may be less aggressive and more submissive than others. People in agricultural societies, frequently encountering lots of people, are likely to suffer a lot more from being overly aggressive than people in hunter-gatherer societies. Rulers have also always been quick to eliminate those breaking laws or otherwise opposing the current rule, selecting for submissiveness.
The fifth chapter discusses various ways (trade, warfare, etc.) by which different genes have spread through the human population throughout time. The sixth chapter discusses various historical encounters between humans of different groups. Amerindians were decimated by the diseases Europeans brought with them, but the Europeans were not likewise decimated by American diseases. Many Amerindians have a very low diversity of genes regulating their immune system, while even small populations of Old Worlders have highly diverse versions of these genes. On the other hand, Europeans had for a long time difficulty penetrating into Africa, where the local inhabitants had highly evolved genetic resistances to the local diseases. Also, Indo-European languages might have spread so widely in part because an ancestor protolanguge was spoken by lactose tolerant herders. The ability to keep cattle for their milk and not just their flesh allowed the herders to support larger amounts of population per acre, therefore displacing people without lactose tolerance.
The seventh chapter discusses Ashkenazi Jews, whose average IQ is around 112-115 and who are vastly overrepresented among successful scientists, among other things. However, no single statement of Jews being unusually intelligent is found anywhere in preserved classical literature. In contrast, everyone thought that classical Greeks were unusually clever. The rise in Ashkenazi intelligence seems to be a combination of interbreeding and a history of being primarily in cognitively challenging occupations. The majority of Ashkenazi jews were moneylenders by 1100, and the pattern continued for several centuries. Other Jewish populations, like the ones the living in the Islamic countries, were engaged in a variety of occupations and do not seem to have an above-average intelligence.