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The 10,000 Year Explosion

How civilization accelerated human evolution

Gregory Cochran and Henry Harpending

Arguing that human evolution has accelerated over past 100,000 years rather than slowing or stopping, and that, as a result, we have changed significantly in mind and body even over recorded history.

Mainly because humans have experienced radical changes to their environment since moved out of Africa. Inventions of new hunting tools such as spear throwers and bows meant lithe Bushman type bodies better suited than heavier muscular bodies. Fire and cooking changed demands on teeth and digestive systems. Language altered both way sounds heard and way processed in brain.

We already know some genetic changes very quick. Dogs started from wolves 15,000 years ago, now come in more varied shapes and sizes than any other mammal. Russian geneticist Dmitri Belynev bred tame silver foxes in just 40 generations, simply by selecting for approachability. Found that other traits came along with the behaviour - lighter coat color, rounder skulls, floppy ears. And plants - corn is derived from a wild grass called teosinte over 7000 years.

All dog behavioural adaptations have recognizable precursors in wolves, but the breeds recognizably different. Biting is disproportionally distributed - US dog bite records for show 1110 attributed to pit bull terriers, just 1 for border collies. They also vary greatly in learning power - a border collie can learn a new command after 5 repetitions with 95% accuracy rate, while a basset hound takes 80-100 reps for a 25% accuracy rate.

Most of the changes from wolf to domestic dog breeds can be thought of as turning switches on or off or twirling knobs, and the same things apply to humans. The Duffy mutation is a malaria defence that turns off the caspase 12 gene in red blood cells, but leaves it operational everywhere else. Or the knob can be turn up the volume as in groups which have an extra copy of the gene which produces amylase, which aids in digesting starch.

Some, like William Flew, argue that although we all look very different on the outside, we are exactly the same on the inside. Argue that there is more genetic variation within groups than between them. Yet almost the same genetic variation exists within breeds of dogs (about 85% of human genetic variation within-group, 15% is between-group; dogs the split is 70-30%). But this reasoning implies that there is more variation between individual Great Danes than between Great Danes and Chihuahuas, which obviously wrong. The problem with just looking at the amount of genetic variation tells you nothing about the size or significance of trait differences.

Sometimes a single mutation on one gene is enough to change everything. The most common cause of dwarfism is caused by such a single change.

Since humans outside of Africa have a very recent common ancestry (50,000 years) the observable differences between populations must have evolved rapidly, which can only have happened if the alleles (gene variants) underlying these differences had strong selective advantages. Includes gene for skin color (SLC24A5), eye color (HERC2) and lactose tolerance (LCT).

Differences in a single gene can produce completely different life strategies. Fire ants with one version of pheromone receptor live in independent colonies with a single queen, whereas ones with a different version live in a huge megacolony with many queens.

We are significantly different to the humans who lived 40,000 years ago, in that we are far more inventive in almost every part of life. And we are a lot different to humans who lived 10,000 years ago, particularly in the diseases we are now resistant to, because we are the survivors.

Human genome project shows us that again and again over past few thousand years a favorable mutation has occurred in some individual and spread widely until a significant fraction of the human race carries the mutation.

60,000 years ago, before the expansion out of Africa, guess that population around 250,000. By the Bronze Age, 3000 years ago, about 60 million. Favorable mutations that had before appeared every say 100,000 years, now showing up every 400 years.

Agriculture imposed a new way of life - new diets, new diseases, new need to co-operate with more people, much bigger need for forward planning - all things for which hunter-gatherers were poorly adapted. Societies which adapted poorly were eliminated by more efficient competitors.

Lactose tolerance in adults started Scandinavia about 7-8000 years ago, and has spread to 95% of Danes and Swedes. A different mutation that had same effect appeared among Tsutsi cattle rearing tribes East Africa, and is found in about 90% of them.

Acne and tooth decay are both caused by infectious bacteria, but only cause trouble with a high carbohydrate diet (ie after farming).

Populations which never farmed (Australian Aborigines) or only recently adopted (Amerindians) have characteristic health problems when exposed to Western diets. Most severe is adult-onset diabetes. Polynesians also disproportionately suffer, even though they farm taro, breadfruit etc. Suggest that their island-isolated populations too low to throw up the protective mutations that Western cultures evolved.

We now know quite a lot about the mutations affecting skin color. In most cases the genes driving the change from dark to light skin are completely different in Europe and Asia. And where the same gene involved, different mutations making changes.

Gene called OCA2. It controls melanin pathways, affecting skin and eye color. Conventional explanation of selection for lighter skin color is that it allows more sunlight which is essential for Vit D production. But some doubts as to whether this is the full story. Mutations of this same gene cause cave fish to become blind albinos after several generations. Since there is no light in the caves, there must be some other function of the (mutated) gene which is being selected for. If a human has 2 broken copies they are an albino. They have problems such as cancer and blindness as well as social rejection and/or persecution. Yet it is the single most common genetic disease in southern Africa - over 1% of the population have a single copy of the mutation. It's persistence strongly suggests that the single copy confers some evo advantage to carriers.

Again, the underlying genetic mutations are quite different in Europe and Asia. The mutations have somehow created lots of variety in Europeans - redheads and blondes, blue and green eyes. In all the rest of the world, everyone has dark hair and dark eyes.

A gene which, in its ancestral form conserves salt, is almost universal in tropical Africans. Probably explains why African Americans suffer from high blood pressure. A mutated variant, which does nothing to hold onto salt, becomes increasingly common as move north into Europe.

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When humans first became farmers their bodies were poorly adapted to new (no meat)diet. We know because skeletons from 10,000 years ago were much shorter and showed signs of rickets and other protein deficiencies.

Skeleton has changed. We are more lightly built, our jaws have shrunk, and our brow ridges have disappeared (except in Aust Aborigines). Bronze Age skeletons in Europe from about 3000 years ago still had the same brow ridges. Even in last 1000 years there have been changes, as measured from Black Death skeletons (650 years ago) and crew of the Mary Rose, the Tudor warship which sank 450 years ago. The shape of the skull has changed, with the part holding the frontal lobes getting bigger.

Selecting for work habits: Hunter-gatherers had to share - your kill would rot before you cd eat it all so made sense to give some to rest of yr group in the expectation that they wd reciprocate. ("The only way to store surplus food is in someone else's stomach.") Farmers needed to be selfish about keeping seed grain or breeding stock. Hunter gatherers had no way of storing wealth - if you had enough food and yr weapons ok then might as well take it easy. But a farmer who worked hard on improving land increased his family's survival chances, especially as rising population densities made it harder to get a living.

The cultures which led the Industrial Revolution and later innovations are the ones which first adopted intensive agriculture. Suggest that the genetic changes that accommodated people to a dense hierarchical society were the things like ability to plan and negotiate. If just cultural changes, which have to be relearned by each new individual, expect other countries to have caught up. But fact that they haven't suggests an innate cause.

We instinctively think ancestry is like mixing paint - we assume if a population starts put 90% Norwegian and 10% Nigerian, it will stay that way. But in fact, if you put that population in Nigeria, certain alleles that help Nigerians stay alive (and prosper enough to reproduce), such as malaria resistance and dark skin to protect against skin cancer, will become more common. And after many generations almost everyone in the population will have the Nigerian version of those genes.

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