I was reading (can’t remember where) that the speed of human population differentiation really seems to have speeded up since the introduction of agriculture, for multiple reasons. Essentailly, the main reason is that due to our revolutionary changes in lifestyle (diet, sedentism) we’ve been put through more selective sweeps in the past ten thousand years than you’d have seen in the past tfifty thousand.
I’ve decided to list a few of the causes, and some of the effects. Anybody who spots anything I’ve missed let me know.
Diet:
Hunter gatherers eat a diet that is primarily made of animal flesh, fat and protein. Farmers eat mainly carbohydrate. There are several changes needed to adapt to this new diet, and variations of it.
- Lower insulin resistance… insulin resistance plus a high carb diet equals diabetes, infertility and obesity. This is seen very clearly in recently integrated hunter gatherer peoples like the Aborigines. There is also the suggestion that many degenerative diseases are attributable to high carbohydrate diet and it’s lower levels of antioxidants (vitamins and uric acid).
- Lighter skin (cooler climes)….grain based diets are much lower in vitamin D. Only a minor issue if you are a hunter gatherer with access to plenty of fresh offal, but critical if you eating a subsistence grain based diet.
- Omega three oils… plentiful in a hunter gatherers diet, but rare and only of poor quality in vegetable sources. This is going to encourage the development of people with brain development less sensitive to low levels of O3 oils (mental health and behavioural issues are made worse/introduced on a low O3 diet).
- Lactose tolerance… The lactase persistence trait in Europeans is approaching fixation in Northern Europe, but it’s only about 8,000 years old.
- Gluten tolerance.. crucial if you are eating a wheat based diet. Gluten intolerance will make you quite sick, and appears to have been rapidly selected out of Europeans, with places like Ireland and Finland showing just a couple of percent of the population being gluten intolerant now.
- Alcohol tolerance.. Living in a sedentary fashion gives rise to dense populations and contaminated water. In Europe, the standard method of sterilising the water was to add alcohol to it. In the far East the method of sterilising it was to boil it and make tea. This seems to have made Europeans a little better at processing the toxic by-products of alcohol.
Diseases;
Living in much greater population densities than hunter gatherers ever could, farmers are exposed to more pathogens because;
- More people to harbour a new virulent disease mutation (twice the population, twice the risk of a nasty new disease).
- More people to act as host and carrier to new and old diseases.
- Lack of mobility. Mobile people can much more easily quarantine a sick individual by moving on.
- Contaminated water, not such an issue for lower population densities, or mobile people.
- Continuous contact with livestock introducing new pathogens (bird flu, poxes, etc) and parasites.
Reproductive strategies;
Female hunter gatherers tend to space out pregnancies to every three or three and a half years, usually by prolonged breastfeeding. It’s impractical to have two babies that need carrying, so the elder child needs to be walking well before the new baby comes.
However, agricultural communities don’t have to abide by this. Babies can be weaned early onto goat or cows milk, fed soft soaked grains at an earler date than pre-chewed meat, and they don’t need to be able to keep up with the grown ups before the next baby is born. This makes the spacing for the births closer together. Slightly detracting from this higher fertility rate is the higher mortality rate from poor nutrition and disease, but the farmers will still have an increasing population.
Also, male reproductive strategies are different. Land can be owned, wealth accumulated and more than one wife can be supported by a succesful man. Also, a new wife later in life is possible, as a landowner doesn’t have to personally do the labour, and lives longer and can provide for children even at a later age (not an option for an ageing hunter). This will reduce the amount of males that reproduce.
All this recent selective pressure, probably starting around 15,000 years ago in several locations, might explain the recent rapid changes in the areas that have farming. The dentition has certainly changed; Europeans and Asians generally have much smaller teeth than their ancient ancestors, and less developed jaws as food has become more processed.
One of the effects this kind of selection would also lead to is the success of groups that have some kind of genetic adaption to it’s new lifestyle. Like the development of a resistance to malaria in the Bantu people. If an adaptation is a trait to survive in a new environment is due to multiple genes, the new population will have a massive advantage. A ‘single gene’ trait like lactose tolerance will spread very rapidly under it’s own selective pressure, probably outrunning the spread of the original population that it arose in. However, if a trait (like a disease resistance) requires multiple genes to be effective, the original population will expand massively, partially absorbing but most replacing the people there before. This would drastically alter the appearance of the people living in an area.
This might explain the fast, massive expansions of the Mongoloid east Asians and the Bantu Africans. Each appears to have expanded very rapidly from a relatively small core population, effectively replacing the people who were there before. Also, this could explain the colonisation pattern of the Americas, where Australoid people seem to have been the first settlers. The Jomon-like Asians appear to have moved in later in North America, as well as the European Solutreans at a similar date. Then the Mongoloid Americans (place of origin uncertain) arose and expanded over the entire continent.
There also appears to have been a population expansion originating from Southern Turkey about 10,000 years ago, which spread across North Africa, Southern Asia, Western China and Southern Europe. This appears to have been the expansion of the Neolithic revolution, oddly in two languages, Afro-Asiatic and Indo-European. This is probably why Southern Europeans don’t look a lot like the earlier inhabitants from 15,000 years ago, they were overwhelmed by colonists from Turkey. Northern Europeans do still seem to bear a strong resemblance to ‘Cro Magnon’ people, generally just being slightly smaller in proportion, so it seems that expansion stalled in Southern Europe.
Mathilda's Anthropology Blog. 