oh no, my bizarre moment has come, cause lactose tolerance is actually A Thing I Know About because it’s played a fascinating role in human evolution for thousands of years. This chart displays some of the broad trends, but it’s giving near continental averages, which doesn’t showcase how this kind of thing really breaks down and some of the surprising exceptions.
Lactose tolerance is the majority trait for only a very few population groups: North Europeans (and therefore populations that draw heavily from that stock, such as America,) nomadic central Eurasians, and sub-Saharan pastoralist Africans, but that latter group is often overlooked. The vast majority of Africans cannot process lactose, but certain people groups whose lifestyles have revolved around cattle for thousands of years will have 80% and even approaching 100% lactose tolerance rates. They’d be spots of dark green amidst a sea of orange and burgundy on the above chart.
Our hunter-gatherer ancestors were almost entirely lactose intolerant, that is definitely the biological norm (and people groups who maintained that lifestyle, such as Native Americans, remained as such – along with groups who transitioned to sedentary agricultural lifestyles, but I’ll get into that). As such, lactose tolerance is an adaptive trait that only became prevalent in environments that exerted strong selective pressure for it. So, cows were domesticated some 10,000 odd years ago in the Middle East (and some have contended for an independent domestication event in Africa as well). In either case, cattle quickly spread across the continent and we know there was milking and cheese production at least 6,000 years ago in both the Nile and Mesopotamia. While cow meat would have been enjoyed by all, in agricultural societies milk and cheese would have been options, but hardly staples as there were plenty of other things to eat as well, and therefore there would have been no selective pressure for processing lactose. Also, sedentary societies had ways of processing milk and cheese that allowed lactose intolerant people to drink/eat dairy products. Fermenting milk or aging cheese breaks down lactose, making it a non issue once ingested. This is why fermented milk may seem utterly foul to many Westerners, but is extremely common in other parts of the world. But, fermentation and aging requires time, and the ability to store things in a single location for weeks or even months. Sedentary societies adapted the milk to fit their biology, but nomadic societies did the reverse.
There are still mobile pastoralist societies in Africa today, and there have been for thousands and thousands of years. For many of them, cows are not one of many dietary options, they are the single dietary staple around which their lifestyle revolves. Biologically, this means you gotta get with the program if you wanna survive. For most mobile tribes, fermentation and aging weren’t options, so there would have been strong selective pressure favoring those who could drink milk straight outta the cow, as they would have had an additional, highly nutritious food source available to them. Milk also allowed for a marked shortening of the weaning process, transitioning children from breastmilk to cow’s milk, which would again be advantageous for groups where both the men and women work and are always on the move. Over generations these populations specialized into essentially cow-based lifestyles, creating a survival niche highly advantageous to them, and fast forward thousands of years and there are groups in Africa with near ubiquitous lactose tolerance, while the rest of the continent (and the world really) is nearly entirely intolerant.
Many of these same factors would have influenced the central Eurasian populations, which is why Mongolians and other descendants of nomadic steppe peoples are largely lactose tolerant, as mare’s milk would have been a dietary staple (though they also developed efficient ways to ferment it).
North Europeans developed lactose tolerance in response to deficiencies in certain nutrients. The northern climate limited Vitamin D production, and the agricultural products available to them were often low on calcium and protein, and so dairy farming developed alongside agriculture to create a more rounded diet (and this was limited to Northern Europeans, as Mediterranean peoples such as the Romans wrote about their great confusion at the northern barbarians’ ability to drink fresh milk)
And I promise all of this is fascinating because the ability to process lactose evolved independently in several different population groups and in response to different factors: lifestyles revolving around cows, lifestyles revolving around horses, deficiencies in climate and agriculture. Besides providing insight into human history and biology, lactose tolerance is also a great example of convergent evolution, where different genetic populations in different environments produce similar results.
And uh, that’s my rant about the role of milk and lactose tolerance in human evolution.
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