We adult mammals shouldn’t be eating milk, in theory. Milk is for babies, who produce an enzyme called lactase that breaks down the milk sugar. Mammals lose the ability to produce lactase by adolescence.
Yet many modern humans – not all by a long shot – continue to produce lactase in adulthood. They are blessed with a “lactase persistence” mutation, and are called “lactose tolerant.”
People who don’t produce lactase in adulthood are called “lactose intolerant,” and when they consume milk they may suffer from flatulence, bloating and diarrhea as the bacteria in their digestive systems eat the milk and emit gas.
It has been assumed that in prehistory, lactase persistence emerged roughly at the same time as we adopted the cow, sheep and goat and discovered that they’re good for more than the barbecue because otherwise we couldn’t have consumed the milk.
But that is not so. A new and very different theory was published Wednesday in Nature by Richard Evershed of southwest England’s University of Bristol, Mark Thomas and colleagues, who created the first comprehensive map of prehistoric dairy exploitation and the ability to digest it.
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Sneak peek at one big surprise: lactose intolerance is a thing, but not much of one.
So, one major discovery is that in Europe, the mutation enabling us to digest milk only arose and become widespread thousands of years after the cow, sheep and goat were captured and penned.
A second is that based on a UK Biobank medical and genetic database of 300,000 modern Britons, no real difference was found between “lactose tolerant” and “lactose intolerant” people – both drink milk more or less equally, and the “intolerant” evinced short- or long-term harm from it.
A third is that indicators best explaining the spread of the tolerance mutation isn’t whether or not milk was there: it was famine and exposure to infectious disease.
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Radiating from Turkey
In the Near East and western Eurasia (not Africa, China, the Americas), the Neolithic revolution began in southeast Turkey about 10,500 years ago and radiated from there. This spread was not linear or straightforward, in the sense that different places adopted agriculture and animal husbandry at different times, but the pattern is clear.
The researchers sought evidence of prehistoric dairy consumption by detecting milk fat residue on over 13,000 pieces of pottery from over 550 sites around Europe, going back 9,000 years, with the emergence of the mutation detected by conducting DNA analysis of 1,786 ancient people.
Now, it is possible that milk consumption began the day the first cow or sheep or goat was trapped, but 10,500 years ago was the pre-pottery Neolithic era, so no pottery fragments with or without fat residue on them could be checked, Evershed explains. Pottery began about 9,000 years ago in the Northern Levant (and much earlier in eastern Asia), but the point is: when pottery began milk was apparently already being consumed in Turkey, and also in Italy and the Balkans.
In Kazakhstan and the British Isles, for instance, domestic animals arrived about 6,000 years ago, and that is when dairying began there, according to a separate study of plaque on ancient Britons’ teeth.
The earliest evidence (so far) of lactase persistence mutation is about 6,700 years ago in central Europe. “It pops up occasionally about 6,000 years ago but starts to really rise only in the late Bronze Age,” says Evershed.
But the lactase persistence mutation only became common in Europe (varying by place) about 3,000 years ago, though it is highly variable. In modern Europe, almost 100 percent of today’s northerners have it, versus around a fifth of Greeks and 14 percent of Sardinians. In Asia, the rate ranges from about a tenth to a third of the population. In Turkey too, only about a third of the people are lactase-persistent.
Given how common it is in some parts, the variant gene experienced what co-author Prof. Thomas from University College London called “turbocharged natural selection.” The question is what caused this powerful natural selection, if it wasn’t the arrival of milk.
What doesn’t kill you
Here in Israel we are a melting pot, so lactase mutation prevalence isn’t enlightening. We can say, though, that dairy is so popular that serial hikes in the price of cottage cheese helped trigger mass cost-of-living protests in 2011.
So what we have is that some Europeans were drinking milk for thousands of years before the mutation enabling them to digest it comfortably became widespread. Ergo, the mere presence of milk on the table, though key to the mutation’s “rationale,” apparently was not a key driver of this “turbocharged” evolution.
Are we to conclude that for thousands of years prehistoric peoples ate milk and simply put up with flatulence, diarrhea and stomach ache?
Possibly; possibly they didn’t even notice the effects; possibly they felt that was within the range of normal. Do note the persistence of the cigarette and sugared sodas despite widespread recognition of the risks involved. Cancer and diabetes are deadly, but farting won’t kill you – as Prof. George Davey Smith of Bristol Medical School and his team show based on the UK Biobank data.
“Our findings show milk use was widespread in Europe for at least 9,000 years, and healthy humans, even those who are not lactase persistent, could happily consume milk without getting ill,” Davey Smith says.
Enter the cow, into the living room
Even so, how to explain a roughly 4,000-year gap between the arrival of dairy and lactase persistence?
If you’re healthy, lactase non-persistent and drink milk, you may bloat but you are not going to die of it, Thomas explains. But if you're not...
One theory has been that early dairy eaters were making cheese, which is low on lactose. Indeed, Evershed says, cheese-making pots from about 7,500 years ago were found in Poland. Also, if you’re milking a goat on a hot summer day in Turkey or Kazakhstan or France and it is prehistory and you have no fridge, it will ferment very fast and the yeast will eat the lactose. But, as said, this is apparently an irrelevance because the database shows present-day non-tolerant people drinking milk with no real ill effect.
But back in prehistory, if there is famine, if you are malnourished and milk becomes a key food, and you do not have lactase to digest it – then you may become very ill, Evershed explains.
Or, if your health is compromised because you are living in a densely populated settlement with your animals, and getting zoonotic diseases due to poor sanitation, the same applies: you are vulnerable, and wolfing down milk products without lactase can push you over into a death state.
But if you are one of the lucky ones with lactose persistence, that milk will help you weather the hard times or sickness, Evershed sums up.
And there we have it. Prehistoric people unable to digest milk and eating a lot of it when sick died, while people who could digest the milk had an advantage, of surviving and being fruitful and multiplying, and thus the gene spread rapidly, if spottily, through humanity.
And there goes the theory that the earliest farmers who couldn’t digest milk had to decide whether to become dairy farmers, Evershed says. That was not apparently a dilemma for them.