In our book Greg and I devote a chapter to the hypothesis, developed with John Hawks and Doug Jones, that the expansion of Indo-European languages was driven by a biological change, an advantageous mutation, that enjoyed a large fitness advantage. Mammals do not ordinary drink milk as adults, while all mammals drink milk as infants. Lactase, the enzyme that digests the milk sugar lactose, ordinarily stops being produced after the age of weaning or so. That is why your kitten loves to lap up milk while your adult cat, after a bowl of milk, may fail to reach the litter box in time.
Several versions of a broken regulator of lactase production are found in human populations leading to lactase persistence after childhood. Individuals with a broken regulator are called lactose tolerant (LT) because they are able to digest milk sugar. Without it milk may cause intestinal discomfort, diarrhea, or worse. On the other hand other people experience no discomfort at all: the difference is likely due to differences in the population of microorganisms in the gut. (There also exist other ways in which people are discomfited by milk, so the mapping between LT and discomfort is not clean).
There is a large region of homogeneity on European haplotypes with the mutation, telling us that it has arisen to high frequency within the last few thousand years. It must have enjoyed a huge selective advantage, but what was it? Seems unlikely that this prominent example of fast evolution was driven by flatulence.
Simple calorics seems to provide a simpler and better answer. In a dairy culture where fresh milk was readily available, children who could drink it obtained about 40% more calories from milk than children who were not LT.
Consider that 1 Liter of cow’s milk has
* 250 Cal from lactose
* 300 Cal from fat
* 170 Cal from protein
or 720 Calories per liter. But what if one is lactose intolerant? Then no matter whether or not flatulence occurs that person does not get the 250 Calories of lactose from the liter of milk, but only gets 470.
Many pastoralists consume a lot of milk (like the Herero I discussed in another post) but they must first ferment away the lactose to make the milk edible. It can be converted to yogurt or cheese or several other prepared milk products. This photo shows a Herero girl churning a gourd of fresh milk (along with way too much calf saliva and dead flies), converting it to something like buttermilk.
We also do that. Our 1 Liter of milk yields 100 g. of cheddar cheese, yielding 400 Cal of energy, cf. 720 in the fresh milk. We have discarded 300 or so Calories, about 40% of the energy, in the conversion.
Whenever food became tight LT children must had enjoyed a tremendous advantage over other children, an advantage that we can see in the genetic evidence of very rapid evolution of LT.