The discussion orbits around a recent
pay-per-view study by O.O. Sverrisdóttir, which claims, with some soundness for what I can discern, that LP in Europeans must have gone through positive selection.
Actually the study, as most of its kind, deals only with one LP marker, the well known SNP
rs4988235, whose T variant allows adults (in dominant fashion) to digest milk, an ability often lost after weaning.
As
I discussed back in 2010, there must be other such SNPs because actual LP phenotype only partly corresponds with the known LP alleles. But for whatever is worth, this is the (2010) "known allele" LP map:
In Europe at least, it essentially corresponds with the T variant of rs49235, which concentrates in Scandinavia, Atlantic Islands and the Basque/SW French area.
At first I boarded the discussion with perplexity, because, even if the positive selection argument seems sound, it seems hard to find a reason for it: milk is not such a "great" source of food, excepting the issue of calcium and a high content of protein and fat, and the occasional claims that it is related to vitamin D deficiency seem extremely feeble because this vitamin is present at extremely low frequencies in natural milk, being rickets (where milk's extra calcium could play some role) only a "less important" side effect of vitamin D deficiency, because its main harmful effect is to impair early brain development, a most serious problem for which calcium seems quite meaningless. So why would the ability to digest milk would have become such a matter of life or death to be actively selected for generation after generation until near-fixation?
A key piece of information is that not a single sequenced Neolithic farmer has ever been found to carry the relevant LP allele (being all CC) and only since Chalcolithic we begin to find some TT and CT individuals. These are found in Sweden and in the southern areas of the Basque Country (see
here for a lengthier discussion):
- In Götland (Pitted Ware culture) only 1/20 alleles was T (i.e. 1/10
persons had the CT combo, all the rest being CC and therefore likely
lactose intolerants).
- In Longar (Navarre, dated to c. 4500 BP)
1/7 individuals was TT, while the other six were CC (intolerant). There
were no CT cases.
- In San Juan Ante Porta Latinam (SJAPL, Araba, dated to c. 5000 BP), 4/19 were TT, 2/19 were CT, while the remaining 13 were CC.
In the Basque cases we can appreciate that there must have already been two different populations regarding this SNP, because the CT cases are rare, implying that the two groups were only beginning to mix. It is worth mentioning that the Basque sites are odd in several aspects: on one side they seem to be
military cemeteries (mostly males, arrow injuries and arrow points) and, on the other, they are rather exceptional
in the Basque historical sequence of mtDNA pools (a lot more K and some other lineages than usual, less H and U).
But a key finding in this study is that a Neolithic sequence from Atapuerca (near Burgos city, historical Basque SW border) was again CC for the relevant SNP (and therefore likely lactose intolerant). So it is very possible that proto-Basques did not have the T allele in notable frequencies either (although I keep some reservations for lack of larger samples).
Whatever the case, if the T allele was selected positively as it seems, there must be a powerful reason for it. Was it cows, as some have claimed a bit too vehemently? I doubt it.
Why? Because for all we know from the Middle Ages, a period very similar in many aspects to the Metal Ages, it were goats and not cows the main providers of milk. This makes total sense because the hardy goats are rather inexpensive to rear, while cows are more costly and were often reserved for traction jobs. In most cases, cow produce, be it milk or meat, was an expensive luxury apt only for the upper echelons of a society that was becoming more and more hierarchical and unequal since precisely the Chalcolithic period.
Some oral accounts I have heard tell that not so long ago "acorn bread and goat milk" were often staple for the poor. In other areas maybe it was not acorn bread but, say, oat meal (or whatever else), but almost certainly the milk came almost invariably from goat udders, which very efficiently transform leaves and almost any vegetable, even thorny ones, into milk (and meat) for our consumption.
It is crucial to understand that only if milk was a key survival staple, LP would have become fixated. Otherwise people would have preferred alternative foods and survived in similar shape, so positive selection would never have happened at this locus (non-LP individuals would have survived easily, selection would never have happened or would have been mild enough to retain much greater diversity).
It is also crucial to understand that, for all we know, this positive selection only happened since the Chalcolithic, i.e. when social stratification, inequality and private aristocratic property became common. Obviously the upper classes (or castes) had no problems accessing high quality foods, including meat, but the masses probably had growing problems in this aspect as the land and cattle became more and more concentrated in few hands.
Even where a wide class of free peasants existed, as was probably the case in much of Atlantic Europe, these were surely often not well-off enough to afford dairy cows. Instead goats would have been available for almost everybody, even the poorest of farmers. And very likely they were the only steady supply of proteins and fat, mostly via milk.
Plausibly this need of extra nutrients of animal origin was more intense in the Atlantic areas of Europe because cereals do not perform so well in the prevalent humid conditions. Also before the medieval development of the heavy plough, the deep Atlantic soils were not at all as productive as they are now (and that's why NW Europe only got its economic prominence in the last millennium, being before a peripheral area to the much more productive Mediterranean climate).
But climatic and agricultural issues aside, I strongly suspect that the main driver of LP positive selection, were goats, because these and their dairy produce were almost certainly available for almost everyone and, in the Metal Ages, the vast majority of people were farmers, often rather poor peasants who had to rely on their goats for survival, very especially in the bad times.
I really do not see any other explanation that fits the data.
PS- This social inequality & goats argument makes sense assuming that the positive selection theory is correct. However before I fully embrace it, I would need a half-decent sample of aDNA sequences from the Atlantic areas of Europe, notably Britain & Ireland, the Basque Country & SW France and mainland Scandinavia, where the T allele peaks. I say because what we find in some Chalcolithic sites, notably in the Basque Country, rather strongly suggests that there was already a TT population somewhere and we have not yet found it. So maybe some of the premises of the positive selection theory are not as sound as I said above - but we do not know yet.
