This month we get to know a bit more about the ability of humans to digest milk sugars (lactose) in adulthood, with particular emphasis on Africa:
Alessia Ranciaro et al., Genetic Origins of Lactase Persistence and the Spread of Pastoralism in Africa. AJHG 2014. Pay per view (free access after 6-month embargo) → LINK [doi:10.1016/j.ajhg.2014.02.009]
AbstractIn humans, the ability to digest lactose, the sugar in milk, declines after weaning because of decreasing levels of the enzyme lactase-phlorizin hydrolase, encoded by LCT. However, some individuals maintain high enzyme amounts and are able to digest lactose into adulthood (i.e., they have the lactase-persistence [LP] trait). It is thought that selection has played a major role in maintaining this genetically determined phenotypic trait in different human populations that practice pastoralism. To identify variants associated with the LP trait and to study its evolutionary history in Africa, we sequenced MCM6 introns 9 and 13 and ∼2 kb of the LCT promoter region in 819 individuals from 63 African populations and in 154 non-Africans from nine populations. We also genotyped four microsatellites in an ∼198 kb region in a subset of 252 individuals to reconstruct the origin and spread of LP-associated variants in Africa. Additionally, we examined the association between LP and genetic variability at candidate regulatory regions in 513 individuals from eastern Africa. Our analyses confirmed the association between the LP trait and three common variants in intron 13 (C-14010, G-13907, and G-13915). Furthermore, we identified two additional LP-associated SNPs in intron 13 and the promoter region (G-12962 and T-956, respectively). Using neutrality tests based on the allele frequency spectrum and long-range linkage disequilibrium, we detected strong signatures of recent positive selection in eastern African populations and the Fulani from central Africa. In addition, haplotype analysis supported an eastern African origin of the C-14010 LP-associated mutation in southern Africa.
The study detects in essence four alleles that explain lactase persistence in Africa: one (Fulani and other Central Africans) is the same as in Europe, another (Afroasiatics and some Nilotics from NE Africa) is related to Arabia but at least two other variants are African-specific:
|Figure 3. Contour Maps of Africa Show the Allele Frequency Distribution for the Four Primary SNPs Associated with the LP Trait in the Current Study|
|from Fig. 4 (haplotype network)|
Instead the C-14010 allele, present in East Africa around Lake Victoria, and to some extent also in SW Africa, seems very much unrelated.
Alleles and their variants
The European T-13910 allele as three main haplotype variants in Africa:
- The Mozabite one is related to West Asia
- The main Fulani and Bulala one is related to Europe
- A third variant includes only Fulani and Arabic Baggara (strictly African therefore)
Judging on the haplotype tree to the right, the Mozabite/West Asian variant probably derives from Europe but the case for the Central African ones is much less clear.
The authors estimate the allele to have an age between 12,000 and 5,000 years, i.e. approximately early Neolithic. This would allow it to have arrived to Africa and Europe separately before booming.
The G-13915 allele is attributed by the authors to Arabian origins c. 4000 years ago, based on previous studies and its presence only in Afroasiatic and Nilo-Saharan speakers. However I can't but notice that the haplotype structure above is centered in East Africa, with Arabian and Middle Eastern sequences being found only at the branches, what makes me cast a prudential question mark on that claim.
The G-13907 variant is clearly NE African in origin and distribution, with the greatest frequency being among the Beja (Afroasiatic pastoralists of coastal Sudan). This one is restricted to Afroasiatic speakers only, mostly of the Cushitic branch. The authors did not produce an age estimate for this branch.
The C-14010 allele could be as old as 23,000 years or as recent as only 1200 using the 95% C.I. Again it is essentially present among Nilo-Saharan and Afroasiatic (Southern Cushitic) populations, rather than among Bantu speakers, suggesting again association with pastoralism.
Notably this allele has reached Southern Africa but it seems more directly related to the Khoe-Khoe pastoralist history and maybe other populations of possible East African origins (Himba, Herero) than to anything Bantu.
Lactase persistence in hunter-gatherers and other roles of the enzyme
To the surprise of the authors the Hadza hunter-gatherers have high frequencies (47%) of the lactase persistence phenotype (not associated to any of the previously mentioned alleles), something they try to explain on the grounds that the same enzymes that allow lactose digestion, also works as the enzyme phlorizin-hydrolase, which uses the substance phlorizin, found in many Rosaceae (the main group of fruit-producing plants). Besides its anti-diabetic properties, phlorizin has also been used to fight against malaria, what may well be another pathway of positive selection, alternative to usefulness of lactose digestion.
This is a particularly interesting note because it could potentially feed also alternative explanations for the strong selection of lactase persistence alleles in other populations, particularly Europeans. Just tentatively, it might have helped to fend off diabetes (type II) in high carbohydrate diets typical of the Neolithic onwards. The fact that people of West Eurasian and African ancestry are somewhat less prone to this type of diabetes than people of other Asian and Native American ancestry may support this function of lactase persistence.
However in the case of the Hadza a role against malaria (same enzyme) seems more likely.
Still plenty of room for further research
There are still way too many pockets of lactase persistence phenotype which remain unexplained and more rarely (Afghan Tajiks particularly) also pockets of lactose intolerance lacking explanation, as I discussed in 2010. Map of worldwide actual LP phenotype from Yuval Itan 2010:
In the case of Africa the Western Sahel pocket is most intriguing. Also the alleged 88% LP phenotype in the Sudanese (not apparent in the map but data from the same study - maybe they meant Western Sudanese from the Sudan region, i.e. the Sahel and not the Republic of Sudan?), which is only explained in half. Even Germans and Italians have much greater ability to digest milk than what their known alleles can explain so far.