The peak distribution of lactase persistence matches an area also showing the highest diversity of cattle milk genes. Notably, this region corresponds to the land of the Neolithic farmers of the Funnel Beaker Culture from the third millennium BC suggesting a remarkable gene-culture co-evolution. (..)
From a purely nutritional side it is therefore not clear why such an extremely strong selective advantage is conferred by the lactase persistence phenotype. However, this mutation is characteristic for cultures that share the traits of animal domestication and adult milk consumption. Pastoralist populations in Africa also display the lactase persistence phenotype (90% in Tutsi, 50% in Fulani), they likewise show haplotype homozygosity extending for > 2 Mb around the lactase gene. The changes are again point mutations in the lactase enhancer region, but they are molecularly distinct from those found in European people (G/C-14010, T/G-13915, C/G-13907) (Tishkoff et al., 2007). We have here a remarkable case for convergent adaptive evolution. Is it possible that the lactase-associated haplotype is adaptive for something else in addition to fresh milk digestion? Might it confer protection against infection like another sugar-degrading enzyme, glucose-6-phosphate dehydrogenase, against malaria? (..)
The close contact between domesticated animals and the farmers created problems: humans were in too close contact with sick animals and trans-species infections became much more likely. The hunter met his prey only at distance and when he could touch the prey, the animal was already dead. All the mechanisms, which microbes induced in the infected host to assure their transmission like sneezing, coughing or diarrhoea are not any longer operative in the dead animal. The farmer probably also discovered quite early the value of animal dung for burning and as fertilizer on the field, which recycles animal pathogens into the human food chain. We can thus safely anticipate that the early farming society was plagued by new diseases (Diamond, 1997). Zoonosis was feeding new pathogens into the human population (Weiss, 2001).
Notably, important human pathogens, which happen to belong to the most transmissible agents like measles and smallpox, have their closest relatives in viruses from domesticated animals. Sequence analysis revealed that measles, which today circulates exclusively in the human population, is a close relative of rinderpest virus of cattle and the Peste des petits ruminants virus of sheep and goat (all belong to the morbillivirus group of paramyxoviruses) (Griffin, 2001; Lamb and Kolakofsky, 2001). If viruses had co-evolved with their hosts during evolution, we would expect the closest relatives of measles virus in paramyxoviruses of primates. In the morbillivirus group only a Tupaiavirus is known, but the systematic attribution of tupaias (tree shrews) to the primates is disputed (some zoologists classify them with the insectivores). (..)
The close relationship of smallpox (variola) virus (Moss, 2001) with the cowpox virus permitted one of the biggest success stories of medicine. Milkmaids who had acquired cowpox were resistant to smallpox. Based on this observation, Jenner developed modern vaccination by inoculating cowpox lesion material into humans. The very name vaccination recalls the cow (Latin vacca) and underlines again the close relationship between a human and a bovine virus. The precise origin of the vaccinia virus is not known. In fact smallpox seems to have emerged perhaps 5000 years ago presumably from wild animals like rodents (Esposito et al., 2006). Cattle are only an intermediate host. However, this does not change the argument. Rats and mice, attracted by the cereal stores of the early farmers, belong as a pest also to the undesired content list of the Neolithic package like fleas and lice. The closest existing relative of smallpox is camelpox, but this does not go against the argument either since camel bedouins also have developed lactase persistence characterizing them as members of the dairy cultures. After the eradication of smallpox, cases looking like smallpox have been traced to monkeypox, demonstrating ongoing trans-species infection and a possible reservoir independent of dairy animals.
