Humans first domesticated cattle about 8.000 years ago. Possibly this was first for the meat, but at some point someone (or rather, several someones in several different regions) started also making use of the milk given by lactating cows. Which raises some interesting questions, as many people can't digest the milk sugar, lactose, found in cows' milk. Some of these were addressed in a recent study that examined the variation in cattle milk protein genes, lactose tolerance in modern humans, & stone-age cattle-farming sites (Beja-Pereira et al. 2003). The authors concluded that their data showed evidence of a 'gene-culture evolution between cattle and humans.'
In their study of lactose tolerance in Europeans, Beja-Pereira et al. found a high level of genetic diversity in Northern European native cattle. The distribution of this diversity was similar to the distribution of the allele that gives the ability to digetst lactose - and also with the distribution of Stone-Age cattle farms. Their explanation? 'Gene-culture' evolution between cattle & human culture. The ability to digest lactose opened up a new energy source to human populations with that ability. These populations kept larger dairy herds and actively selected for higher milk yields, changing the frequency of the milk protein genes in cattle - and this in turn would have affected the frequency of the lactase gene in humans. Gibbons notes that the same process led to the evolution of lactose tolerance in at least three different African groups in response to the domestication of dairy cattle.
A point to remember: the mutation allowing lactose digestion would have popped up multiple times in the past - but it was highly unlikely to be established in any human population in the absence of a selective advantage (the ability to make use of a novel food source). Unless, that is, it was carried along with some other feature under selection pressure, which doesn't seem to have been the case here.
A. Beja-Pereira, G. Luikart, P.R. England, D.G. Bradley, O.C. Jann, G. Bertorelle, A.T. Chamberlain, T.P. Nunues, S. Metodiev, N. Ferrand & G. Erhardt (2003) Nature Genetics 35: 311-313
A. Gibbons (2006) There's more than one way to have your milk and drink it too. Science 314: 1672
This post uses material that was originally written for an item on the Science on the Farm website.