Scientists found the gene that controls our ability to digest milk was missing from skeletons of European ancestors living between 5840 and 5000 BC.
The discovery, a joint effort by University College London and Mainz University, appears to dispel previous theories that Europeans had adapted to eat dairy much earlier in their history.
The finding sheds more light on how lactose tolerance has developed, and in a commercial sense, again reinforces the argument that those who are lactose intolerant may still be 'trained up' to eat dairy.
Humans need the lactase gene to break down lactose in milk, something many people in northern Europe and America possess but others, such as many in Asia, do not.
Those without the gene may experience serious discomfort if they eat dairy products.
Greater exposure to dairy products helped Europeans to leave their lactose intolerance behind, the team claimed, after analysing bone DNA in the region's ancestors.
"We found that the lactose tolerance variant of the lactase gene only became common after dairy farming, which started around 9,000 years ago in Europe."
It may have spread so quickly, at least in evolution terms, because milk's healthy properties meant those who drank the white stuff had a better chance of survival, the scientists said.
The claim is unlikely to be greeted kindly by anti-dairy lobbyists, who claim milk can cause a variety of health problems for some consumers.
But Dr Mark Thomas, of UCL's Biology Department, said early Europeans who had the lactase gene and so were able to drink milk had a "massive survival advantage".
"To go from lactose intolerance being rare or absent seven to eight thousand years ago to the commonality we see today in central and northern Europeans just cannot be explained by anything except strong natural selection."
Scientists have known for some time that all humans were once lactose intolerant.
A previous study has suggested the lactase gene may have been widespread but merely 'turned off'.
Now, the UCL and Mainz researchers want to examine why such huge disparity still exists between different peoples.
For example, around five per cent of northern Europeans and Americans are considered lactose intolerant, yet this rises to eighty per cent in parts of southern Europe, where the first dairy farms are thought to have been.
Source : Proceedings of the National Academy of Sciences Published 26 February – 2 March "Absence of the lactase-persistence-associated allele in early Neolithic Europeans."
Authors: M G Thomas (UCL), Joachim Burger, M Kirchner, B Bramanti and W Haak (Mainz).