Earliest humans were lactose intolerant for 5,000 years
Some of the world's earliest humans remained lactose intolerant for thousands of years after they first introduced dairy foods into their diets.
As hunter-gatherers became farmers, and milk and cheese became a more staple part of their daily diet, early humans had difficulty adapting to the dietary changes for up to 5,000 years.
Researchers from University College, Dublin, and Trinity College have documented the findings, which have now been published in the scientific journal Nature Communications.
The scientists involved came to the conclusion after extracting DNA samples from the petrous temporal bone, the thickest bone in the body, found at the base of the skull.
Fragments from this particular bone offered scientists samples with between 12pc to 90pc of human DNA.
The skeletons sampled date from the Early Neolithic period to the Iron Age, 5,700 BC to 800 BC.
"Our findings show progression towards lighter skin pigmentation as hunter and gatherers and non-local farmers intermarried, but surprisingly no presence of increased lactose persistence or tolerance to lactose," said Professor Ron Pinhasi from UCD Earth Institute, a joint senior author of the paper.
"This means that these ancient Europeans would have had domesticated animals like cows, goats and sheep, but they would not yet have genetically developed any sort of a tolerance for drinking large quantities of milk."
Lactose intolerance occurs when the body is unable to break down lactose and absorbs it into the blood, leading to symptoms like bloating, flatulence and diarrhoea.
Professor Dan Bradley, from the Smurfit Institute of Genetics in Trinity College, said that their results also implies that "the great changes in prehistoric technology, including the adoption of farming, followed by the first use of hard metals, bronze and then iron, were each associated with the substantial influx of new people.
"We can no longer believe that these fundamental innovations were simply absorbed by existing populations, in some sort of cultural osmosis," he added.
The study was carried out on 13 individuals in burial sites along the Great Hungarian Plain, "an area known to have been at the crossroads of major cultural transformations that shaped European prehistory".