Early Europeans had an issue drinking milk from cows and goats for 5,000 years after the introduction of farming, according to a new study.

It took at least 5,000 years for their genes to evolve until they weren't intolerant to lactose, the natural sugar in mammalian milk.

"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 lead researcher Professor Ron Pinhasi, from University College Dublin's Earth Institute, in a statement.

"This means that these ancient Europeans would have had domesticated animals like cows, goats and sheep, but they would not yet have genetically developed a tolerance for drinking large quantities of milk from mammals," Pinhasi added, according to LiveScience.

Researchers examined ancient DNA extracted from 13 individuals buried at different archaeological sites in the Great Hungarian Plain, a region known to have been at the crossroads of cultural change in European prehistory.

Samples were taken from the petrus bone, which is a hard part of the skull protecting the inner ear, and dated from 5,700 BC to 800 BC.

"The high-percentage DNA yield from the petrous bones exceeded those from other bones by up to 183-fold,"the study's joint senior author Ron Pinhasi, an archaeologist at University College Dublin in Ireland, said in a statement. "This gave us anywhere between 12 percent and almost 90 percent human DNA in our samples, compared to somewhere between 0 percent and 20 percent obtained from teeth, fingers and rib bones."

The scientists also discovered that great changes in prehistoric technology, like the adoption of farming, and the first use of hard metals such as bronze and then iron, were each associated with the substantial influx of new people.

Scientists are now sequencing even more ancient human genomes dating back 13,000 years from the Caucasus and other parts of Europe "to find out about genetic diversity that existed before and after the Ice Age," Pinhasi said, according to LiveScience. "We are also analyzing ancient farmers to find out who the first farmers truly were."

The findings appear in the online Oct. 21 edition of the journal Nature Communications.

See Now: OnePlus 6: How Different Will It Be From OnePlus 5?