European men and women, starting about 5,000 years ago, started looking for partners with fairer skin and lighter hair and eyes, although researchers don't exactly know why.

That's the basis of a new study --- published in the journal Proceedings of the National Academy of Sciences --- that asserts up until that transitional period in history, the human archetype throughout the continent sported dark skin along with darker features.

"Prehistoric Europeans in the region we studied would have been consistently darker than their descendants today," said Sandra Wilde of the Palaeogenetics Group at the Johannes Gutenberg University Institute of Anthropology in Mainz, Germany, and first author of the study. "This is particularly interesting as the darker phenotype seems to have been preferred by evolution over hundreds of thousands of years. All our early ancestors were more darkly pigmented."

Wilde was one of several anthropologists from Johannes Gutenberg University Mainz and geneticists from University College London, in collaboration with archaeologists from Berlin and Kiev, who analyzed DNA samples from ancient skeletons and found selection has had a significant effect on the evolution of the human genome even in the past 5,000 years, resulting in sustained changes to the appearance of the people of Europe.

The researchers in Mainz and London decided to break with traditional ways of studying changes in the genome, which couldn't pinpoint when specific mutations had taken place.

Their new approach involved analyzing DNA from archaeological skeletons and then comparing the prehistoric data with that of contemporary Europeans using computer simulations. And, where the genetic changes could not be explained by the randomness of inheritance, the researchers were able to determine positive selection played a role, i.e., that frequency of a certain mutation increased significantly in a given population.

"In Europe we find a particularly wide range of genetic variation in terms of pigmentation," said co-author Karola Kirsanow, also a member of the Palaeogenetics Group at Mainz University. "However, we did not expect to find that natural selection had been favoring lighter pigmentation over the past few thousand years."

The signals of selection that the Mainz palaeogeneticists discovered are some of the most notable discovered to date in the human genome.

"Perhaps the most obvious is that this is the result of adaptation to the reduced level of sunlight in northern latitudes," Professor Mark Thomas of UCL, a corresponding author of the study.

"Most people of the world make most of their vitamin D in their skin as a result UV exposure. But at northern latitudes and with dark skin, this would have been less efficient. If people weren't getting much vitamin D in their diet, then having lighter skin may have been the best option," he added.

On the other hand, said Wilde, "this vitamin D explanation seems less convincing when it comes to hair and eye color ... Instead, it may be that lighter hair and eye color functioned as a signal indicating group affiliation, which in turn played a role in the selection of a partner."

Such sexual selection is common in other living creatures, the paper noted.

"We were expecting to find that changes in the human genome were the result of population dynamics, such as migration. In general we expect genetic changes due to natural selection to be the exception rather than the rule. At the same time, it cannot be denied that lactase persistence, i.e., the ability to digest the main sugar in milk as an adult, and pigmentation genes have been favored by natural selection to a surprising degree over the last 10,000 years or so," said Professor Joachim Burger, senior author of the study.

"But, it should be kept in mind that our findings do not necessarily mean that everything selected for in the past is still beneficial today," he continued. "The characteristics handed down as a result of sexual selection can be more often explained as the result of preference on the part of individuals or groups rather than adaptation to the environment."