March 12, 2014
Europeans’ Appearance Altered Over Five Millennia Of Natural Selection
April Flowers for redOrbit.com - Your Universe Online
A great deal of research has been focused on the factors that have influenced the human genome since the end of the last Ice Age.
An international team of scientists—including anthropologists at Johannes Gutenberg University Mainz (JGU), geneticists at University College London (UCL), and archaeologists from Berlin and Kiev—has analyzed ancient DNA from skeletons, finding that natural selection has had a major effect on the human genome even in the past 5,000 years. The result of this selection has been sustained changes to the appearance of people. The study findings were published this week in Proceedings of the National Academy of Sciences.
For a while now, geneticists have been able to detect echoes of natural selection in the genomes of living humans. Those techniques, however, are typically not very accurate about when that natural selection took place. The researchers decided to try a new approach, which involved analyzing DNA from archaeological skeletons and then using computer simulations to compare that data to that of contemporary Europeans. In places where the genetic changes could not be explained by the randomness of inheritance, the research team was able to infer that natural selection played a role. For example, that the frequency of a particular mutation increased significantly in a given population.
Sandra Wilde, of the Palaeogenetics Group at the JGU Institute of Anthropology, was investigating various genetic markers in archaeological and living individuals when she noticed remarkable differences in genes associated with hair, skin, and eye pigmentation.
"Prehistoric Europeans in the region we studied would have been consistently darker than their descendants today," says Wilde. "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 says that things had to have changed over the last 50,000 years as humans began their migration to northern latitudes.
"In Europe we find a particularly wide range of genetic variation in terms of pigmentation," adds co-author Dr. Karola Kirsanow, who is 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 selection signals identified by the Mainz / UCL team are comparable to those for malaria resistance and lactase persistence, making them among the most pronounced that have been discovered to date in the human genome.
According to the researchers, there are several possible explanations.
"Perhaps the most obvious is that this is the result of adaptation to the reduced level of sunlight in northern latitudes," says Professor Mark Thomas of UCL. "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."
"But this vitamin D explanation seems less convincing when it comes to hair and eye color," Wilde continues. "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."
In animals, sexual selection of this kind is common. The research team believes it may also have been one of the driving forces behind human evolution over the past few millennia.
"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," adds Professor Joachim Burger. "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. 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."