How Neanderthal DNA Changed Humans

People of European and Asian descent today retain Neanderthal DNA that may affect their hair, skin, fertility, predisposition to certain diseases and possibly other characteristics, a new study in the journal Nature suggests.

The genetic material inherited from Neanderthals combined with that of humans when the two species interbred 40,000 to 80,000 years ago, the study holds. The research further supports that indigenous Africans possess little or no Neanderthal DNA because their ancestors did not breed with Neanderthals, which lived in Europe and Asia.

It now appears that mating between the two species was much more prevalent than was previously suspected.

Some genetic mutations introduced by Neanderthals were not beneficial to humans. Neanderthals' contribution to modern DNA was partially removed by natural selection over time.

"Given the large amount of Neanderthal alleles (gene variants) that were swept away by selection, we think that there was a larger fraction of Neanderthal ancestry initially," lead author Sriram Sankararaman explained to Discovery News, adding that "we think that this ancestry was reduced by a third."

Sankararaman, a post-doctoral researcher at Harvard Medical School's Department of Genetics, co-authored the paper with leading evolutionary geneticists, such as Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology and Harvard's David Reich.

The researchers analyzed genetic variants in 846 people of non-African heritage, 176 people from sub-Saharan Africa and a 50,000-year-old Neanderthal.

The clearest indicator that a gene variant came from a Neanderthal was if the variant appeared in some non-Africans and the Neanderthal, but not in the sub-Saharan Africans.

Levels of Neanderthal ancestry differ in European and Asian groups, according to the study. Han Chinese people in Beijing, for example, have the most such ancestry while Puerto Ricans have the least.

All Europeans and Asians, however, retain Neanderthal genes affecting keratin, which is a fibrous protein in skin and hair. Reich explained that keratin makes skin, hair and nails tougher and better able to withstand cold temperatures.

"It's tempting to think that Neanderthals were already adapted to the non-African environment and provided this genetic benefit to humans," Reich said.

It's possible that the lighter skin and hair of many Europeans was also influenced by Neanderthal ancestry, but Sankararaman indicated that further research is needed to fully make that connection.

The study additionally found that genetic variants passed down from Neanderthals also affect an individual's disposition toward type 2 diabetes, Crohn's disease, lupus, a liver condition known as biliary cirrhosis and even whether a person is likely to smoke.

X chromosome-related genetic mutations from Neanderthals, along with DNA associated with male sexual organs, were more likely to have been incompatible with the human genome. These were less easily exchanged between the two species. People who still retain such Neanderthal DNA, like East Asians, experience reduced fertility as a probable result.

"This suggests that when ancient humans met and mixed with Neanderthals, the two species were at the edge of biological incompatibility," Reich said.

Daven Presgraves, an associate professor in the University of Rochester's Department of Biology said the Nature paper implies humans migrating out of Africa were able to "borrow" genes associated with skin, hair and nails from Neanderthals "perhaps accelerating adaptation to a Eurasian environment that was new to them."

The findings also strongly support the theory that Neanderthals were simply absorbed into the human gene pool. They are now, in essence, a part of many people.

A replica of a Neanderthal is dressed in modern clothes at the Neanderthal Museum in Mettmann, Germany.

Back in the Beginning

To put a human face on our ancestors, scientists from the Senckenberg Research Institute used sophisticated methods to form 27 model heads based on tiny bone fragments, teeth and skulls collected from across the globe. The heads are on display for the first time together at the Senckenberg Natural History Museum in Frankfurt, Germany. This model is Sahelanthropus tchadensis, also nicknamed "Toumai," who lived 6.8 million years ago. Parts of its jaw bone and teeth were found nine years ago in the Djurab desert in Chad. It's one of the oldest hominid specimens ever found.

Australopithecus afarensis

With each new discovery, paleoanthropologists have to rewrite the origins of man's ancestors, adding on new branches and tracking when species split. This model was fashioned from pieces of a skull and jaw found among the remains of 17 pre-humans (nine adults, three adolescents and five children) which were discovered in the Afar Region of Ethiopia in 1975. The ape-man species, Australopithecus afarensis, is believed to have lived 3.2 million years ago. Several more bones from this species have been found in Ethiopia, including the famed "Lucy," a nearly complete A. afarensis skeleton found in Hadar.

Australopithecus africanus

Meet "Mrs. Ples," the popular nickname for the most complete skull of an Australopithecus africanus, unearthed in Sterkfontein, South Africa in 1947. It is believed she lived 2.5 million years ago (although the sex of the fossil is not entirely certain). Crystals found on her skull suggest that she died after falling into a chalk pit, which was later filled with sediment. A. africanus has long puzzled scientists because of its massive jaws and teeth, but they now believe the species' skull design was optimal for cracking nuts and seeds.

Paranthropus aethiopicus

The skull of this male adult was found on the western shore of Lake Turkana in Kenya in 1985. The shape of the mouth indicates that he had a strong bite and could chew plants. He is believed to have lived in 2.5 million years ago and is classified as Paranthropus aethiopicus. Much is still unknown about this species because so few reamins of P. aethiopicus have been found.

Paranthropus boisei

Researchers shaped this skull of "Zinj," found in 1959. The adult male lived 1.8 million years ago in the Olduvai Gorge of Tanzania. His scientific name is Paranthropus boisei, though he was originally called Zinjanthropus boisei -- hence the nickname. First discovered by anthropologist Mary Leakey, the well-preserved cranium has a small brain cavity. He would have eaten seeds, plants and roots which he probably dug with sticks or bones.

Homo rudolfensis

This model of a sub-human species -- Homo rudolfensis -- was made from bone fragments found in Koobi Fora, Kenya, in 1972. The adult male is believed to have lived about 1.8 million years ago. He used stone tools and ate meat and plants. H. Rudolfensis' distinctive features include a flatter, broader face and broader postcanine teeth, with more complex crowns and roots. He is also recognized as having a larger cranium than his contemporaries.

Homo ergaster

The almost perfectly preserved skeleton of the "Turkana Boy" is one of the most spectacular discoveries in paleoanthropology. Judging from his anatomy, scientists believe this Homo ergaster was a tall youth about 13 to 15 years old. According to research, the boy died beside a shallow river delta, where he was covered by alluvial sediments. Comparing the shape of the skull and teeth, H. ergaster had a similiar head structure to the Asian Homo erectus.

Homo heidelbergensis

This adult male, Homo heidelbergensis, was discovered in in Sima de los Huesos, Spain in 1993. Judging by the skull and cranium, scientists believe he probably died from a massive infection that caused a facial deformation. The model, shown here, does not include the deformity. This species is believed to be an ancestor of Neanderthals, as seen in the shape of his face. "Miquelon," the nickname of "Atapuerca 5", lived about 500,000 to 350,000 years ago and fossils of this species have been found in Italy, France and Greece.

Homo neanderthalensis

The "Old Man of La Chapelle" was recreated from the skull and jaw of a Homo neanderthalensis male found near La Chapelle-aux-Saints, in France in 1908. He lived 56,000 years ago. His relatively old age, thought to be between 40 to 50 years old, indicates he was well looked after by a clan. The old man's skeleton indicates he suffered from a number of afflictions, including arthritis, and had numerous broken bones. Scientists at first did not realize the age and afflicted state of this specimen when he was first discovered. This led them to incorrectly theorize that male Neanderthals were hunched over when they walked.

Homo floresiensis

The skull and jaw of this female "hobbit" was found in Liang Bua, Flores, Indonesia, in 2003. She was about 1 meter tall (about 3'3") and lived about 18,000 years ago. The discovery of her species, Homo floresiensis, brought into question the belief that Homo sapiens was the only form of mankind for the past 30,000 years. Scientists are still debating whether Homo floresiensis was its own species, or merely a group of diseased modern humans. Evidence is mounting that these small beings were, in fact, a distinct human species.

Homo sapiens

Bones can only tell us so much. Experts often assume or make educated guesses to fill in the gaps in mankind's family tree, and to develop a sense what our ancestors may have looked like. Judging from skull and mandible fragments found in a cave in Israel in 1969, this young female Homo sapien lived between 100,000 and 90,000 years ago. Her bones indicate she was about 20 years old. Her shattered skull was found among the remains of 20 others in a shallow grave.