People With Most Diverse Ancient Human DNA Found

Melanesians retain both Neanderthal and Denisovan ancestry in their genes, according to new research that finds they are about 2 percent Neanderthal and between 3-4 percent Denisovan.

This means that Melanesians - people native to Vanuatu, the Solomon Islands, Fiji, Papua New Guinea, New Caledonia, West Papua and the Maluku Islands - have the highest degree of archaic human ancestry ever documented. Neanderthals and Denisovans went extinct several thousand years ago, so the family roots of Melanesians go incredibly deep.

The discovery, reported in the journal Science, adds to the growing body of evidence that all humans alive today are basically hybrids.

Photos: Faces of Our Ancestors

"I think there is no such thing as a purebred modern human," senior author Joshua Akey of the University of Washington's Department of Genome Science told Discovery News. "All of our genomes are a mosaic of different ancestries, and admixture is a recurring theme throughout human evolutionary history."

For the study, Akey and his team analyzed the genomes of 1,523 individuals from around the world, including 35 Melanesians. They confirmed prior findings that all non-African people inherited roughly 1.5 to 4 percent of their genomes from Neanderthals. Melanesians, however, were the only population that also had significant Denisovan ancestry.

Such seemingly small percentages are misleading, because the researchers also found strong evidence that recurrent natural selection against archaic DNA sequences happened as the various human groups interbred. As a result, today's Neanderthal and Denisovan heritage percentages likely don't reflect the true amount of interbreeding that took place.

Ancient Human With 10 Percent Neanderthal Genes Found

As Akey said, "Our data suggests matings with archaic individuals may be more prevalent than currently thought."

The regions of the modern human genome that appear particularly depleted of archaic sequences are those that play a role in speech and language development, suggesting that these skills could have set modern humans apart from Neanderthals and Denisovans.

Anne Stone, a professor and director of the Center for Bioarchaeological Research at Arizona State University, points out that regions of our genome that are less likely to contain archaic DNA are those areas that "encode important adaptations for modern humans, i.e. what makes us modern human, while those that contain more archaic DNA may represent regions where archaic adaptations were advantageous."

Based on earlier research, such advantages might have included changes to immunity, hair and skin to better match the needs of out-of-Africa environments.

Neanderthal-Human Sex Happened Earlier

Akey and his team also mapped out the genetic flow of the Neanderthal and Denisovan sequences and found that Neanderthal gene flow (admixture) with modern human DNA occurred at least three distinct times in modern human history. The researchers only found evidence for one such admixture event with Denisovans.

Anthropologists are not entirely sure where and how the fateful matings between modern humans migrating out of Africa and Neanderthals and Denisovans took place. Akey, however, said that he and his colleagues "can order the times of distinct admixture events, including admixture that occurred in all non-African populations shortly after the out-of-Africa dispersal; another pulse of admixture that occurred in the ancestors of Europeans and East Asians after they diverged from Melanesians; and a third pulse of admixture with the ancestors of present day East Asians after they diverged from Europeans."

He suspects that modern humans mated with Denisovans in Southeast Asia before some of those individuals traveled to Melanesia.

Got Allergies? Blame Neanderthals

Kirk Lohmueller, an assistant professor in the Department of Ecology and Evolutionary Biology at the University of California, Los Angeles, commented that the new research "is significant because it shows that there were multiple waves of admixture between modern human populations and archaic hominids."

Information about interbreeding that took place in Africa is forthcoming; Akey and his colleagues are working on that research now.

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.