Neanderthal-Human Sex Happened Earlier

Remains of a Neanderthal woman who lived around 100,000 years ago in the Altai Mountains of Siberia reveal that human and Neanderthals mated much earlier than previously thought.

One or more of her relatives were actually humans, a new study shows.

It has been known that Neanderthals contributed DNA to modern humans, so people today of European and Asian descent retain Neanderthal DNA in their genomes, but the Neanderthal woman offers the first evidence that gene flow from interbreeding went from modern humans into Neanderthals as well.

Photos: Faces of Our Ancestors

The study, published in the journal Nature, "is also the first to provide genetic evidence of modern humans outside Africa as early as 100,000 years ago," Sergi Castellano, who co-led the study and is a researcher at the Max Planck Institute for Evolutionary Anthropology, told Discovery News.

Given the now closely intertwined histories of Neanderthals and Homo sapiens, Castellano added that "it is better to refer to Neanderthals and modern humans as two different human groups, one archaic and one modern, and not different species."

Earlier research determined that Neanderthals and modern humans met and mated outside of Africa sometime between 47,000–65,000 years ago. The Siberian woman shows that such interbreeding could have happened as early as 120,000 years ago, since it is now believed modern humans and Neanderthals were both then present in the region around the Persian Gulf and in the area where the following countries are now located: Cyprus, Egypt, Israel, Lebanon, Syria and Turkey.

Photos: Are You Related to Neanderthals?

Castellano, Martin Kuhlwilm and colleagues analyzed the Siberian woman's DNA and identified portions of her genome that match sequences taken from people who are now living in Africa. The researchers also analyzed the remains of a Denisovan individual and the remains of two Neanderthals that were found in European caves (one from Croatia, and the other from Spain). No modern human DNA was detected in those three other individuals.

The scientists therefore think that a population of Neanderthals likely migrated from Europe through the Near East, where mating with modern humans occurred. They then continued to travel up into the Altai Mountains, where the climate was much milder 125,000 years ago than it is now.

Castellano and his team further believe that the modern humans who were in the Near East at this early time eventually died out and did not contribute to the present day Homo sapiens genome.

Nevertheless, Castellano said, "Some Asians and Oceanians (living today) have more Neanderthal DNA because a second pulse of admixture might have happened with Neanderthals."

The evidence throws a wrench in the widely held view that modern humans first left Africa about 60,000 years ago. Instead, it seems that one or more groups of modern humans were leaving Africa tens of thousands of years earlier, and apparently at least some were mating with Neanderthals along the way.

Montgomery Slatkin is a professor in the Department of Integrative Biology at the University of California at Berkeley. He told Discovery News, "I think the results (of the new study) are important because they show how complex the relationships between the ancestors of modern humans and Neanderthals were. It will be interesting to find out whether the early interbreeding between Neanderthals and humans affects only the Neanderthals found in Asia, or all Neanderthals."

Chris Stringer, a research leader in Human Origins at the Natural History Museum, London, wonders how the interbreeding happened. He speculated that "the possibilities range from relatively peaceful exchanges of partners, to one group raiding another and stealing females -- which happens in chimps and some modern hunter-gatherers -- through to adopting abandoned or orphaned babies."

Humans Vs. Neanderthals: How Did We Win?

"Eventually, geneticists should be able to show if the transfer of DNA in either direction was mainly via males, females or about in equal proportion, but it will need a lot more data before that becomes possible," he added.

Stringer also wonders if all of the modern human dispersals out of Africa before 60,000 years ago failed, such that all of those individuals died out and made no contribution to the present Homo sapiens genome.

"If both the early modern human groups and the early intermixed Neanderthals went completely extinct, then this was still ultimately a failed expansion," he said. "But the increasing resolving power of genetic studies means that the search is now on for further traces of these mysterious early moderns, and their Neanderthal relatives in Asia."

Neanderthals and modern humans may have intermixed much more than previously thought.

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.