Mauro Cutrona
Humans may have interbred with Neanderthals long ago, but pairings probably rarely produced offspring.
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updated 9/12/2011 4:19:58 PM ET 2011-09-12T20:19:58

We may have interbred with Neanderthals in the past, but only rarely was that sex successful in producing offspring, scientists now suggest.

Any such dalliances might either have been scarce or only rarely produced offspring, or both, researchers explained.

Recent analyses of Neanderthal genes revealed that many of us have this extinct lineage within our ancestry. Estimates suggest that Neanderthal DNA makes up 1 to 4 percent of modern Eurasian genomes.

To learn more, scientists designed a computer model that estimated how much Neanderthal ancestry would be present in modern humans based on different levels of interbreeding, simulating potential interactions after our ancestors expanded into Neanderthal territory from Africa starting about 50,000 years ago. They next applied this model on the level of Neanderthal DNA seen in modern French and Chinese groups.

Based on this data and model, the researchers discovered the interbreeding success rate was probably less than 2 percent in most scenarios. Assuming that both lineages interacted for about 10,000 years, this means successful interbreeding would have, on average, happened just once every 23 to 50 years, they calculated.

"If more exchange had occurred, we would have become Neanderthals," researcher Laurent Excoffier, a population geneticist at the University Of Berne in Switzerland, told LiveScience.

One possibility is that interbreeding with Neanderthals was more common than such findings suggest, but that any resulting hybrid populations died off before they could leave a significant imprint on the modern human genome in general. However, even if this were true, the amount of Neanderthal DNA making its way into modern human genomes would appear more patchy and variable than it currently does, Excoffier said.

This new model also suggests that Europeans and Asians should have different components of Neanderthal ancestry, due to different ways these genes would have mixed after these populations split. They calculate that Asians and Europeans share about one-third of their Neanderthal DNA, but the rest should be unique to each continent.

"By being able to identify which Neanderthal fragments are found in different populations and trying to relate these fragments to different Neanderthal populations established in different parts of Eurasia, we would perhaps be able to infer where those admixture events occurred and thus precisely retrace past routes of migrations of our ancestors," Excoffier said.

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Future research could also investigate the newfound lineage of the Denisovans, which lived in what is now Siberia. Genetic analysis of their fossils suggests their DNA makes up 4 to 6 percent of modern Melanesian genomes. Recent research has also suggested that modern humans had sex regularly with a mysterious, now-extinct relative in Africa.

Excoffier and colleague Mathias Currat detailed their findings online Monday in the Proceedings of the National Academy of Sciences.

Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.

© 2012 LiveScience.com. All rights reserved.

Interactive: Before and after humans

Explainer: Ten fossils that tell the human tale

  • Frank Franklin II  /  AP file

    Where did we come from? Many truth seekers turn to faith and religion and find their answers therein. Others approach the question through a scientific lens and the theory of evolution. They have pieced together a tale of human origins from the fossils of our ancestors. The tale is incomplete and its telling reshaped with fresh interpretation of the growing fossil record.

    Click on the "Next" label to learn about ten fossil discoveries that have evolved the scientific rendering of human origins. In this image, a reconstructed Neanderthal skeleton is compared to a modern human.

    — John Roach, msnbc.com contributor

  • Toumai: Earliest-known ancestor of modern humans?

    MPFT

    Jaw fragments, isolated teeth and a skull excavated from the Sahel desert of Chad dated to between 6 million and 7 million years old may re-cast the opening chapter in the story of human origins. The fossils, revealed in 2001 and shown in this reconstruction, put the split in the evolutionary tree that eventually led to chimps on one branch and humans on the other more than 1,500 miles northwest of east Africa's Rift Valley, the current epicenter of research into human ancestors. But some scientists are not yet convinced the creature, named Sahelanthropus tchadensis and nicknamed Toumai, walked upright, which many scientists consider a key characteristic that distinguishes hominids from non-human primates.

  • Thigh bone suggests earliest two-legged walker

    Courtesy John Gurche, Brian Richmond via Science

    Analysis of a thigh bone amongst a clutch of fossils discovered in Kenya in 2000 and dated to nearly 6 million years ago may provide the earliest definitive evidence of a human ancestor that walked on two legs. Several detailed analyses of the femur, or thigh bone, shown here, have revealed it was adapted for upright walking. The bone belongs to a species known as Orrorin tugenensis. Most recently, U.S. scientists concluded the strategy first exhibited by this species for walking upright persisted for 4 million years, the majority of evolutionary history.

  • Middle Awash discovery fills gap in evolution story

    Tim D. White / Brill Atlanta

    Teeth and bones of the hand, foot, and thigh, shown here, are among the fossils of a 4.2 million-year-old Australopithecus anamensis specimen found in Ethiopia's Middle Awash region that has allowed scientists to link together their most complete chain of human evolution to date. The discovery helped fill a gap in the story, showing a likely transition between an earlier human ancestor known as Ardipithecus ramidus to the more recent australopithecines. The Middle Awash has yielded eight species in the story spanning 6 million years.

  • Lucy, the world's most famous fossil

    Dave Einsel  /  Getty Images

    Lucy, a 3.2 million-year-old Australopithecus afarensis named after the Beatles song "Lucy in the Sky with Diamonds," is perhaps the world's most famous fossil. She was discovered in the Afar region of Ethiopia in 1974 and remains among the most complete skeletons of an erect-walking human ancestor ever found, with about 40 percent of her bones intact. Her discovery allowed scientists for the first time to determine that upright walking predated the big brains of modern humans. Lucy's brain case is about the size of a chimp. In this photo, visitors view the Lucy skeleton at a Houston museum.

  • Taung child hailed as 'missing link' in 1924

    Denis Farrell  /  AP

    The diminutive fossil skull of 3.5-year-old early human ancestor, known as Taung child, was hailed as the "missing link" between apes and humans when it was discovered in 1924. Known scientifically as Australopithecus africanus, the discovery of the 2 million-year-old child also provided the first evidence that early humans evolved in Africa, rather than Europe, as many scientists believed at the time. In this photo, a researcher holds a replica of the skull as he makes the case that an eagle killed the Taung child.

  • Turkana boy, most complete skeleton found

    Sayyid Azim  /  AP

    Turkana boy, a nearly complete 1.6 million-year-old fossil of what some scientists call Homo ergaster, an early African population of Homo erectus, is considered the most complete skeleton of a prehistoric human ever found. The boy, who was discovered in 1984 in Kenya's Turkana region, stood 5-foot-3, indicating that hominids had gotten considerably taller and lankier since the days of Lucy, 3.2 million years ago. Plans to unveil Turkana boy at the National Museum of Kenya, shown here, in 2007 caused a stir between creationists and scientists.

  • Fossil discovery splinters human family tree

    Image: Casa Rinconada
    National Museums Of Kenya / F. Spoor  /  National Museum of Kenya via AP

    Many cartoons of evolution show a humpbacked ape slowly, linearly, progressing to a tall and erect modern human. Scientists long ago concluded that was too simple of a view, preferring instead to use a branching, thorny and knotted tree to depict the process. A discovery announced in 2007 threw yet another splinter in the picture. Many scientists had believed Homo habilis gave rise to Homo erectus who gave rise to modern humans. But the new finding shows habilis and erectus lived side by side for half a million years, raising doubt that habilis is a direct human ancestor. The scientists also found that erectus exhibited large size variation within the species, as shown in this image comparing two erectus skulls.

  • Neanderthals' relationship to modern humans fuzzy

    Image: Chankillo
    Image courtesy of National Academy of Sciences, PNAS

    The 1856 discovery of a skull cap and partial skeleton from a cave in Germany's Neander valley was the first recognized fossil human form. But exactly how the species, named in 1864 as Homo neanderthalensis, is related to modern humans remains the subject of fierce academic debate. Neanderthals occupied Europe and Asia from about 200,000 years to 30,000 years ago, overlapping in places with modern humans. Recent genetic analyses suggest little, if any, interbreeding between the species. Skeletal evidence, however, suggests Neanderthals were not very different than their modern human cousins. Even their brains were comparable to, if not bigger, than ours, as depicted in this Neanderthal reconstruction. Other studies have shown that like modern humans, Neanderthals used tools, wore jewelry, hunted, and buried their dead.

  • Hobbit discovery stuns the world, stirs debate

    Richard Lewis  /  AP

    As modern humans spread around the world over the past 160,000 years or so, a hobbit-like ancestor was holed up on the Indonesian island of Flores until at least 12,000 years ago, scientists announced at a press briefing in 2004, shown here. The stunning find has been scrutinized ever since. Some scientists agree the fossils represent a new species, Homo floresiensis. Others suggest the fossils belong to a diminutive race of modern humans, perhaps afflicted by one of several diseases associated with dwarfing.

  • Oldest modern humans found in Ethiopia

    Courtesy of Michael Day  /  AP

    The two partial skulls shown here of modern humans, Homo sapiens, were unearthed in Ethiopia in 1967. At the time, they were given a preliminary date of 130,000 years old. A 2005 revision using more modern dating techniques found them to be about 195,000 years old, making them the oldest known fossils of modern humans. Genetic evidence suggests modern humans arose in Africa about 200,000 years ago and then spread around the world, though other scientists hypothesize modern humans arose in parallel in Africa, Europe, and Asia.

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