New findings from an international team of researchers show that most Neanderthals in Europe died off around 50,000 years ago. The previously held view of a Europe populated by a stable Neanderthal population for hundreds of thousands of years up until modern humans arrived must therefore be revised.
The results indicate that most Neanderthals in Europe died off as early as 50,000 years ago. After that, a small group of Neanderthals re-colonised central and western Europe, where they survived for another 10,000 years before modern humans entered the area. Tribes of Homo sapiens sapiens emerged from Africa, passed through the Mideast and thence to Europe and Asia.
"The fact that Neanderthals in Europe were nearly extinct, but then recovered, and that all this took place long before they came into contact with modern humans came as a complete surprise to us. This indicates that the Neanderthals may have been more sensitive to the dramatic climate changes that took place in the last Ice Age than was previously thought", said Professor Love Dalen of the Swedish Museum of Natural History in Stockholm.
Working with DNA samples taken from the remains of Neanderthals found in northern Spain, researchers noted that the genetic variation among European Neanderthals was extremely limited during the last ten thousand years before their disappearance. Older European Neanderthal fossils, as well as fossils from Asia, had much greater genetic variation, on par with the amount of variation that might be expected from a species that had been abundant in an area for a long period of time. "The amount of genetic variation in geologically older Neanderthals as well as in Asian Neanderthals was just as great as in modern humans as a species, whereas the variation among later European Neanderthals was not even as high as that of modern humans in Iceland", says Anders Gotherstrom--associate professor at Uppsala University.
The study used severely degraded DNA, which required advanced laboratory and computational methods. Scientists from several countries, including statisticians, experts on modern DNA sequencing, and paleo-anthropologists from Denmark, Spain and the US were consulted. Upon review, the international research team concluded that the available genetic data reveals an important and previously unknown part of Neanderthal history. "This type of interdisciplinary study is extremely valuable in advancing research about our evolutionary history. DNA from prehistoric people has led to a number of unexpected findings in recent years, and it will be really exciting to see what further discoveries are made in the coming years", says Juan Luis Arsuaga, professor of human paleontology at the Universidad Complutense of Madrid.
Human beings, that is to say Homo sapiens sapiens, are known to have interbred with Neanderthals in the ancient past, but only rarely was that sex successful in producing offspring, scientists now suggest. This may be due to a low number of pairings among the two related species, or that the couplings rarely produced viable offspring. Recent analyses of Neanderthal genes revealed that Neanderthal DNA makes up 1 to 4 percent of modern Eurasian genomes.
The issue of hybridization of Neanderthals and Homo sapiens is not a settled issue among anthropologists. In earlier studies, 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.
The researchers concluded that 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. Interbreeding with Neanderthals may have been more common than such findings suggest, but that any resulting hybrid offspring died off before they could leave a significant imprint on the modern human genome. 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.
Future research may examine the 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.
At least one anthropologist theorizes that ancient humans actually ate Neanderthals. Based on a single Neanderthal jawbone, which appears to have hack marks consistent with an ancient method of flensing or removal of tissue from bones, it is believed that Neanderthal may have fallen victim to cannibalistic humans. The verdict is still out.