An international research team has find first genetic evidence of modern human DNA in a Neanderthal individual. The discovery –published in Nature– confirms the interbred between Homo sapiens and Homo neanderthalensis and suggests that it happened 100.000 years ago, before the 65.000 years ago previously documented. This earlier genetic exchange, which may have taken place in the Near East, has not been detected in European Neanderthals. The study, led by the Max Planck German Institute, counts among its authors with researchers of CNAG-CRG located at the Barcelona Science Park (PCB).  

Consistent with previous studies, modern humans appeared 200,000 years ago in Africa and spread through Eurasia around 65,000 years ago. Thanks to a unidirectional signal of Neanderthal DNA found in the genomes of humans outside Africa, it was also known that Neanderthals and modern humans had interbred by that time. The present study, however, brings to light the first evidence of the genetic contribution by modern humans to Neanderthals, a process that probably occurred during a previous migration.

Researchers suggest that some modern humans left Africa earlier than what we thought and mixed with Neanderthals. These modern humans probably became extinct and are therefore not among the ancestors of present-day humans.

The evidence of gene flow from modern humans into the Neanderthal genome are based on the analysis of one specific Neanderthal, whose remains were found in the Denisova Cave, located in the Altai Mountains in southern Siberia, near the border between Russia and Mongolia. Two Neanderthals from European caves that were sequenced for this study — one from Croatia, another from Spain — both lack DNA derived from modern humans. The team also analysed the genome of another extinct human, a Denisovan, whose remains (a finger and a tooth) were found in the same cave in the Altai Mountains as the Neanderthal bone. Unlike the Neanderthal individual, the Denisovan individual did not carry any modern human DNA. The researchers do not conclude that modern humans never mated with Denisovans or European Neanderthals, but that the signal they found in the Altai Neanderthal probably comes from an interbreeding event that occurred after a linage with other modern humans from Europe little over 100,000 years ago.

The scientific team responsible for these findings consists of more than 20 researchers belonging to several European research centres including the Max Plank Institute for Evolutionary Anthropology (Germany) – leading the study –, the CSHL’s Simons Center for Quantitative Biology (USA), the Herzliya Interdisciplinary Center (Israel), Cornell University (USA), the Institute of Evolutionary Biology (IBE, CSIC-UPF), the Centre Nacional d’Anàlisi Genòmica (CNAG-CRG) and the Catalan Institution for Research and Advanced Studies (ICREA).

The modern human DNA sequences in the Altai Neanderthal appear to derive from a modern human group that separated early from other humans. The modern human who contributed genes to this particular Neanderthal individual must have come from a population that left Africa long before the migration of the ancestors of present-day Europeans and Asians from Africa less than 65,000 years ago, the scientists say. Thus, there must have been a long lag between when this group branched off the modern human family tree, roughly 200,000 years ago, and when they left their genetic mark in the Altai Neanderthal, about 100,000 years ago, before themselves being lost to extinction.
 

• Reference article: Kuhlwilm M, Gronau I, Hubisz MJ, de Filippo C, Prado-Martinez J, Kircher M, Fu Q, Burbano HA, Lalueza-Fox C, de la Rasilla M, Rosas A, Rudan P, Brajkovic D, Kucan Ž, Gušic I, Marques-Bonet T, Andrés AM, Viola B, Pääbo S, Meyer M, Siepel A, Castellano S. Ancient gene flow from early modern humans into Eastern Neanderthals. Nature. 2016 Feb 17. (doi: 10.1038/nature16544)