Sometime around 45,000 years ago Homo sapiens established themselves in Europe. We know very little about this first wave of immigrants, but two studies on some of the oldest modern human DNA ever obtained have revealed a few clues.
“Modern humans” in this context is used to indicate people who were related to us and to distinguish them from Neanderthals, who had been in Europe for 200,000 years already. Nevertheless, that distinction is a somewhat fuzzy line. As we know, people living today, unless their ancestry is exclusively African, have inherited some Neanderthal DNA, sometimes to their cost.
Such intermingling had already happened by the time three people who represent our oldest modern European fossils lived in Bacho Kiro Cave, Bulgaria. The three individuals have been dated to between 46,000 and 42,500 years ago. All three had 3-3.8 percent Neanderthal DNA, a new paper in Nature reports.
The way another population’s DNA is distributed through the genome can tell us most than the raw percentage. Dr Matjea Hajdinjak of the Max Planck Institute for Evolutionary Anthropology and co-authors conclude the location of Neanderthal genes, in this case, indicate the cave inhabitants' Neanderthal ancestors were quite recent, just five or six generations back for one individual, seven for the other two.
Previous evidence of Neanderthal genes entering the modern human line is from 50-60,000 years ago. This discovery and comparisons with DNA from other early humans, makes the authors think Neanderthal/modern human mating may have been more common than we realized. Our inheritance from them may represent many encounters across the divide, rather than a single Neanderthal ancestor whose genes proved particularly enduring.
Although the inhabitants of Bacho Kiro are the oldest modern human Europeans of which we have evidence, their ancestors may not have stuck around. Hajdinjak reports their closest surviving relatives are not in Bulgaria or western Europe, but in East and Central Asia, and even the Americas. It seems at some point the Bacho Kiro descendants migrated east, and some kept on moving until they crossed the Pacific.
A related paper in Nature Ecology and Evolution explores DNA extracted from the skull of a modern human in Zlatý kůň, Czechia. It's possible this individual preceded the Bacho Kiro three but radiocarbon dating was unsuccessful as a result of contamination, leaving us less certain of her age.
Like the others, the Zlatý kůň woman was 3 percent Neanderthal, genetically speaking. However, it seems she, and the population she belonged to, died out without contributing to the surviving gene pool in either Europe or Asia. That isn't all that unusual; fossils have been found of modern humans of similar age in Romania and Siberia whose branch of the human tree left no detectable genetic trace today. Unsurprisingly, perhaps, Europe in the Ice Age was a difficult place to establish oneself, with some early-arriving populations dying out, while others migrated east.