An international research team, including scientists from the Centro Nacional de Investigación sobre la Evolución Humana (CENIEH), has sequenced the genomes of the most ancient modern humans in Europe, who occupied the Bacho Kiro Cave (Bulgaria) about 45,000 years ago, and has discovered that this primitive human group contributed its genes to present-day East Asians.

Long stretches of Homo neanderthalensis DNA were also identified in the genomes of the inhabitants of this Bulgarian cave, showing that they had Neandertal ancestors five to seven generations further back. As the authors of this study, which is published today in Nature, explain: “This suggests that when the first modern humans arrived in Europe, mixing with Neandertals was the norm, not the exception”.

This investigation started when a team led by scientists from the National Institute of Archaeology with Museum at the Bulgarian Academy of Sciences and the Max Planck Institute for Evolutionary Anthropology in Germany discovered remains of modern humans in direct association with stone tools from the Initial Upper Paleolithic at the Bacho Kiro Cave site. Direct radiocarbon dating of the oldest remains found in the cave yielded an age of between 43,000 and 46,000 years. Therefore, this is the earliest known dispersion of modern humans in the mid-latitudes of Eurasia.

Neandertal DNA

The first inhabitants of Bacho Kiro lived at a time when Neandertals were still around. For this reason, the researchers scanned their genomes in search of Neandertal DNA fragments. ”We found that the individuals from the Bacho Kiro Cave had higher levels of Neandertal ancestry that almost any other primitive human, except for one individual from around 40,000 years ago found in the Pestera cu Oase Cave in Romania. The greater part of this Neandertal DNA appears in extremely long stretches. This shows that, about five to seven generations earlier, these individuals had Neandertal ancestors”, states Mateja Hajdinjak, lead author of this work.

Although only a few genomes of modern humans who lived in the same epoch as some of the last Neandertals in Eurasia have been recovered, almost all of them have recent Neandertal ancestry. “The results point to a habitual mixing between the first modern humans to reach Eurasia and Neandertals. It might even be that they were absorbed by the resident Neandertal populations. Only later on did larger groups of modern humans arrive and replace the Neandertals", explains Svante Pääbo, coordinator of the genetic investigation.

Genetic link with Asia

It was previously thought that humans from the Initial Upper Paleolithic had died out without contributing to the modern human genome. However, the researchers have now demonstrated that the most ancient individuals from the Bacho Kiro Cave, or the groups more closely related to those individuals, contributed their genes to modern humans. “Surprisingly, this contribution is found mainly in East Asia and in America, rather than in Europe, where the inhabitants of Bacho Kiro lived”, says Mateja Hajdinjak.

This genetic link with Asia mirrors the relationships observed between the stone tools and personal ornaments from the Initial Upper Paleolithic, found in this cave, and the tools and jewelry found all over Eurasia. And, most importantly of all, the most recent individual found (from about 35,000 years ago) belonged to a group genetically distinct from the earlier inhabitants of the cave. According to the authors, this shows that the first steps by modern humans in Europe were tumultuous and they were subjected to population replacements.

Pestera cu Oase Cave

Before this study, the first modern humans from the Romanian cave of Pestera cu Oase were the ones exhibiting the strongest degree of mixing between Homo sapiens and H. neanderthalensis.

“We now see that they were not the only ones, and the advances made in dating methods and genetic research have allowed us to learn the chronology and the dynamics of the first modern humans to a level of detail that would have been inconceivable a few years ago”, states Silviu Constantin, geochronologist at the CENIEH, who dated the Romanian specimens indirectly using the uranium series method (U-Th), and has directed the geological research at Pestera cu Oase, which was employed as comparative material for this study.

For her part, Oana Teodora Moldovan, an affiliated researcher at the CENIEH, was the first to recognize the value of the hybrid human specimens from this Romanian site, and carried out the initial excavations.