Interactions between early modern humans and Neanderthals were a lot more common than we thought

The Neanderthal DNA in East Asians today can be traced back to interactions between Neanderthals and early modern humans in Europe 45,000 years ago.

The oldest known remains of modern humans in Europe have been identified in the Bacho Kiro Cave in Bulgaria, according to new research published last week in the journal Nature.

These remains, which belong to five individuals, were radiocarbon dated between 42,580 and 45,930 years ago. Neanderthals died out about 40,000 years ago.

By analyzing the ancient genomes from the Bacho Kiro remains, researchers were able to determine their relation to humans today, as well as the complexity of populations as humans migrated from Africa to Eurasia thousands of years ago.

Human ancestors

The three oldest of the individuals whose remains were found in the cave are most closely connected to current populations in East and Central Asia as well as the Americas. Their genomes revealed between 3% and 3.8% of Neanderthal DNA. In decoding the genetic material researchers concluded that the individuals had Neanderthal ancestors five to seven generations back across their family histories.

“These individuals represent an early expansion of modern humans into Europe that was previously unknown in the genetic record, with implications for the broader out-of-Africa expansion,” said Mateja Hajdinjak, study author and Marie Skłodowska-Curie Individual Fellow within the Ancient Genomics Laboratory at The Francis Crick Institute in England.

“Crucially, the genomes of Bacho Kiro Cave individuals point out that the early human groups in Europe commonly mixed with Neanderthals.”

The researchers believe that mixing between modern humans and Neanderthals was much more common than they had previously thought, especially when the first modern humans arrived on the scene in Europe.

“They may even have become absorbed into resident Neandertal populations. Only later on did larger modern human groups arrive and replace the Neandertals,” said Svante Pääbo, study coauthor and Director for the Max Planck Institute for Evolutionary Anthropology in Germany, in a statement.

Remains of four of these individuals were found together with stone tools linked to the Initial Upper Paleolithic Era, or the earliest known stone tool culture that is associated with modern humans in Eurasia. The Upper Paleolithic began about 40,000 years ago.

These stone tools, along with personal ornaments made from animal teeth recovered from the cave, are similar to those found across Eurasia.

A fifth individual found in the cave, dated to about 35,000 years ago, was found to belong to a group that’s genetically distinct from the others and belonged to a different population.

“This shifted our previous understanding of early human migrations into Europe in a way that it showed how even the earliest history of modern humans in Europe may have been tumultuous and involved population replacements,” Hajdinjak said.

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The Bacho Kiro Cave was first excavated by archaeologist Dorothy Garrod in 1938 with later excavations occurring in the 1970s. Researchers revisited and excavated the cave in 2015, investigating a layer that was rich with thousands of animal bones, tools made from stone and bones, pendants, beads, and the fragmented remains from five Homo sapiens.

Hajdinjak said the next steps would be accessing genomic data from other sites that are associated with the Initial Upper Paleolithic across Eurasia.

“The emerging picture of early human groups shortly after out-of-Africa migration is a complex one, and it would be amazing to see if this archaeological culture found across vast geographical area (spanning from East Europe to Mongolia) is made by different human groups, how they relate to each other, and importantly how they relate to resident archaic groups,” she said via email.

A different population

More research released last week shed light on another population of early modern humans.

A skull recovered in the Czech Republic belonged to one of the earliest modern human populations in Eurasia that stemmed from migrations out of Africa.

The skull was found at the Zlatý kůň site and belonged to a woman who carried 3% Neanderthal ancestry. Her group in Eurasia did not contribute genetically to later populations in Europe or Asia. This is likely the oldest reconstructed modern human genome to date.

Efforts to secure radiocarbon dating of the skull failed due to collagen contamination from cattle, likely animal glue used to preserve parts of the skull in the past century. However, Neanderthal segments within the genome suggest that the skull may have be older than 45,000 years.

Parts of the skull were first recovered in 1950. Researchers initially thought it was only 15,000 years old.

“We have found out that the Zlatý kůň woman belonged to group of modern humans that were among the first to settle in Europe after modern humans left Africa more than 50,000 years ago,” said Kay Prüfer, co-lead author of a study that published last week in the journal Nature Ecology & Evolution. Prüfer is also the group leader at the Max Planck Institute for Evolutionary Anthropology.

The Zlatý kůň woman belonged to a separate lineage of non-Africans that went extinct later on, Prüfer said. She represents a population that formed in the dawn before ancestors of those in modern-day Europe and Asia split apart.

Previous research has suggested that modern humans had already migrated to southeastern Europe between 43,000 and 47,000 years ago, but the fossil record has been lacking support for this.

This finding, together with the Bacho Kiro Cave individuals, suggests an intriguing theory.

“It is possible that Europe was settled by two different groups of modern humans in the time before 45,000 years,” Prüfer said. “The Bacho Kiro and Zlatý kůň finds tell us different pieces of the story of how Europe was first settled, but we do not know how the groups represented by these individuals were related. That is a clear next question for us to answer.”