Gene flow between Neanderthals and early modern humans may have been a one-way street, researchers have found.

While the presence of Neanderthal DNA in modern human genomes is well attested, comparatively little is known about variations among Neanderthal genomes themselves. This remains the case, even with so-called “late” Neanderthals: individuals known to have been alive after the time modern humans moved into their territory.

Now, a team led by geneticist Mateja Hajdinjak of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, has succeeded in compiling the genomes of five Neanderthals from the late period, all of whom lived between 39,000 and 47,000 years ago.

Four of the specimens were born after the best-established date for the arrival of Homo sapiens, but, the researchers find, “we do not detect any recent gene flow from early modern humans in their ancestry”.

Previous research has shown that the majority of gene flow from Neanderthals into humans originated from Neanderthal populations that diverged from a group that lived in the Altai mountains of the Caucasus about 150,000 years ago.{%recommended 1362%}

The Neanderthal genomes sequenced by Hajdinjak and colleagues descend from the same Altai lineage, and the absence of human-derived DNA came as a surprise.

“Interbreeding between Neanderthals and early modern humans is likely to have occurred intermittently, presumably resulting in gene flow in both directions,” they write.

The absence of human genetic material was thus unexpected. However, the researchers point out that five genomes comprise only a small sample and thus human-to-Neanderthal gene flow cannot be excluded at this stage.

It is possible, too, they say, that despite interbreeding, human DNA simply wasn’t transferred into the Neanderthal’s baggage.

“This may indicate that gene flow affected the ancestry of modern human populations more than it did Neanderthals,” they write, and urge further research to resolve the issue.

The study is published in the journal Nature.