Around 40,000 years ago, the Neanderthal line died out. But for many millennia prior, these ancient humans migrated and mingled with other lineages, including the one that eventually begat our own.

Now, that picture is getting even more complex. In a study published today in the journal Science Advances, researchers unveil the unusual trajectories represented by two Neanderthal fossils that trace their ancestry as far back as 120,000 years ago.

Both of the bones, which are from different individuals, appear to display a lot of genetic similarity to fossils some 80,000 years younger, hinting that some Neanderthal lineages maintained remarkable continuity over tens of thousands of generations. Additionally, one of them appears to contain traces of another group of early humans in its DNA, hinting that this line once interbred with another, still unknown, population.

Neanderthals arose between 400,000 and 500,000 years ago, but much of what’s known of their story comes from more recent fossils. The new research represents “the first time we can look at Neanderthals in Europe across a long period of time,” study author Stéphane Peyrégne, an anthropologist at the Max Planck Institute for Evolutionary Anthropology in Germany, told Alison George at New Scientist.

In the study, a team of scientists led by Peyrégne and Kay Prüfer, also at the Max Planck Institute, extracted genetic material from two 120,000-year-old bones: a femur from Germany’s Hohlenstein-Stadel Cave and a jawbone from Belgium’s Scladina Cave. When the researchers compared the DNA within to other, more recent Neanderthal remains, they were surprised to see that both of the newly uncovered fossils bore a stronger genetic resemblance to 40,000-year-old European Neanderthals than to a (much older) Siberian Neanderthal that had been their contemporary.

The enduring genetics of this particular lineage suggest that a substantial migration took place around 120,000 years ago, with a group of European Neanderthals moving east to replace existing populations in Siberia. This points to the idea that Europe was something of a Neanderthal nexus, repeatedly spawning populations that then dispersed into other regions, Katerina Harvati, a paleoanthropologist at the University of Tübingen, Germany who was not involved in this study told New Scientist’s George.

For the femur, however, there was more to the story. An analysis of this fossil’s mitochondrial DNA—a kind of genetic information that’s inherited only from the mother—revealed that the Hohlenstein-Stadel individual’s maternal lineage was unlike any known Neanderthal genome. The results build on the findings of 2017 study from another group of researchers who, in their own analysis of the Hohlenstein-Stadel specimen, proposed that Neanderthals might have mated with our own ancestors on their way out of Africa more than 200,000 years ago.

That’s just one possibility. Alternative candidates for interbreeding exist, including what may be a long-gone Neanderthal population that researchers have yet to characterize. For this to be the case, a group of wayfaring Neanderthals would have needed to split off, ventured far from their brethren, and remained isolated long enough to accumulate genetic differences. Bouts of freezing in Europe might have been enough to motivate such an exodus, Carles Lalueza-Fox, a paleogeneticist at the Institute of Evolutionary Biology in Spain who was not involved in the study, told Bruce Bower at Science News.

Many questions remain. In the meantime, it’s clear that the tapestry of human history will only get more intricate. As Qiaomei Fu of the Chinese Academy of Sciences in Beijing, who specializes in ancient DNA but was not involved in the study, told Maya Wei-Haas at National Geographic, “More and more, [we’re learning] that admixture in hominin history is quite complex, and may have occurred quite often.”

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