A new study proposes that most interbreeding between human species paired Neanderthal men and early modern human women.
In 2010, Swedish paleogeneticist and Nobel laureate Svante Pääbo and his colleagues determined that tens of thousands of years ago Neanderthals and early modern humans interbred. They found that the DNA of Neanderthals left a genetic legacy in living humans. In a recent paper in the journal Science, researchers in the lab of Sarah Tishkoff, the David and Lyn Silfen University Professor in Genetics and Biology, have produced compelling evidence that the interbreeding was strongly sex biased. They show that men with a lot of Neanderthal ancestry and women with a lot of modern human ancestry had a penchant for mating with each other.
Genetic evidence indicates that the anatomically modern women who fancied or succumbed to Neanderthal men (considered “archaic” in the sense that their body shape did not fall within modern-day variation) came out of Africa in at least two main waves about 250,000 and 50,000 years ago. The Neanderthals descended from a population that had left Africa much earlier and had established populations in western Eurasia by around 400,000 years ago. The Penn geneticists can’t tell if the attraction between genders in the two groups was mutual or the mating was coerced, but they submit that the coupling took place following the two major migrations, the most recent episode some 47,000 years ago, a few thousand years before Neanderthals went extinct in Europe.
The mystery confronting Tishkoff and senior research scientist Alexander Platt C’99, an evolutionary geneticist in her lab and the co-lead author of the study, was that over generations only tiny portions of Neanderthal DNA survived in modern humans and even less is found in the X chromosome (one of the two that help determine the sex of an embryo) than others. The Penn scientists wondered why.
A conventional explanation held that so-called “Neanderthal deserts” arose because certain Neanderthal alleles were deleterious when paired with modern human genes, leading to their disappearance through the mechanism of natural selection. Maybe hybrid children who inherited modern human genes on their X chromosomes didn’t survive long enough to pass them on. But maybe some aspect of the original interbreeding could help explain the barren chromosomal stretches. Since males contribute half as many X chromosomes to future generations as females do, if most of the interbreeding between modern humans and Neanderthals involved female modern humans and male Neanderthals, we would have started with fewer Neanderthal X chromosomes in the first place.
To test whether natural selection was the primary factor accounting for the deserts or ancient mating patterns were a more probable cause, the Tishkoff team, which also included Daniel Harris, a research associate in the Perelman School of Medicine, devised an experiment to examine “the other half of the picture,” according to Platt. “We don’t have to look only in our own gene pool,” he says. “By looking at the other side of these interactions, at Neanderthals, we hoped to discover more.”
They examined DNA recovered by Pääbo from the fossilized toe bone of a Neanderthal female who lived in the Altai mountains of Siberia some 122,000 years ago. To their surprise, they found about 1.6 times more modern human DNA in her X chromosomes than in her other chromosomes. Similar excesses showed up in the X chromosomes from female Neanderthal genomes reconstructed from bones recovered from two other sites, dated to 80,000 and 52,000 years ago, respectively. The researchers compared those data to genetic material from a reference panel of women living in sub-Saharan Africa today—who have little or no Neanderthal ancestry because they are the descendants of anatomically modern humans who remained in regions south of the Sahara after the exodus of the small group who encountered Neanderthals in the Near East some 47 millennia ago.
If mixing anatomically modern human and Neanderthal DNA was harmful, the researchers reasoned, the sequence of information encoded in the Neanderthal genome would show large gaps devoid of modern human DNA similar to the relative lack of Neanderthal DNA on the X chromosomes of contemporaneous African women in the reference panel. The mirror-image genetic imbalance they discovered—the expected faint traces of Neanderthal DNA on the X chromosomes, compared to the other chromosomes, of the women in the reference panel and the significant amounts of modern human DNA on the X chromosomes of the Neanderthals—led them to their mating game hypothesis: the powerful attraction of men in Neanderthal societies to women with a modern human parent or grandparent—and/or vice versa. As a result of these sexual liaisons, Platt says, “the women would have passed on their X chromosomes to Neanderthal populations. The subsequent abundance of modern human DNA in the Neanderthal genomes we studied belies natural selection as the most probable cause for missing swaths of Neanderthal DNA in the X chromosomes of humans today. The simplest explanation that fits all the available evidence is sex bias in mating.”
To further test their hypothesis, the Tishkoff team modeled different demographic processes, but those alternatives “required more complex, shifting scenarios across time and geography and fall short of accounting for the amounts of modern human DNA actually found in the Neanderthal bones,” Platt says. “They also are insufficient to explain the lack of Neanderthal X chromosomes in the modern human gene pool without male Neanderthals relocating into anatomically modern human populations. But Neanderthal men preferring anatomically modern women gets you there.” He is quick to say, however, it “does not rule out more complicated scenarios combining natural selection and sex biases, as differential migration and male preferences may have been at play simultaneously.” As Tishkoff points out, her team’s findings “can’t rule out complex demographic processes that would involve female modern humans migrating to the location of Neanderthal males.”
What is clear is that the pattern postulated by the Penn scientists remained consistent in mating episodes separated by 200,000 years. Their findings show that studying ancient DNA can illuminate ancient social behavior and help us understand our ancestry. Tishkoff sees the work she and her colleagues have undertaken as a “testament to the power of integrating molecular genetics with anthropology to shed light on an early chapter in human history.”
—Mary Ann Meyers Gr’76
