Previous to the early 2010s, scientists were conflicted on the question of Homo sapiens interbreeding with Neanderthals, but since then, multiple studies have shown the incredible overlap between Neanderthal and human DNA - enough that approximately 1-4% of modern human DNA traces directly back to Neanderthal ancestry.
Neanderthals coexisted with modern humans for tens of thousands of years, and the two groups certainly met. Evidence shows that Homo sapiens not only procreated with Neanderthals - they also slew and ate them.
It's not surprising that prehistoric humans mated with Neanderthals since prehistoric people were notoriously promiscuous. But while science proves humans mated with Neanderthals, the evidence still leaves some questions open. When did they begin to interbreed? Was it consensual? And were there human-Neanderthal relationships?
There's not a huge difference between prehistoric Homo sapiens and Neanderthals, it turns out. While Neanderthals were slightly stockier and had thicker bones, the two were compatible in many ways. According to a study published in Science, on average, up to 4% of a person's genetics trace directly to the Neanderthals.
While the Neanderthals vanished 30,000 years ago, the study provides solid genetic evidence that prehistoric Homo sapiens procreated with Neanderthals.
Homo sapiens migrated from Africa in large numbers around 65,000 years ago. When they spread to the Middle East and Europe, Homo sapiens encountered Neanderthals. When the two hominids met, they also mated.
Recent scientific evidence shows Homo sapiens and Neanderthals interbred multiple times, including early mixings that may have occurred over 100,000 years ago after small groups of Homo sapiens left Africa. Interbreeding with likely Neanderthals shaped the DNA of subsequent generations.
Scientists recently solved a prehistoric DNA mystery: how did a Neanderthal living in southwest Germany 100,000 years ago have modern human mitochondrial DNA, or mtDNA? As recently as 2010, scholars believed that interbreeding between Homo sapiens and Neanderthals likely only stretched back 60,000 years ago, when Homo sapiens left African to slowly spread to the Middle East and Europe.
But research in 2017 shows that a female human in Africa mated with a Neanderthal male over 220,000 years ago. This coupling spread mtDNA to an entire lineage of Neanderthals, who may have met Homo sapiens in Europe tens of thousands of years later.
Was mating between humans and Neanderthals consensual? Scientific data provides scant evidence on that question. However, one clue would come from examining exactly who was doing the procreating. Was it Neanderthal males impregnating human females or vice versa?
Genetics researcher Adam Siepel says, "All we really know is that some offspring of humans and Neanderthals eventually got incorporated into human populations, because what we see is small fragments of genomes in human populations."
Author Robert Sawyer takes a slightly darker view on consent. "I wish I could paint a more romantic tale of candlelit dinners over mammoth steak, but it seems much more likely that the gene flow was mostly unintentional." Or, in short, the groups didn't mate intending to cause pregnancy.