How much contact did humans have with other human species? Research in the last few years shows that there was a lot of contact between species, and there is evidence of it in our DNA.
The first humans (Homo sapiens) to reach Europe quickly “befriended” the Neanderthals they met there. A series of recent studies have not only confirmed that Homo sapiens and Neanderthals reproduced regularly, but also that they did so more often than previously assumed. The evidence could be useful for understanding how humans migrated out of Africa.
Neanderthals are thought to have disappeared around 40,000 years ago, but it is still unclear exactly how much contact this extinct archaic species had with Homo sapiens individuals spreading out of Africa. Basic questions about how they became extinct and how much genetic and cultural overlap there was between the two species remain unanswered.
One of the issues hindering our understanding is that radiocarbon dating methods are limited to remains more than 50,000 years old. However, recent research has helped fill in some of these gaps, showing that Homo sapiens and Neanderthals were quite “close”.
In April 2021, a research team discovered early human remains in Bulgaria’s Bacho Kiro Cave. The bones belonged to three individuals and contained a tooth and bone fragments that radiocarbon dating dated between 43,000 and 46,000 years old. The researchers also found Late Stone Age stone tools in the sediments surrounding the remains. According to the study, these remains constituted the earliest known modern humans in the Late Pleistocene in Europe. They were also more closely related to ancient populations in East Asia and the Americas than to later populations in Western Eurasia.
Genetic analysis revealed that the three humans had Neanderthal ancestors in their family history going back only a few generations. This “confirms that early European modern humans mixed with Neanderthals and suggests that such mixing may have been widespread,” the authors wrote.
In another study published in 2021, evolutionary geneticist Kay Prüfer of the Max Planck Institute for the Science of Human History in Jena, Germany, and colleagues showed that DNA from a female Homo sapiens skull dating to the same time period had Neanderthal ancestry.
Prufer and his team examined the nearly complete skull of a woman discovered in 1950 in the Zlatý kůň cave in what is now the Czech Republic. They found that about 2-3 percent of the woman’s genome contained Neanderthal DNA, but the Neanderthal segments were longer than those seen in other samples from humans. According to the researchers, this suggests that the woman was one of the earliest Homo sapiens in Eurasia after we left Africa. This is because later humans had shorter sections of Neanderthal DNA, effectively diluting it with each successive generation.
Taken together, these studies paint an interesting picture. The remains analyzed in both studies are probably the oldest, if not the only, specimens with Neanderthal DNA in their genomes. Given the evidence of earlier hybridization between humans and Neanderthals examined in both studies, this means that the two species could have done so as early as 60,000 years ago. But not all of these ancient humans influenced the DNA of subsequent generations.
The woman whose skull was analyzed represents a genetic dead end, as she did not contribute any DNA to humans who lived around 40,000 years ago. In contrast, the DNA of three individuals from Bacho Kiro was part of a newly discovered population of Homo sapiens with strong genetic links to modern humans living in East Asia and Native America.
Until relatively recently, it was believed that modern humans in Africa would not have Neanderthal DNA in their genomes, because Neanderthal DNA evolved outside Africa. But another study published in 2020 shows that all African Homo sapiens have evidence of Neanderthal DNA. This, it is argued, reveals new insights into human migration patterns. Instead of a mass exodus from Africa, as previously hypothesized, humans appear to have left the continent in various stages, with gene flow going back and forth, taking Neanderthal DNA with them.
The results of these various studies refute the idea that anatomically modern humans displaced archaic human species such as Neanderthals and Denisovans without interbreeding with them. Although much of our genome still comes from Africa, ancient DNA transitions show that the species were much more “friendly” than previously thought.