Over the last decade, researchers have analyzed an enormous amount of genetic information gathered from both modern humans and our distant, ancestral forebears. This research has reshaped our understanding of gene transfers between various groups of Homo sapiens, including now-extinct groups like the Neanderthals and Denisovans. Now, researchers have found something truly remarkable in the same Siberian cave that gave us our first glimpse into Denisovans in the first place — a female child who was half-Neanderthal, half-Denisovan, nicknamed Denisova 11.
Neanderthals are likely familiar to most people, but the Denisovans are a much more recent discovery. First discovered in 2010 in the Denisova Cave (hence the name) and known only from a small number of fossils, the Denisovan people lived some 41,000 years ago. Genetic analysis has shown that they shared a common ancestor with Neanderthals and that the last common ancestor between humans, Neanderthals, and Denisovans lived some 744,000 years ago. Both Neanderthal and Denisovan DNA are preserved in humanity’s genome, but the distribution is quite different. Neanderthal DNA is found in all populations of humans outside sub-Saharan Africa and makes up an estimated 1.5 – 2.1 percent of our own genome, with approximately 20 percent of the Neanderthal genome believed to be preserved across all humans.
Chris Stringers’ hypothesis of the family tree of genus Homo, published in Stringer, C. (2012). “What makes a modern human”. Nature 485 (7396): 33–35. doi:10.1038/485033a. Image by Wikipedia
Denisovan DNA, in contrast, is concentrated mainly in populations from Melanesia, with much smaller amounts of DNA found in both mainland Asians and Native American populations. We already knew there were significant overlap and interbreeding between Neanderthals and Denisovans — 17 percent of the Denisovan genome near Denisova Cave represents DNA donated by the local Neanderthal population. But there’s a far cry between finding evidence of interbreeding in the abstract and actually finding an individual who was a 50/50 cross between the two (Neanderthal mother, Denisovan father) is still an incredible feat.
The current known path of Denisovan migration. Image by Wikipedia
The research team extracted DNA on six separate occasions, just to make certain that their analysis wasn’t being thrown off by external contamination. One of the basic ground rules in paleontology is that you never actually see the first or last of anything. Because such a tiny fraction of material from Earth was preserved across hundreds of thousands of years, you can only nail things down to within certain date ranges. Finding an F1 50/50 cross between the two species was very lucky and implies that such crosses may have been relatively common at the time — particularly since there’s evidence that Denisova 11’s father had some distant Neanderthal ancestry in his own bloodline as well.
For decades, scientists have debated why Homo sapiens is the only extant species in the genus Homo. One common explanation is that we killed off our own competition, while another argument claims that interbreeding may have ultimately led to one population being absorbed into the other. We’ve never found a Neanderthal-human hybrid analogous to this Neanderthal-Denisovan hybrid, but finding Denisova 11 suggests that hybridization between different types of humans may not have been so rare.
“When you find a needle in a haystack, you have to start wondering if what you’re really looking at is a needlestack,” John Hawks, an anthropologist at the University of Wisconsin-Madison, told The Atlantic. “This genome shows that hybrids were nowhere near as rare as people have been assuming. They must have been really common.”
And they must have lived, collectively, long enough and well enough to reproduce. It’s the only reason we’re still finding fragments of our distant ancestors embedded in ourselves.
Feature image by Wikimedia Commons
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