For the majority of our 300,000-year history, we shared the world with several other species of humans, says Will Newton. And we didn’t just live alongside them, we bred with them too…
It’s a common misconception that we, anatomically modern humans (Homo sapiens), evolved relatively straightforwardly from a single common ancestor, be that Homo erectus, Homo heidelbergensis, or perhaps a so-far unknown species of hominin.
However, that is far from the case. In reality, our species is the product of millennia of interbreeding between many different groups of humans. You, reading this article today, are a genetic hotchpotch who owes your existence to the fact that many, many generations ago, a long-lost relative slept with a similar-looking but crucially genetically distinct human.
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In 2010, scientists published the first draft of the Neanderthal (Homo neanderthalensis) genome, based on the analysis of four billion base pairs of Neanderthal DNA. This study was groundbreaking as it discovered a mixture of genes that were shared by both Neanderthals and modern humans, suggesting the two species regularly interbred.
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Thanks to these kinds of genetic studies, we now know that modern humans of non-African descent carry roughly 2% Neanderthal DNA in their genomes. Those of African descent don’t typically display this genetic signal as their ancestors remained in Africa during this period of prehistory and likely never met Neanderthals and other human species based in Eurasia.
Shortly after the first draft of the Neanderthal genome was published, scientists sequenced the genome of a new, previously unknown group of hominins. This genome was sequenced from a tiny fragment of a finger bone discovered deep in the Altai Mountains of Siberia, Russia, at a site known as Denisova Cave.
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The results of these genetic analyses revealed a group we now call the Denisovans. Since that particular discovery, more Denisovan remains have been uncovered, including a jawbone, teeth, and skull fragments. There are also hominin remains found elsewhere in East Asia that, while previously unclear in terms of their affinities, are now starting to be assigned to the Denisovans.
Like we did with Neanderthals, we bred with Denisovans too.
“This was one of the most exciting discoveries in human evolution in the last decade,” said Trinity College Dublin’s Linda Ongaro, the lead author of a review paper published last year that studied the history of interbreeding events between Denisovans and modern humans.
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“Unlike Neanderthal remains, the Denisovan fossil record consists of only that finger bone, a jawbone, teeth, and skull fragments. But by leveraging the surviving Denisovan sediments in modern human genomes scientists have uncovered evidence of at least three past events whereby genes from distinct Denisovan populations made their way into the genetic signatures of modern humans.”
In this study, Ongaro and co-author Emilia Huerta-Sanchez also outlined evidence that suggests Denisovans, who ranged from Siberia to Southeast Asia and Oceania, were adapted to several different types of environments. Some of these genes are present in populations of humans living today and, like they did the Denisovans tens of thousands of years ago, give them certain advantages.
“Among these [mixture of advantageous genes] is a genetic locus that confers a tolerance to hypoxia, or low oxygen conditions, which makes a lot of sense as it is seen in Tibetan populations; multiple genes that confer heightened immunity; and one that impacts lipid metabolism, providing heat when stimulated by cold, which confers an advantage to Inuit populations in the Arctic,” said Ongaro.
Denisovans weren’t the only group that passed advantageous genes onto modern humans, so did Neanderthals. It’s thought Neanderthals passed on genes that are associated with skin and hair, perhaps giving our ancestors increased resilience against the cold in the cooler environments of northern Europe and the vast steppe that stretched across Siberia.
However, not all the genes we inherited from Denisovans, Neanderthals, and other groups of humans were beneficial. According to some studies, certain genes believed to have been inherited from Neanderthals are associated with conditions such as diabetes, lupus, and Crohn’s disease. We may also be able to blame our increased risk of nicotine addiction on our distant Neanderthal relatives.
The discovery of new remains and the continual improvement of genetic testing techniques are shedding more and more light on our ancient past, but there’s still so much we don’t know.
“There are numerous future directions for research that will help us tell a more complete story of how the Denisovans impacted modern day humans, including more detailed genetic analyses in understudied populations, which could reveal currently hidden traces of Denisovan ancestry,” said Ongaro. “Additionally, integrating more genetic data with archaeological information – if we can find more Denisovan fossils – would certainly fill in a few more gaps.”
At the time of writing, the Denisovans aren’t considered a distinct species; rather a population of humans that were genetically distinct from Neanderthals and modern humans. That said, our understanding of this ancient group is changing all the time - in June of this year, DNA recovered from the mysterious ‘Dragon Man’ skull uncovered in Harbin City, China, in 1933 revealed the 146,000-year-old fossil belonged to a Denisovan.
This was a monumental discovery and renewed calls for the Denisovans to be formally recognised as a distinct species.
However, there’s a possibility that, as we find more Denisovan remains and associated DNA, we discover they’re not so dissimilar to us and their close cousins Neanderthals. The fact that modern humans, Neanderthals, and Denisovans could all successfully interbreed with one another certainly suggests that we are very, very closely related, perhaps by some definitions the same species.
The hominin family tree is - to put it simply - a confusing mess; rather than resembling an evenly branched pine, it’s more like a chaotic knot of ivy. We’re beginning to untangle some of our tree’s branches, but it’ll be decades, maybe even centuries before we’re able to properly prune it and ‘solve’ the mystery that is our ancestry.
The study conducted by Trinity College Dublin’s Linda Ongaro and Emilia Huerta-Sanchez was published in the journal Nature Genetics. The quotes from Ongaro come from an associated press release published on Trinity College Dublin’s website.
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