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During most of the past 2 million years or so, there were several species of hominins on Earth at any given time. Like, you might be familiar with our famous extinct cousins, the Neanderthals. But since 2010 we’ve been uncovering evidence of another mysterious cousin that we lived alongside, the Denisovans!

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Our species, Homo sapiens, are the only humans around on Earth these days, but up until recently, that wasn't the case. You've probably heard of our famous extinct cousins, the Neanderthals, who lived in Europe and Asia.

But since 2010, incredible evidence has revealed that another mysterious cousin lived alongside early humans and Neanderthals: the Denisovans. ...or de-NEE-so-vins, depending on whether… I don't know. I've always said deh-nih-SOH-vins, but now I'm being told it's de-NEE-so-vins. And everything we know about them comes from a few small bits of bone and the genetic legacy they left behind from generations of interbreeding with our species.

Our species and our cousins are hominins, a group of animals which includes us and our ancestral relatives basically, anything more closely related to us than to chimpanzees. During most of the past 2 million years or so, there were several species of hominins on Earth at any given time. And in the past few hundred thousand years, before our species outlasted, or outcompeted, or just killed all the rest, our ancestors lived alongside Neanderthals and at least one other species of human.

The first evidence of these extinct hominins came from a single finger bone discovered in Denisova Cave in Siberia. From the shape of the bone, researchers could tell it was some sort of hominin, but that bone by itself wasn't enough to identify what species it was. So, they sequenced the DNA from the bone, hoping to determine if it came from a Neanderthal or a member of our species.

But the DNA found was strikingly different from both of those. It belonged to a whole other branch of our family tree. Researchers named the newly-discovered hominins Denisovans after the cave where that revealing pinky bone was found.

And since then, only a few scattered fossils of them have been found… and almost all have come from that same cave. There were three molars, for example, which were identified as Denisovan from their DNA. There was also a small fragment of a limb bone reported in 2015 that turned out to be from an individual with a Neanderthal mom and a Denisovan dad!

And in 2016, scientists discovered a fragment of braincase from Denisova Cave. In fact, the only Denisovan fossil we've found to date anywhere else is a partial jaw. It came from the Tibetan Plateau, around 1,500 miles south of cave!

So it seems these cousins of ours were pretty widespread across Asia. And yet, that's all the physical evidence we've got that they existed. A few teeth and some bone fragments are all of the fossils known for Denisovans as of the summer of 2019.

If it weren't for DNA analysis, there's no guarantee we would have ever realized they were different! And with so few remains, paleoanthropologists can't actually properly describe them as a new species just yet. In order to do that, they would need enough bones to compare and contrast with other species, to clearly list what features make them distinct.

This lack of fossils also makes it difficult to understand what Denisovans were really like. We know they inhabited Denisova Cave for a long time, since the fossils found there range from 300,000 to 50,000 years old. And there are also tools and artifacts known from those same deposits, but it's impossible to say so far whether those were the work of Denisovans or Neanderthals, whose remains have also been found in the cave sediments.

Some of those tools might even have been the work of our own species, although so far, no definite Homo sapiens fossils have been recovered from the cave. With only DNA to go on, it's tough to answer these questions about Denisovan lifestyle, culture, or appearance. But that DNA alone can tell us quite a bit—and not just about Denisovans.

If you've ever had your own DNA tested, you may have learned that you have a bit of Neanderthal in you the result of a long history of our ancestors interbreeding with Neanderthals. Well, you may have some Denisovan DNA in there as well! You see, the DNA we've sequenced from these bones has revealed a history of gene-swapping between Denisovans, Neanderthals, and our own ancestors.

Denisovan DNA is especially common in the modern human populations of. Southeast Asia and Melanesia, making up as much as 4% of their genome. In fact, a study in 2019 more closely examined the Denisovan genes found in the people of Papua New Guinea and Southeast Asia, and found evidence of three separate lineages of Denisovans.

That means Denisovans were much more diverse than scientists originally thought. There might even have been multiple species of them! Those same analyses also suggested some interbreeding wasn't all that long ago, as recent as 15 to 30,000 years ago.

If true, that would mean Denisovans survived at least as long as Neanderthals, and only disappeared right before the end of the Ice Age and the rise of modern civilization. And even though they're long gone, they still help us out today in unexpected ways. For example, there are specific genetic mutations found in modern Tibetan people that help them regulate hemoglobin levels in their blood so they can survive better at high altitudes.

These mutations aren't seen in most other living humans, but they are found in Denisovan genomes. Researchers think that's where these genetic traits first evolved, perhaps for their own high-altitude lifestyles, and then our species picked them up through interbreeding. Similarly, there are gene variants found in Inuit populations that promote the development of heat-storing brown fat, which comes in handy in cold climates.

Those genes, too, bear a striking resemblance to Denisovan genes, and may also have passed by them to us. Other genetic variants relating to things like our immune systems and skin color have also been identified as possible Denisovan genes. And since these traits ultimately became common in our species, researchers suspect they may have helped our ancestors defend against diseases or withstand new environments as they spread through Europe and Asia.

But until we find more fossils, the exact extent of their influence on our genomes will remain unclear. Since 2010, there has been a whirlwind of new discoveries and revelations about Denisovans, but these relatives of ours still remain shrouded in mystery. Scientists are on the lookout for more fossils, especially ones that would help us nail down what they looked like and what their culture may have been like.

With enough of them, scientists might even be able to give Denisovans an official species name. And more genetic studies will help us further sort out how our species benefited from consorting with other human species. Because one thing is increasingly clear: we definitely did not come to be the way we are today all by ourselves.

Thanks for watching this episode of SciShow! If you liked learning about Denisovans and their enduring genetic legacy, you might like our episode on what Neanderthal DNA is doing to our genomes. Ancient humans, amirite?

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There? Is it there?! [♩OUTRO].