When We Met Other Human Species

You and I belong to the only group of hominids that currently exists on the planet. We are the only twig left on our branch of the family tree. But we were not always alone. 100,000 years ago, Eurasia was home to

other

hominid

species

, some of which we know our ancestors met and had a good time with. We've known some of them for a long time, like the Neanderthals, whose fossils we've been unearthing since the 19th century. But some of them are more recent additions to the family tree, like the Denisovans, who on this channel we may have called Denis-ovans but have been informed that they are De-nis-o-vans.

The Denisovans were discovered almost by accident in 2008, and we know them only from a few fossil bones and the DNA of their living descendants. That surprising discovery has opened our eyes to the fact that our ancestors knew and even mated with

other

hominids. So now anthropologists are following the genetic traces of these ancient interbreeding events, traces that many of us carry with us today. Thanks to this research, we are beginning to better understand how and even where modern

human

s mated with other hominids, giving us a more complete picture of the history of our

species

. And we're starting to address some really interesting questions, like: What is our heritage from that time

when

we encountered other

human

species?

And why are we the only ones left today? As we get closer to answering those questions, we begin to see that perhaps part of our success as a species has to do with those other hominids we encounter on our travels around the planet. Neanderthals lived throughout Europe and from southwest to central Asia. We have found their fossils from Portugal and the United Kingdom in the west to the Altai Mountains of Siberia in the east and as far as Israel in the south. The oldest Neanderthal-type fossils come from a site in northern Spain called "Sima de los Huesos" - literally, the Well of Bones - dating back about 450,000 years, while the most recent ones come from a handful of sites throughout Western Europe.

That date dates back to about 40,000 years ago. Anatomically, Neanderthals were very similar to us, with some differences. They were relatively short and stocky, with robust limbs and large brains. They had strong eyebrows, large noses, and skulls that were more oval in shape than the round ones of Homo sapiens. And we know they weren't just dumb cavemen. They controlled fire, created spears and stone tools, made jewelry from eagle claws, and cared for wounded members of their groups. And our ancestors clearly recognized that they were like us, enough for us to interbreed with them! We know this because researchers sequenced the Neanderthal nuclear genome, originally in 2010, from bone fragments found in a cave in Croatia.

And by comparing that genome to that of modern humans from many different populations, we can discover how much Neanderthal DNA some of us still carry. Although original estimates were around 4%, more recent studies have suggested that living people of European and East Asian ancestry have between 1 and 2% Neanderthal DNA in their genes. Meanwhile, people native to sub-Saharan Africa do not have Neanderthal DNA, indicating that their ancestors never encountered Neanderthals. And as we find more fossils to sample, we'll be able to say that these interbreeding encounters occurred more than once. And our genomes may even shed light on

when

they happened.

For example, genetic material extracted from the left femur of a modern human male who lived in Siberia about 45,000 years ago has been found to contain Neanderthal DNA. And the researchers were able to measure the length of Neanderthal segments of their genome, compared to the same segments in living people. It turned out that his Neanderthal sections were longer than those of modern humans, suggesting that he was not many generations removed from his Neanderthal ancestor. In fact, researchers were able to estimate that this Siberian man was the product of interbreeding between Neanderthals and Homo sapiens that occurred between 50,000 and 60,000 years ago, just 10,000 years before he was born, give or take a few thousand years.

And this probably occurred when modern humans migrating out of Africa encountered Neanderthals in the Middle East. Likewise, a 40,000-year-old jawbone was found in Romania in 2002, providing some of the first evidence of the existence of modern humans in Europe. And it was discovered that it had some anatomical similarities with Neanderthals. When his genome was sequenced more than a decade later, it was discovered that the human had had a Neanderthal ancestor only four or six generations ago. Between 6 and 9% of his genome was Neanderthal! So, using fossils like these, researchers have been able to determine that Homo sapiens bred with Neanderthals several times in different locations.

But how did all this mixing change us? Well, sometimes not much. The Romanian jaw bone genome suggested that the population did not contribute much to the DNA of living modern humans. But sometimes these encounters had a great impact. For example, two genes that play important roles in our immune response appear to have passed from Neanderthals to people of Eurasian ancestry. One of these genes, known as STAT2, is part of our immune system's signaling response when we contract a viral infection. And we know that the Eurasian version of STAT2 came from Neanderthals, because it is not found in sub-Saharan Africans.

Furthermore, molecular clock studies have found that the Neanderthal version of this gene appeared in the Eurasian genome long after the evolutionary split between Homo sapiens and Neanderthals, so it must have arisen from interbreeding. Meanwhile, members of many East Asian populations have been found to carry a gene known as HYAL2, which is involved in repairing skin cells after the skin has been exposed to the sun's UVB rays; in other words, sunburn. And this gene also seems to come from Neanderthals, probably a useful adaptation for modern humans who expanded into Asia. Both genes are examples of a phenomenon known as adaptive introgression, when genetic material from one species passes into the gene pool of another species and is then selected to stay.

But these genetic contributions can also have a downside. Introgressed genes that were once beneficial may become less beneficial over time, as the environment in which natural selection occurs changes. For example, there is a gene involved in rapid blood clotting, which used to be really beneficial before medical care existed. But now it has been discovered that this gene increases the risk of blood clots. And this gene also seems to come from interbreeding with Neanderthals. But Neanderthals are not the only hominids we come to know so intimately. In 2010, paleogeneticists announced a shocking discovery: a site known as Denisova Cave in southern Siberia had yielded ancient mitochondrial DNA from a previously unknown hominid.

Previous work there had found evidence of modern humans and Neanderthals, so researchers expected that the DNA they extracted from a pinky bone found there would also be Neanderthal, but that was not the case. Almost a decade later, we have seven fossil bones with this unique genetic signature: there's the tip of a little finger, three molars, a sliver of long bone and a piece of skull cap from Denisova Cave, all dated between 52,000 and 195,000 years old. back. And there is also a single partial jawbone from the Tibetan Plateau, dating back to at least 160,000 years ago. This new group of hominids has not yet been given a scientific name, because it lacks a type specimen, a fossil that is complete enough to compare in future finds.

For now, they are informally known as the Denisovans. And they are essentially a phantom lineage within our own ancestry: a branch of the hominid family tree that lacks a fossil record. Now, it could be that there are other Denisovan fossils in collections around the world that simply haven't been identified. But since there is no type specimen and most fossils do not have DNA that can be extracted, we simply don't know for sure. But we have sequenced enough of the Denisovan genome to be able to tell part of their story. For example, their mitochondrial DNA, which is passed from mothers to children, suggests that the last common ancestor we share with the Denisovans lived about 1 million years ago.

But the Denisovans' nuclear DNA, which makes up the majority of their genome, is actually more similar to ours. So this could be a sign that Denisovans also interbred with other hominids within our lineage, such as Homo erectus. Today, we find small amounts of Denisovan DNA in East and South Asian populations, and up to 6% Denisovan DNA in some populations of Melanesians in the southwest Pacific. And the variations we see between the DNA of modern people and the ancient genomes we have from Denisova fossils suggest that there were interbreeding events with at least three different groups of Denisovans.

So it didn't happen just once or in one place. And some of these genetic contributions are really important. Take for example the gene known as EPAS1, which is found in many people native to the Tibetan Plateau. This gene is associated with differences in hemoglobin concentrations. And at high altitudes, more hemoglobin means more efficient oxygen transport. This gene appears to have been introduced by the Denisovans and was strongly selected for because of the advantages it offered to modern humans living at high altitudes. So, we know that many of the hominids that we used to live and coexist with were very well adapted to many environments.

So why are there no populations of these other hominids today? Well, anthropologists have been thinking about that for a long time, especially when it comes to Neanderthals. The oldest explanations for its disappearance have been that climate change, competition from modern humans, or some combination of the two caused its downfall. And there is some evidence that there were cycles of intense cold and dryness in Europe between 44,000 and 40,000 years ago, which could have caused Neanderthal populations to decline, leaving them vulnerable to extinction. Other researchers have modeled the distribution of Neanderthals and their habitats, and suggest that those habitats were becoming more fragmented by changes in climate.

As for the impact of modern humans, we know we encountered Neanderthals, but there is no evidence of violence or direct competition between the two groups. So some researchers have suggested that it was simply the continued migration of modern humans from Africa to Eurasia that pushed Neanderthals to extinction, and that we were not better adapted than them, we were simply more numerous. Other researchers think it could have been that we had better clothing and technology, and that social factors, such as long-distance trade, may have given us an advantage. Or maybe Neanderthals were just about to disappear anyway.

The genetic information we have suggests that their populations were smaller than those of modern humans and that inbreeding may have occurred more frequently, resulting in less genetic diversity. This generally makes populations less adaptable to changing environmental pressures. As for the Denisovans, well, we barely realized they existed, but it's possible that some of the same factors that brought about the end of the Neanderthals also affected them. As a result, we are the only hominid species left. But it's surprising to me how close we came to that not being the case. But this does not necessarily mean that Homo sapiens were somehow better suited to survive from the beginning.

Because we really weren't. The fact is that our species was better adapted to local conditions precisely because we interbred with other hominids that had evolved to adapt to those environments. Other species such as Neanderthals and Denisovans contributed to our survival in these new landscapes. They helped us tolerate new conditions, such as high elevations and intense sunlight. They helped our bodies improve the ability to signal when we were sick and helped our blood clot faster when we were injured. The genetic legacy they left us is part of the secret of our success. And in a sense, those hominids didn't completely disappear, because parts of them continue to live today, in us.

They live in our own genes and remind us of a time when we were not alone. Thanks to this month's eontologists: Patrick Seifert, Jake Hart, Jon Davison Ng, and Steve. If you would like to join them and our otherPatrons to support what we do here, visit patreon.com/eons and make your contribution. And if you want to join us for more deep-time adventures, head over to youtube.com/eons and subscribe. Thank you for joining me today in studying Konstantin Haase, and if you'd like to learn more about our hominid predecessors, watch our companion episode, "The Humans Who Lived Before Us."