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  • You and I belong to the only group of hominins on the planet today.

  • Were the lone twig left on our branch of the family tree.

  • But we weren’t always alone.

  • 100,000 years ago, Eurasia was home to other hominin species, some of which we know our

  • ancestors met, and spent some quality time with.

  • Some of them weve known about for a while, like the Neanderthals, whose fossils weve

  • been digging up since the 1800s.

  • But some of them are more recent additions to the family tree, like the Denisovans, who

  • who we may, on this channel, have called Denis-ovans but we have been informed

  • that it's De-nis-o-vans

  • The Denisovans were discovered almost by accident in 2008, and we know them from only a few fossil bones

  • and from the DNA of their living descendants.

  • That surprising discovery has opened our eyes to the fact that our ancestors met, and even

  • mated with, other hominins.

  • 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, were starting to better understand how and even where modern

  • humans paired up with other hominins, giving us a more complete picture of the history

  • of our species.

  • And were starting to tackle some really exciting questions, like: What’s our inheritance

  • from that time when we met up with other human species?

  • And why are we the only ones left today?

  • As we get closer to answering those questions, were starting to see that maybe part of

  • our success as a species has to do with those other hominins that we encountered in our

  • travels around the planet.

  • Neanderthals lived throughout Europe, and from southwestern to central Asia.

  • Weve found their fossils from Portugal and the UK in the west all the way to the

  • Altai Mountains of Siberia in the east, and down into Israel in the south.

  • The oldest Neanderthal-like fossils come from a site in northern Spain called theSima

  • de los Huesos” - literally, the Pit of the Bones - dated to about 450,000 years ago,

  • while the most recent come from a handful of sites across western Europe that date to

  • around 40,000 years ago.

  • Anatomically, the Neanderthals were very much like us, with a few differences.

  • They were relatively short and stocky, with robust limbs, and big brains.

  • They had heavy brow ridges, large noses, and braincases with more of an oval shape than

  • the round ones of Homo sapiens.

  • And we know they weren’t just dumb cavemen.

  • They controlled fire, created stone tools and spears, made jewelry from eagle talons,

  • and cared for injured members of their groups.

  • And our ancestors clearly recognized them as being like us -- enough so that we interbred

  • with them!

  • We know this because researchers have sequenced the Neanderthal nuclear genome, originally

  • in 2010, from bone fragments found in a cave in Croatia.

  • And by comparing that genome to those of modern humans from many different populations, we

  • can find out 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 descent have between 1 to 2% Neanderthal DNA in their

  • genes.

  • Meanwhile, people native to Sub-Saharan Africa don’t have any Neanderthal DNA, indicating

  • that their ancestors never encountered Neanderthals.

  • And as we find more fossils to sample, we can tell that these interbreeding encounters

  • happened more than once.

  • And our genomes can 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 researchers were able to measure how long the Neanderthal segments of his genome were,

  • compared to the same segments in living people.

  • It turned out that his Neanderthal sections were longer than modern humans’, suggesting

  • that he wasn’t that many generations removed from his Neanderthal ancestor.

  • In fact, the researchers were able to estimate that this Siberian man was the product of

  • an interbreeding event between Neanderthals and Homo sapiens that occurred 50,000 to 60,000

  • years ago -- just 10,000 years before he was born, give or take a few thousand years.

  • And this was likely 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, which provided some of

  • the earliest evidence of modern humans in Europe.

  • And it was found to have some anatomical similarities to Neanderthals.

  • When its genome was sequenced over a decade later, that human was found to have had a

  • Neanderthal ancestor only some four to six generations back.

  • Between 6 and 9% of its genome was Neanderthal!

  • So, using fossils like these, researchers have been able to determine that Homo sapiens

  • bred with Neanderthals several times in different places.

  • But how did all of this interbreeding change us?

  • Well, sometimes, not very much.

  • The genome from the Romanian jaw bone suggested that population didn’t contribute much to

  • the DNA of living modern humans.

  • But sometimes, these encounters had a big impact.

  • For example, two genes that play important roles in our immune response seem to have

  • passed from Neanderthals to people of Eurasian descent.

  • One of these genes, known as STAT2, is part of our immune system’s signaling response

  • when we get a viral infection.

  • And we know that the Eurasian version of STAT2 came from Neanderthals, because it isn’t

  • found in sub-Saharan Africans.

  • Plus, 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 mustve come from interbreeding.

  • Meanwhile, members of many East Asian populations have been found to carry a gene known as HYAL2,

  • which is involved in skin-cell repair after skin has been exposed to the sun’s UVB rays

  • -- in other words, sunburn.

  • And this gene also seems to have come from the Neanderthals, likely a helpful adaptation

  • for modern humans spreading across Asia.

  • These genes are both examples of a phenomenon known as adaptive introgression, when genetic

  • material from one species moves into the gene pool of another species, and then is selected

  • for, so it sticks around.

  • But these genetic contributions also can have a downside.

  • Introgressed genes that were once beneficial can become less-so over time, as the environment

  • in which natural selection is taking place changes.

  • For example, there’s a gene that’s involved in the rapid coagulation of blood, which used

  • to be really beneficial before medical care was available.

  • But now that gene has been found to increase the risk of blood clots.

  • And this gene, too, seems to have also come from interbreeding with Neanderthals.

  • But Neanderthals aren’t the only other hominins that we got to know so intimately.

  • In 2010, paleo-geneticists announced a shocking discovery: a site known as Denisova Cave in

  • southern Siberia had yielded ancient mitochondrial DNA from a previously unknown hominin.

  • Earlier work there at had turned up evidence of modern humans and Neanderthals, so the

  • researchers were expecting that the DNA they extracted from a pinky bone found there would

  • also be Neanderthal - but it was not.

  • Almost a decade later, we have seven fossil bones with this unique genetic signature:

  • There’s the tip of a pinky, three molars, a sliver of long bone, and a piece of skullcap

  • from Denisova Cave, all dated to between about 52,000 to 195,000 years ago.

  • And there’s also a single partial jawbone from the Tibetan Plateau, dated to at least

  • 160,000 years ago.

  • This new group of hominins has yet to be given a scientific name, because it lacks a type

  • specimen, a fossil that’s complete enough for future finds to be compared to.

  • So for now, theyre informally known as the Denisovans.

  • And they are essentially a ghost lineage within our own ancestry: a branch of the hominin

  • family tree that lacks a fossil record.

  • Now it could be that there are other Denisovan fossils in collections around the world that

  • just haven’t been identified.

  • But because there’s no type specimen, and most fossils don’t have DNA that can be

  • extracted, we just don’t know for sure.

  • But we have sequenced enough of the Denisovan genome to be able to tell some of their story.

  • For instance, their mitochondrial DNA -- which

  • is passed down from mothers to their offspring -- suggests that the last common ancestor

  • that we shared with the Denisovans lived about 1 million years ago.

  • But the nuclear DNA of Denisovans -- which accounts for most of their genome -- is actually

  • more similar to ours.

  • So this might be a sign that Denisovans also interbred with some other hominins within

  • our lineage, like Homo erectus.

  • Today, we find small amounts of Denisovan DNA in populations in East and South Asia,

  • and up to 6% Denisovan DNA in some populations of Melanesians in the southwest Pacific.

  • And the variations we see between the DNA in modern people and the ancient genomes we

  • have from Denisovan fossils suggests that there were interbreeding events with at least

  • three different groups of Denisovans.

  • So it didn’t happen just once, or in just one place.

  • And some of these genetic contributions are really important.

  • Take the gene known as EPAS1, 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 seems to have been introduced by the Denisovans and was strongly selected for

  • because of the advantages it offered to modern humans living at high elevations.

  • So, we know that many of the hominins that we used to live and hang around with were

  • really well-suited to a lot of environments.

  • So, why aren’t there populations of these other hominins walking around today?

  • Well, anthropologists have been thinking about that for a long time, especially when it comes

  • to the Neanderthals.

  • The longest-standing explanations for their

  • disappearance have been that climate change, competition from modern humans, or some combination

  • of the two caused their 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 mightve caused Neanderthal populations

  • to decline, leaving them vulnerable to extinction.

  • Other researchers have modeled the distribution of Neanderthals and their habitats, and suggest

  • those habitats were becoming more fragmented by changes in the climate.

  • As for the impact of modern humans, we know we met Neanderthals, but there’s no evidence

  • of violence or direct competition between the two groups.

  • So, some researchers have suggested that it was just the continued migration of modern

  • humans from Africa into Eurasia that pushed the Neanderthals into extinction, and that

  • we weren’t better adapted that they were, we were just more numerous.

  • Other researchers think it might have been that we had better clothing and technology,

  • and that social factors, like long-distance trade, may have given us an advantage.

  • Or, maybe the Neanderthals were just on their way out anyway.

  • The genetic information that we have suggests that their populations were smaller than those

  • of modern humans, and that inbreeding mightve occurred more often, resulting in decreased

  • genetic diversity.

  • This generally makes populations less adaptable to changing environmental pressures.

  • As for the Denisovans, well, we only just realized they existed at all, but it’s possible

  • that some of the same factors that brought about the end the Neanderthals affected them,

  • as well.

  • As a result, we are the only species of hominins left.

  • But it's kind of remarkable to me how close we were to that not being the case

  • But this doesn’t necessarily mean that Homo sapiens was somehow more fit for survival

  • from the start.

  • Because, we really weren’t.

  • The fact is, our species became better adapted to local conditions precisely because we interbred

  • with other hominins that had evolved to fit those environments.

  • The other species like the Neanderthals and the Denisovans contributed to our survival

  • in these new landscapes.

  • They helped us tolerate new conditions, like high elevations and intense sunlight.

  • They helped our bodies become better at signalling when we were sick, and they 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 hominins didn’t completely disappear, because parts of them live on today,

  • in us.

  • They live on in our own genes, reminding us of a time when we weren’t alone.

  • Thanks to this month’s Eontologists: Patrick Seifert, Jake Hart, Jon Davison Ng, and Steve.

  • If you’d like to join them and our other patrons in supporting what we do here, then

  • go to patreon.com/eons and make your pledge!

  • And if you want to join us for more adventures in deep time, just go to youtube.com/eons

  • and subscribe.

  • Thanks for joining me today in the Konstantin Haase studio, and if you’d like to learn

  • more about our hominin predecessors, then watch our companion episode, “The Humans

  • That Lived Before Us.”

You and I belong to the only group of hominins on the planet today.

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当我们遇见其他人类时(When We Met Other Human Species)

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    joey joey 發佈於 2021 年 05 月 02 日
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