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  • [♩INTRO]

  • 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 bitand 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?

  • And if you just want to keep on learning all sorts of sciencey things with us,

  • there's probably some related videos in the sidebar.

  • I think they're over that way.

  • There's also the subscribe button.

  • There?

  • Is it there?!

  • [♩OUTRO]

[♩INTRO]

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B1 中級

Denisovans:我們神祕的表親讓我們變得更好 (Denisovans: Our Mysterious Cousins That Made Us Better)

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    林宜悉 發佈於 2021 年 01 月 14 日
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