You and I belong to the only group
of hominins on the planet today. We’re 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 we’ve known about for a while,
like the Neanderthals, whose fossils we’ve 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, we’re 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 we’re 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,
we’re 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. We’ve 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 the “Sima 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 must’ve 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, they’re 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
might’ve 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 might’ve
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.”
Fantastic channel overall. Thanks for posting.
I wonder why they consider them a different species if we could still breed with them.
Interesting!
i think that we would still have slavery if we hadn't killed all other human species, i wonder if neanderthals were smart enough to handle modern day construction work. Homo floresiensis would be big in the entertainment industry, children will beg their parents for one as pet/companion, just imagine the shelters... shudders.