
Traces of two unknown archaic human species turn up in modern DNA - sahin-boydas
https://newatlas.com/archaic-human-species-dna/60601/
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WalterBright
> We knew the story out of Africa wasn't a simple one, but it seems to be far
> more complex than we have contemplated

Things are always going to be much more complex than the evidence shows,
simply because the evidence we have is always going to be incomplete.

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tasty_freeze
I realize the concept of species is hard to pin down, but I thought one first-
order approximation was that if two animals can reliably breed and form
fertile offspring, then they are the same species. There are weird cases like
ring species, and there is often a period of time during speciation where two
lineages could functionally breed fine but don't because of some kind of
ecological or behavioral barrier.

But if all these different human lineages apparently bred successfully and had
fertile offspring (enough to leave a few percent of their DNA in modern
humans), what is to say they were different species?

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btilly
The first approximation is an approximation only. The closer you look, the
fuzzier it gets.

There are many pairs of species which can interbreed, and sometimes be
fertile, and as a result genes can get from one to the other. But for the most
part they don't interbreed, even when they share a territory. An example today
is tigers and lions. (They no longer overlap in the wild, but they used to
before humans shrank both their territories.)

Similarly homo sapiens and neanderthals coexisted for tens of thousands of
years. And yet we only have evidence for a handful of successful interbreeding
events. That's pretty good evidence that they were separate species.

For these new ones, whether they were fertile or not is subject to dispute.
However we can estimate how far back the last common ancestor of their
chromosomes and ours is. And from that gap, we have reason to believe that
they are similarly distant from homo sapiens as Neanderthals were. And so they
likely were a different species, even though some interbreeding was possible.

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stevenjohns
> Similarly homo sapiens and neanderthals coexisted for tens of thousands of
> years. And yet we only have evidence for a handful of successful
> interbreeding events. That's pretty good evidence that they were separate
> species.

I don't think it's as simple as what you're saying. We have evidence that 2-4%
of the entire modern genome of non-Africans came in from Neanderthals, and an
even larger amount of genetics coming in from Denisovans for many in modern
Asia and beyond into Australia.

As I understand it, all Neanderthal genetics that modern man has came in
through the X chromosome. This could indicate that male offspring of
Neanderthals and Homosapiens were infertile, which would be a cut-and-dry
indicator that they're two separate species.

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ahje
It's also worth mentioning that if we assume that interbreeding didn't occur
somewhat frequently then that means that non-African humans evolved from the
same ancestors that the Neanderthals' and Denisovans evolved from, while most
people in Africa evolved from another group or groups.

The earliest modern human remains found outside Africa date to about 200,000
years ago, yet almost all people born outside Africa are descendants of people
migrating from Africa about 70,000-50,000 years ago. We don't really know what
happened to the first waves of emigrants but we do know they didn't leave many
traces. The traces we have found don't indicate hostility so the easiest
explanation is that they simply lived alongside the Neanderthals and
Denisovans and that the Neanderthals and Denisovans were more successful.
Since we know there were interbreeding I find it very likely that those were
isolated incicents, considering the human sexual drive. :)

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mirimir
People do have sex with other species. And if there were other human species,
it'd probably be even more common. There's some non-human prostitution, and
that does include other primates.

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ahje
Precisely. We do have a handful of cases where it is known to have happened,
and I can't see how it could be a few isolated incidents only.

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awt
>For example, all present-day populations show about two percent of
Neanderthal ancestry which means that Neanderthal mixing with the ancestors of
modern humans occurred soon after they left Africa, probably around 50,000 to
55,000 years ago somewhere in the Middle East.

I thought subsaharans lacked Neanderthal admixture?

~~~
deogeo
According to
[https://genographic.nationalgeographic.com/neanderthal/](https://genographic.nationalgeographic.com/neanderthal/)
and the chart on top of
[https://en.wikipedia.org/wiki/Interbreeding_between_archaic_...](https://en.wikipedia.org/wiki/Interbreeding_between_archaic_and_modern_humans),
that seems correct.

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Bucephalus355
There are many fascinating relics and bizarre ancient remnants in human DNA.
Endogenous Viral Elements are a great example of this. Referred to as
“horizontal evolution”, there are a variety of organisms that have the ability
to literally transfer their DNA to other species. Something like 5-8% of the
Human Genome is the result of “injections” from these paleoviruses.

Part of that percentage is junk / redundant DNA strands humans evolved over
time to make it more difficult for viruses to navigate and compromise our DNA.

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Merrill
There is also evidence of admixture with archaic species within Africa. The
picture within Africa is probably even more complex, and analysis is made more
difficult by the absence of ancient DNA due to the warm climate.

"Outstanding questions in the study of archaic hominin admixture" \-
[https://journals.plos.org/plosgenetics/article?id=10.1371/jo...](https://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1007349)

What humans have in common is that we are descended from the most aggressive,
fastest breeding population that killed off all our close competitors.

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menzoic
> What humans have in common is that we are descended from the most
> aggressive, fastest breeding population that killed off all our close
> competitors.

We didn't necessarily kill 100% of all our individual close competitors. We
might've won the majority of resources and protected them from outsiders.

We also could've been the most adaptable and naturally curious group that was
more clever at over coming hardships and willing to travel long distances to
discover new resources even if the risk level was high.

~~~
JoeAltmaier
Also the existing (extinct) populations could have become diluted within the
larger immigrant population, until their mating opportunities went to zero?

