Comment by btilly | Hacker News Reader

Comment by btilly | Hacker News Reader

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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.

> 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.

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. :)

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.

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.

I can imagine that first-generation hybrids were slaves. Maybe with males being castrated.

This was in hunter gatherer times. There weren't any slaves. There much formal hierarchy as we'd understand it. Some people were known to be wiser and/or tougher but that wasn't a formal role. And if you didn't like how things were run you could always try to convince people to go somewhere else with you and split the tribe.

Also, where are you getting castration from? We know these people existed from the genetic traces of their descendants, and people who are castrated don't have kids.

And you know that, how?

The Cree, for example, raided for slaves.

So, you bring up Cree slavery and at first I was really surprised because the Cree lived in hunter-gatherer bands of exactly the sort that I thought never took slaves. But I did some quick research on the internet and the only thing I could find about Cree slaves was them taking slaves in raids and then immediately turning around and selling the slaves to other, settled groups. So they took slaves but didn't keep them.

Or perhaps you meant the Creek instead of the Cree? They certainly took and kept slaves but they were a settled agricultural group of the sort you'd expect could have slavery.

No, I just happened to remember reading about Slavey Lake.

But it is my impression that some Native American groups did engage in slavery. As did many in Africa, even before they started dealing with Arabs and Europeans.

So it's arguably not a stretch that early humans engaged in slavery. Indeed, I recall reading that the male breeding population was substantially smaller than the female breeding population for a while. I can't find the source, but it was maybe 10-50 kyr BP.

Parent said:

> 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.

Castration is an alternative (or additional) explanation.

> We have evidence that 2-4% of the entire modern genome of non-Africans came in from Neanderthals

The actual evidence is pretty weak (Pattersons D-statistic is a rather crude method), and crucially doesn't infer a direction. It's equally possible that 2-4% of the Neanderthals' genomes come from humans, and I think it's more believable that a few percent of the Denisovan's (there is only one!) genome come from (relatives of the ancestors of present day) Melanesians.

I'm not sure I understand. In your last scenario, where would the Melanesians have gotten significant differences from most other humans, to then pass on to Denisovans?

The Melanesians didn't pass anything on, that's chronologically impossible. They are the descendants of a migration wave out of Africa, through Asia, to Australia. If some interbreeding with the Denisovan (there is only one!) happened, the Melanesians are the closest extant relatives of whoever did the interbreeding. Outside Melanesia, that branch of humanity was later replaced by the expansion of another migration wave, and the again by the Han expansion.

Yes, their ancestors is what I meant. I get it now, thanks.

https://www.nationalgeographic.com/science/2019/03/ancient-i... during the bronze age it's speculated that all the native men were replaced by men from the steppes. so since all the neanderthal genetics came from the X chromosome, then that means the original european inhabitants were decedents of neanderthals. however, isn't it so that east asians have more neanderthal dna than europeans? is the notion that neanderthal dna is only express in the X chromosome true for east asians as well? if that's the case, maybe a similar event occurred there but at a much earlier date. or were the original european women captured and brought back to east asia?

the notion that a hybrid offspring maybe infertile is imo dominating these discussions too much.

what I find lacking in these discussions is the real possibility that genocide and slavery (the sexual and work kind) would explain so much.

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