Why did humans nearly die out around 70,000 years ago and how did they survive?
Toba or not Toba, that is the question.
Global cooling?
Good one!
Keep in mind that there weren’t all that many of us to begin with, and now that we know the different populations of what we call Homo was actually one, large interbreeding population we should keep in mind that our little branch of H. sapiens wasn’t the only game in town. It’s quite possible that Neanderthals and Denisovans would have gone extinct even if we had never left Africa, but then again, maybe not.
Are Neanderthals and Denisovians extinct now? After all, they have hundreds of millions of living descendants today.
I think it’s reasonable to say that Neanderthals, as a distinct, recognizable population, are extinct. But yeah, nature doesn’t generally cooperate with our hyoo-man need to put things in distinct boxes.
The first part has been answered (Toba). For the second part,
the answer is, the same way most species repopulate: a rigorous routine of unprotected sex.
While some tiny bits of Neanderthal and Denisovian DNA are floating around in our gene pool (evidence that there was at least some interbreeding going on at some point), I don’t think that anyone would call a modern human with those tiny bits of DNA a Neanderthal or a Denisovian.
If only one of your 10,000 ancestors was a Neanderthal and the other 9,999 were all Homo Sapiens, you’re a Homo Sapiens and not a Neanderthal. I don’t know the actual numbers off the top of my head, but the amount of Neanderthal and Denovian DNA in modern humans is some tiny percentage like that.
Actually, it is more like 2 to 4 percent (in certain populations–Europeans more Neanderthal, Asians more Denisovian, Africans tiny amounts of either but with some sticky ghost spit.) So about the same degree of relation as a great-great-great grandparent.
Got a cite for that? I was under the impression that it was a lot less than that. I may be going off of old information, though.
Thanks for the cites.
You got the numbers and distribution a bit wrong. From your first cite:
In particular, note that Denisovan DNA is found mostly in South Asians and “Oceana” populations. But then, Oceana has always been at war with East Asia. 
Largest percentage, but not “most.” The natives of Tibet, for instance, posses a Denisovian gene that helps them survive at high altitudes.
I think you are missing the point. Neanderthal genes are found in Europeans and Asians (not just Europeans) and when we talk about Denisovan genes in Asians, we are talking mostly about South Asians and Oceanians. Someone reading your post who did not know better would think:
Neanderthal genes -> Europeans
Denisovan genes -> Asians.
That is not correct.
Also, even though an individual person may have only low-single digit percentage Neanderthal genes, different people have different genes from the available pool. Altogether, around 20% of the Neanderthal genome is still around in modern humans.
This is really fascinating stuff, and it seems that the more we look, the more we find. Sub-Sahara Africans generally lack Neanderthal and Denisovan genes, but some have genes from a “ghost species” or “ghost population”. “Ghost” because scientists can tell the genes came from outside our species, but they’re not exactly sure which population/species they came from. The genes seem to predate Neanderthal populations, which makes sense, so there must have been some remnant population of non-spaiens in Africa that bred with some African population, although not the ones that left Africa and became the current non-African sapiens around the world. Link.
I’ve read that Homo-Sapiens may have been down to as low as 40 breeding pairs. What a close call!
That might be an example of the best of the best surviving and repopulating. I did an experiment with guppies years ago where I would breed up a few hundred and then kill off all but a few survivors to breed back up. It was going well until I accidentally wiped out my experiment.
That could have been a figure for minimum effective population (I think that is the proper term) which is the minimum number of breeders needed to preserve all the gene variations available at that time. It is like if (to pull a lame analogy off the top of my head) there were 5 colors of marbles, you would need at least 5 marbles to have an example of each color. But if you have a random selection of 500 marbles, it won’t represent any more than 5 colors, because 5 colors are all there are. So that 40 breeding pairs" may mean “at least 40 pairs, maybe 400 pairs, but the additional pairs didn’t add anything to the mix.”