I imagine the fact that DNA reshuffles a little during every successful sexual reproduction helps prevent that kind of thing.

The FIRST part though, is my favorite part, a careful, reasonable synthesis of historical dissection, geology, chemistry and archaeology that includes some pretty awesome discoveries that as of 2006 (when the book was published) were really new, and as of now, are still bloody awesome (mostly because of how they were found: they found the world’s oldest known intact wooden tools! To do it, they had to send the robot sub that investigated the Titanic, down into the deepest waters of the Black Sea, deeper than any diver can safely go, to a place with waters so chemically toxic to life that even wood doesn’t rot down there. And why are the waters so toxic? Because a flash flood of salt water – an epic natural disaster triggered by glacial flooding, breaking the natural wall of earth that separated the lake from the Mediterranean – turned a fresh water lake into a saltwater sea… killing off all the lake’s freshwater life in a mass extinction that apparently exuded, in its last collective breath, a toxic chemistry that rendered the bottom of the lake into an eerily quiet, literally lifeless dead zone… with the remnants of a coastal civilization silently smothered under it. HELL YEAH. ).

Also (though this isn’t part of that book, it’s just something I read in science mags later), recent genetic discoveries do indicate we were reduced to a population of as little as 5,000 people at one point – and more recent archaeological and paleological discoveries in southern Africa suggest there was bit of a biological oasis on the coastline there, safe from the ravages of glacial flooding or ice ages alike, complete with caves in cliffs that show evidence of not just ancient human habitation approximately 60k-75k years ago, but quite clever humans at that (they appear to have heat-treated their points for added strength! Anyone not familiar with archaeology has possibly no idea of how amazing this is or possibly even what I just said, but trust me, it’s incredibly advanced for tens of thousands of years ago).

Soooo long story short: yes, we had a severe population bottleneck at one point, and yes, there was a period (likely during the last glacial melting period, the end of Younger Dryas Ice Age) which caused a lot of coastal flooding, as well as a severe natural disaster that was very likely the direct inspiration for the oddly specific, oddly common “Noah” family of flood stories (names vary, but it doesn’t just exist in the OT, you also find it mentioned, among other things, in the far older Epic of Gilgamesh).

However, those two events were tens of thousands of years apart – the Black Sea flood IIRC happened about 6,600 BCE, which is less than 9,000 years ago – way more recent than the likely bottleneck.

However, now that I think of it, the chronologically older story of the Garden of Eden? That now makes a lot more sense, if you take into account the southern African population bottleneck… especially since, being the settlement or camps were near a seashore, there was no lack of food at all (though finding freshwater is always a concern, you have to be an idiot or an inexperienced child to starve on the seashore). Though, I should note, a lot of scholars have suggested the Garden was intended to be part of the Middle East and I think they had a general idea of where… of course, others have postulated that it simply was a metaphorical story nostalgically relating our shift from Hunter-Gatherer to Agricultural lifestyles, as of course, the agricultural lifestyle was a great advancement of knowledge and culture (Tree of Knowledge, remember?), but also meant a lot more work – you can actually tell that for quite a long time, the agricultural lifestyle was actually harder and less healthy for our ancestors than hunting and gathering was. But eh. I’m more interested in the Flood stuff anyway. Natural disasters are more interesting by far than nostalgia!

On a completely unrelated note (and a more serious one), nearly every culture has some sort of flood myth, so it is possible there was a great deluge once upon a time and some people survived. For instance, the greeks have one where a man and woman survive and repopulate by stripping to the waist and throwing stones behind them. The stones he threw turned into men, while her stones became women.