I was watching the Nova series, Becoming Human, which said that humans have significantly less genetic variation than other great apes, because our ancestors went through a population bottleneck, possibly being reduced to as few as 600 breeding individuals. The program said this happened because a severe drought hit Africa, leaving only 6 or so habitable coastal regions.
Why weren’t any other great apes affected enough by this drought to reduce their genetic variation? Where did they live during the drought?
Just a guess, but except for humans, all the other Great Apes were jungle animals, were they not? While humans lived on the open plains? I’d imagine the plains would be more devastated by a drought than the jungles.
That’s been suggested before, but it doesn’t really make much sense. Remember, we aren’t talking about a single catastrophic drought here. The human bottleneck must have extended for a minimum of 2, 000 years, and some of the genetic evidence suggests it extended for 100, 000 years.
One thing we do know is that in times of climatic drying it is the rainforest habitat that declines. Conversely savanna habitats decline in times of *high *rainfall and *contract *in wet periods. When you think about it, that’s inevitable. If there is insufficient rain to support woodland, then there must also be insufficient rain to support rainforest. As a result the area of rainforest retreats to refugia such as mountains and coastal belts, and is replaced by savanna woodland. So the savanna species move into areas previously occupied by rainforest, and the rainforest species get crowded together in tiny, fragmented areas.
The only way that reduced rainfall could lead to a reduction in woodland species is if the area of high rainfall was already so tiny that, upon drying, it couldn’t support the influx of woodland species. But we know that this wasn’t the case in Africa. More importantly, if the rainforest regions were so tiny, the effect of their shrinkage on their own species would be even more catastrophic.
This idea that Africa contained only 6 habitable regions is based on some rather silly assumptions, starting with the idea that humans couldn’t survive in regions that receive less than 700mm rainfall. Of course humans are well adapted to deal with arid conditions. We’ve been living happily in the Australian and Southern African deserts, regions that receive less than 400mm of rainfall, for at least 40, 000 years, with just paleolithic technology. The only way that reduced rainfall could produce such a severe bottleneck would be if the whole of the African continent became significantly *drier *than those regions, and we know that never happened.
As for what caused the bottleneck, we really don’t know. I’ve seen it suggested that it was caused by the African wet period that ended ~ 10, 000 year ago, with the expansion of closed forests forcing humans into what are now the desert regions of the Sahara and Kalahari, regions that at the time were savanna woodlands. This theory also neatly explains why humans left Africa at this time: they were forced into the Sahara and then into the Arabian “deserts”. Unfortunately there is no evidence of such a massive increase in African forest cover. A modification of this theory suggested that high rainfall may have inhibited the use of fire as a management tool, forcing humans into drier regions without actually increasing forest cover. But that has just as many problems.
The most plausible explanation, IMO, is that disease caused a sudden massive population reduction, which in turn resulted in drastic technological regression which in turn prevented adaptation to environmental changes.
So, for example, a measles epidemic reduced the population to just a few thousand people in 10 populations scattered across the continent and no longer in contact. That resulted in a massive loss of technology. We can see something similar in the history of Tasmania, where population reduction caused the loss of almost all technology. The lost the ability to make fire, they lost the ability to make clothes, they lost the ability to make freakin’ axes, they lost the ability to make fishhooks and fishnets. If that sort of technological loss is typical of what happens when human population is reduced down to a few thousands over a period of millennia, then a catastrophic reduction to a few hundreds in less than a year would be even more devastating. Humans would have been reduced to knowing how to make stone spears and hand-held knives and little else. Such a loss of technology would have drastically reduced our ability to cope with any subsequent
So having been reduced to the most basic toolkits, human populations recovered for a few hundred years, reaching a population of maybe 100, 000, then a drying event hit. Without any technological or cultural heritage to cope, the population was again devastated and reduced to a thousand or so individuals. Then recovery for another 200 years, then a catastrophic increase in rainfall and regression once again. Then more recovery, then the Toba eruption and so on and so forth.
IMO that much more complicated scenario of an initial disease-mediated population crash producing a technological collapse which in turn rendered us unable to weather climatic changes is much more plausible than the idea that any single event could reduce the human population for thousands of years.
Some new evidence, but the fact is that this was at best a hypothesis, and it was never any more widely accepted than the other hypotheses, such as the drought hypothesis of the OP.
The major problem with blaiming it all on the Toba eruption is the same as blaming it all on drought: lack of correlative effects. If the eruption had such a devastating effect on humans, then why did it have no effect at all on our nearest relatives: chimps, Neanderthals, Erectus and Floresiensis. Those species were all thriving at the time and they don’t appear to have suffered any such bottleneck. If the Toba eruption caused massive hominoid extinctions on the plains of another continent thousands of miles away, why did it have no effect at all on the forests of the same continent, or on the plains of the continent on which it occurred? And of course if it devastated populations of a species as notoriously adaptable as humans, why did it have no effect at all on much less adaptable species such as kangaroos or aardvarks? Thousands of other species of mid-large sized mammals occupied the exact same habitats as humans, and they went through no such bottle neck.
I will defer to the real experts but I have been watching documentaries and reading about human evolution a lot over the last few years from the best experts I can find who write analyses on the state of the art for the masses. One of the main things I have gotten from it is that there is some significant genetic divergence among many groups on the largest and smallest scales. For instance, it may have only been a small group that left Africa and founded most of the rest of the world’s human populations but there are some very distinct groups left in Africa like the San people of South Africa who have genetic roots that diverge from other groups much earlier than the time period we are talking about here and they exist to the present day. Likewise, the Australian Aborigines are another very old group diverging tens of thousands of years ago and exiting to this day.
Spencer Wells has a good documentary on Netflix on Demand the details the National Genome project and the genetic superhighways that created the morphology of different populations over tens of thousands of years. Recent research tells us that people of European and Asian descent aren’t pure homo sapiens at all. We hybridized with the Neanderthals who never went away fully so that a few percent of European and Asian DNA is Neanderthal in origin while that is not true for sub-Saharan Africans.
What I have taken from all this is that the story of evolution is very incomplete and quickly evolving even over the last few years. The idea that all modern humans are very closely related because our ancestors all passed through a tight bottleneck like cheetahs is far from conclusive if you look at the genetic extremes of the known population groups both on a macro and micro level and I would say it is false if you include things like Neanderthal hybridization and long isolated genetic populations.