Peez

And, perhaps, a small extinction event? On the other hand it is possible that the impact of the eruption was relatively weak, but humanity was already stressed for some other reason and was therefore more impacted by this event. On the gripping hand humans were probably pretty widely distributed 70,000 years ago, so it would seem unlikely that all of them were in stressed populations. Of course, if the genetic bottleneck did occur, then humans were not so widespread then as we think.

Peez

ps418

Does anyone know what are the estimates of human population size before and after the ~70k BP bottleneck? Is it even knowable? Are there other hypotheses about what could have caused such a bottleneck? I can think of many mechanisms, but few that would act globally. It woud be interesting to look for evidence of abrupt climate change in the Vostok and GISP2 ice cores. Could a selective sweep somehow cause a major reduction in genetic diversity?

Patrick

Peez

Sorry, I don't have the requested estimates, but in principle it is possible to estimate the impact of the bottleneck, which would relate to both the population size and the length of time. I don't think that it is possible to distinguish between, for example, a small population maintained for 50,000 years leading up to the bottleneck and a slightly larger population maintained for 100,000 years leading up to the bottleneck.

Note that "population size" in these studies invariably refers to "effective population size," which is only equal to actual number of individuals under specific conditions. As Futuyma explains:(Futuyma, Douglas J. 1998. Evolutionary Biology, Third Edition. Sinauer. p. 303)
Effective population size may be smaller (never larger) than the actual number of living individuals due to variance in number of offspring, a skewed sex ratio, overlapping generations, and of course fluctuating population size.

Peez

ps418

Just an update: In my literature seach I haven't found any evidence yet of any signficant extinction coinciding with the Younger Toba eruption. I suppose you could assume that humans were at the wrong end of the distribution of susceptibility to volcanism (e.g. aerosol-induced, short-term cooling), but that reeks of special pleading.

Patrick

Jan Haugland

Does the conclusion that humans were close to extinction necessarily follow from that evidence? It may have been true that the population that eventually became ancestor to all present humans were very small at that time, but can't there have been a large population of other humans around the world at the same time, but whose lineages for some other reason failed to survive to this day?


- Jan

...who rants and raves every day at Secular Blasphemy

RufusAtticus

Yes.

RBH

Regarding the original finding, that extant human populations are apparently descended from a population on the order of 2,000 individuals around 70,000 years BP: Does it follow that there were only 2,000 individuals alive then, or could there have been a larger population but the lineages that made it through the 70,000 years until now are those descended from a subset of a larger population. After all, no one believes that "mitochodrial Eve" was the only female alive around 200,00 years BP (or whenever she was alive).

RBH

Added in edit: Nuts! (Note to self: Read the whole thread before posting, RBH!)

Arken

It's not surprising that humans almost went extinct 70,000 years ago. After all, every other member of genus homo is now extinct and has been for tens of thousands of years (certain Jerry Springer guests not withstanding), what's surprising is that somehow, something was different enough about Homo Sapiens Sapiens that allowed us to survive when none of our taxonomic relatives could. We had evolved some trait that they had not and I hope that one day we will discover what that trait is.

Coragyps

The ability to brew beer.

Arken

You've figured it out! Beer goggles enabled all those ugly cavepeople to actually want to have sex with each other and breed!