Genetic bottleneck 74,000 YA ... nonuman species, too?

[toadspittle]

From studies of mitochondrial DNA, human beings appear to have gone through a bottleneck (with 10,000 or fewer individuals surviving to pass on their genes) some 74,000 years ago. This coincided with an explosion of a supervolcano at Toba, Indonesia (often called the biggest explosion on Earth in the past 2 million years), that spewed out thousands of cubic kilometers of ash and rock and may have caused a 6-year-long volcanic winter.

So the natural assumption is that the Toba eruption caused the near-extinction of our species. But have similar genetic bottlenecks been observed in other species? If not, why did humans fare so poorly?

[rackman]

The cheetah appears to have possibly undergone a similar bottleneck. Here is a link to a site that talks about this phenomenon in cheetahs.

[sunfish]

Cheetahs have experienced a genetic bottleneck, but the estimated date of that bottleneck – 10,000 to 12,000 years ago - would have nothing to do with the Toba eruption, which took place ca. 74,000 years BP. As far as other species go:

The genetic structure of eastern African chimpanzee populations (Pan troglodytes schweinfurthii) (Morinet al., 1994) suggests they survived one or more bottlenecks in the Late Pleistocene (Rogers & Jorde, 1995; Goldberg, 1996). Several populations that reside within the Pleistocene ice age eastern forest refugium of the montane region centered on western Uganda and eastern Zaïre (Hamilton, 1976), have mismatch distributions that suggest they experienced a release from a bottleneck ~67 ka. This date closely coincides with the start of the last ice age (oxygen isotope stage 4, 70–74 ka). … Populations outside of the core Pleistocene refugium seem to have been released from a bottleneck much more recently, possibly after the Last Glacial Maximum (LGM, oxygen isotope stage 2), around 20 ka (Goldberg, 1996). … Bimodal mismatch distributions [in mitochondrial DNA sequences] can be interpreted in other ways (Marjoram & Donnelly, 1994), but the fit of these chimpanzee data with known environmental history suggests the hypothesis that some populations experienced two bottlenecks and releases during or after the two coldest parts of the last glacial.

Mismatch analysis of population structures and histories have been determined for few other species. Cheetahs apparently experienced two severe population bottlenecks in the very recent past (O’Brien et al., 1987). The earlier one, estimated to have occurred ~10 ka, may reflect dramatic reduction in the extent of savanna grassland environments at the beginning of the early Holocene wet phase (isotope stage 1, ~12 ka). Because cheetahs are adapted to open, arid environments, they are unlikely to be affected by environmental change involving reduction of forests. Their population history is thus expected to out of phase with that of chimpanzees. I am not aware of research identifying bottlenecks in other large mammals species.

[Ambrose, Journal of Human Evolution 334, p.628, 1998]

The Toba hypothesis for a human genetic bottleneck is itself interesting, but it’s still a hypothesis rather than a theory owing to the sparseness of data to support it. It was first proposed by Rampino and Self (Science 262:1955, 1993), but really got a boost in the paper by Ambrose quoted above (Journal of Human Evolution 334:623-651, 1998). There are problems with the Toba hypothesis on two fronts:

1. The timing of genetic divergences and bottlenecks has to assume a particular mutation rate. It’s unclear whether the rate assumed by Ambrose is actually correct. Moreover, presumed rates of evolution differ depending on whether you look at mitochondrial DNA or various nuclear DNA variations that suggest divergence prior to 100,000 years ago (e.g., Harris and Hey, Evolutionary Anthropology 8:81-86, 1999).

2. The concept of a “volcanic winter” scenario in relation to the Toba eruption is itself unproven. As pointed out by Oppenheimer (Quaternary Science Reviews 21:1593-1609, 2002):

[T]here remain major gaps in our understanding of the approximate to 74kyr BP Toba eruption that hinder attempts to model its global atmospheric and climatic, and hence human consequences. The tephra record reveals basic aspects of the eruption style but calculations of the duration, and hence intensity and plume height of the event, are poorly constrained. Furthermore, estimates of the sulphur yield of the erupting magma, central to predictions of its atmospheric and climatic impacts, vary by two orders of magnitude (3.5-330 x 10(10) kg). Previous estimates of globally averaged surface cooling of 3-5 degrees C after the eruption are probably too high; a figure closer to 1 degrees C appears more realistic. The volcanological uncertainties need to be appreciated before accepting arguments for catastrophic consequences of the Toba super-eruption.

If the Toba eruption did in fact have a profound effect on global ecology, the evidence ought to exist in other species. Since that evidence doesn’t appear to exist, it’s quite possible that a human genetic bottleneck was provoked by some other event peculiar to human experience, or perhaps glacial-interglacial climate changes (which appear to have impacted other species to varying degrees) at some time other than 74,000 years ago.

[sunfish]

Damn it, I even previewed… ah well, I think it’s okay despite the funky conversion of some symbols.