skepticblog
When Humans Nearly Vanished
by Donald Prothero, Oct 31 2012
According to some estimates, on Halloween of last year (2011), the
population of humans on this planet passed the 7 billion threshold. Today,
humans
(along with their domesticated animals) are the most abundant large
vertebrates on the planet, and the problem of human overpopulation (and its
effects
on the overexploitation of the planet’s resources) is one that vexes
people worldwide. It’s hard to imagine the idea that humans have not always
been
so numerous, or that we have not always been the dominant large species on
the planet. But it was not always so. As I describe in my recent book
Catastrophes!, about 74,000 years ago a volcanic eruption occurred on Mt. Toba
in Sumatra which caused a global “volcanic winter” that nearly wiped out
humans completely.
Studies of the ash deposits in the adjacent ocean floor around Sumatra show
that Toba ejected 2800 cubic km of material. It was believed to be the
largest volcanic explosion in the last 25 million years. It released the
energy equivalent of 1 gigaton of TNT, forty times larger than our largest nuc
lear bomb explosion, and about 3000 times as powerful as the eruption of Mt.
St. Helens. Toba injected so much ash into the stratosphere that the ash
clouds blocked the sun’s radiation. It caused a “volcanic winter” that
lasted almost 10 years, and caused global temperature to drop by 3-5°C
(5-9°F),
further amplifying the cold of the ongoing Ice Ages. The tree line and snow
line dropped 3000 m (9000 feet) lower than today, making most high
elevations uninhabitable. Global mean temperatures dropped to only 15°C after
3
years, and took a full decade to recover to pre-eruption temperatures. Ice
cores from Greenland show the evidence of this dramatic cooling in the
trapped ash and ancient air bubbles, although so far it has not been detected
in
Antarctic ice cores (Rampino and Self, 1993a, 1993b; Robock et al., 2009).
A number of scientists have argued that the Toba catastrophe nearly wiped
out the human race, leaving a genetic bottleneck of only about 1,000 to
10,000 breeding pairs of humans worldwide (_Rampino and Self, 1993b_
(http://www.sciencemag.org/cgi/pdf_extract/262/5142/1955) ; _Ambrose, 1998_
(http://www.bradshawfoundation.com/stanley_ambrose.php) ). In addition to the
geologic evidence of Toba’s size and atmospheric effects, geneticists have
found
evidence from the molecular clocks in our genomes that _human populations
went through a genetic bottleneck _
(http://news.bbc.co.uk/2/hi/science/nature/2975862.stm) at about this time.
Not only is there evidence of reduced human populations about 74,000 years
ago, but there are many human-dependent organisms that show the same
pattern. Scientists found a similar genetic bottleneck in the genes of human
lice, and in our gut bacterium Helicobacter pylori, which causes human ulcers;
both of these date back to the time of Toba, according to their molecular
clocks (_Rogers, 2004_
(http://www.sciencedaily.com/releases/2004/10/041005075751.htm) ; Linz et al.,
2007). There is also genetic evidence that a
number of other large mammals, including chimpanzees (Goldberg, 1996),
orangutans (Steiper, 2006), macaques (Hernandez et al., 2007), cheetahs and
tigers
(Luo et al., 2004), and gorillas (Thalman et al., 2007) suffered population
bottlenecks about the time of the Toba eruption. Humans themselves in the
region around southeast Asia apparently vanished, because the _molecular
clocks in the mitochondrial DNA_
(http://www.sciencedaily.com/releases/2009/06/090604124023.htm) shows that
most humans in the region migrated there
from Africa shortly after the Toba event.
The details of the Toba catastrophe theory is still being argued over, but
it is not unreasonable to think that such a global catastrophe would have
profound effects on the human population. It’s startling to realize how
recently the most common ancestors of all modern human populations came to
inhabit most of the Old World, and how quickly the “races” differentiated over
less than 70,000 years. And the Toba story reminds us that no matter how
dominant and destructive humans are now, our existence on this planet is
precarious and fragile. As historian Will Durant put it, “Civilization exists
by geologic consent, subject to change without notice.”
References
* Ambrose, Stanley H. 1998. Late Pleistocene human population
bottlenecks, volcanic winter, and differentiation of modern humans. Journal of
Human Evolution 34 (6): 623–651.
* Goldberg, T.L. 1996. Genetics and biogeography of East African
chimpanzees (Pan troglodytes schweinfurthii). Harvard University, unpublished
PhD Thesis.
* Hernandez, R.D.; M.J. Hubisz, D.A. Wheeler, D.G. Smith, B. F
erguson, D. Ryan, J. Rogers, L. Nazareth, A. Indap, T. Bourquin, J. McPherson,
D.
Muzny, R. Gibbs, R. Nielsen, C.D. Bustamante. 2007. Demographic histories
and patterns of linkage disequilibrium in Chinese and Indian Rhesus
macaques. Science (316): 240–243.
* Linz, B.; et al. 2007. An African origin for the intimate
association between humans and Helicobacter pylori. Nature 445 (7130): 915–8.
* Luo, S.-J.; J.-H. Kim, W.E. Johnson, J. Van der Walt, J. Martenson,
N. Yuhid, D.G. Miquelle, O. Uphyrkina, J.M. Goodrich, H.B. Quigley, R.
Tilson, G. Brady, P. Martelli, V. Subramaniam, C. McDougal, S. Hean, S.-Q.
Huang, W. Pan, U.K. Karanth, M. Sunquist, J.L.D. Smith, S.J. O’Brien. 2004.
Phylogeography and genetic ancestry of tigers (Panthera tigris). PLoS
Biology (2): 2275–2293.
* Rampino, Michael R.; Self, Stephen. 1993a. Climate–Volcanism
Feedback and the Toba Eruption of ~74,000 Years ago. Quaternary Research 40:
269–
280.
* Rampino, Michael R.; Self, Stephen. 1993b. Bottleneck in the Human
Evolution and the Toba Eruption. Science 262 (5142): 1955.
* Robock, A.; Ammann, C.M.; Oman, L.; Shindell, D.; Levis, S.;
Stenchikov, G. 200). Did the Toba Volcanic Eruption of ~74k BP Produce
Widespread
Glaciation?. Journal of Geophysical Research 114: D10107,
* Steiper, M.E. 2006. Population history, biogeography, and taxonomy
of orangutans (Genus: Pongo) based on a population genetic meta-analysis
of multiple loci. Journal of Human Evolution (50): 509–522.
* Thalman, O.; Fisher, A.; Lankester, F.; Pääbo, S.; Vigilant, L.
2007. The complex history of gorillas: insights from genomic data. Molecular
Biology and Evolution (24): 146–158.
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