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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 nuclear 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; Ambrose, 1998). 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 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; 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 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. Ferguson, 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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9 Responses to “When Humans Nearly Vanished”

  1. Trimegistus says:

    Interesting article. It also gives one a rough estimate of the minimum size for an “ark” if one plans to colonize another world or repopulate the Earth after some Michael Bay-esque catastrophe.

    • Archie Clebberdale says:

      With today’s tech though, it should be possible to make do with a lot less than 10k pairs if you keep banks of stored DNA (probably in the form of sperm and eggs).
      You might not want to though; it’s a lot easier to build a society if you’ve got a lot of working hands: you can shorten your start-up time by skipping the ‘everyone must be a full-time farmer or else we’ll starve’ phase.
      And of course, there are the other species to consider… I think we’re looking at least at something out of an Arthur C. Clarke novel.

      • Ben says:

        Farming in a space colony should be easier if started right, think in terms of not introducing pest species and maintaining an ideal environment year round(if year round will actually mean something after a few years in space!). Aquaponics should have something to offer as well I would expect.
        Hmm, if each home unit were required to maintain a small vegetable garden. Would that also decrease the amount of ‘paid’ farming required? I’m thinking labour intensive vege/herbs. I suppose colony design will effect the above alot.

  2. Karl Withakay says:

    “1 gigaton of TNT, forty times larger than our largest nuclear bomb explosion,”

    The largest nuclear device ever detonated was the 50Mt Tsar Bomba. (1/20th of 1Gt)

    The largest device ever designed but not built or tested was a 100 Mt version of the Tsar Bomba with a Uranium tamper on the secondary. (1/10th of a Gt)

    The largest nuclear device ever detonated by the US was the 15 Mt Castle Bravo test. (1/67th of a Gt)

    The largest nuclear device ever built but not detonated by the US was the Mk41 bomb at 25Mt. (1/40th of a Gt)

  3. john werneken says:

    @Karl yes. And it likely would follow that we could do a Toba ourselves. Hope we don’t lol.

  4. d brown says:

    How about almost all life? A short time ago it was a big deal that it took one big volcanic eruption to cause the great dying that killed about %80 of all life. It was said that it put so much co2 in the air and then the seas that they become so acidic that most of the sea live was killed in a short time. Then it rotted, making gases that killed most land life. The co2 in the seas now is so great that at the rate we and nature were adding to it, it would happen again. In about 2050.

    • James Moore says:

      This is already starting to happen. While it probably won’t kill all of the sea life by 2050 it is estimated that a lot of the worlds coral will have died due to the coral bleaching.

  5. Catzilla says:

    Thanks! Bought the book for my Nook Tablet, after I read this. Found it well written and informative.

    One minor problem the pictures and illustrations (especially charts) are a bit difficult to make out with the e-reader format, not your fault, if you talk to B&N or Kindle people could you mention this.

  6. Buenas noches! Thank you for this article!
    Question: is it ok if I re-post some parts of “When Humans Nearly Vanished | Skepticblog” on a
    different site? Thanks a lot! Have a beautiful day!