Tracking the Course of Evolution


by Richard Cowen

NOTE: This is page 3 of a three-page document.

Repeated, Periodic Mass Extinctions?

Here, I want to dispose of an idea that has garnered a lot of publicity but should be dismissed. In addition to the very largest mass extinctions, Raup and Sepkoski identified smaller ones. They noticed that mass extinctions seemed to have occurred periodically, every 26.2 m.y. since at least the end of the Permian (Figure 6.2). The suggestion was followed immediately by a flurry of claims and counterclaims about cycles in the geological record. Others suggested that the extinction events had a 30 m.y. periodicity; or that the extinctions matched a periodicity of 28.4 m.y. for impact craters on Earth's surface, or that there were periodicities between 30 and 34 m.y. for crater ages, magnetic field reversals, plume eruptions, pulses of mountain-building, and other events.
Then matters got really out of hand. A group from UC Berkeley suggested that the periodic mass extinctions were caused by cometary impacts on Earth (cometary impacts leave no physical evidence!). These periodic cometary impacts were produced by Nemesis the Death Star, a small star that is a distant binary companion of the Sun. Every 28 m.y. or so Nemesis approaches close enough to disturb groups of comets that may orbit around the solar system outside the planets. The comets then bombard the inner solar system, including Earth. There is not a shred of evidence for the existence of Nemesis, however. We can't see it, so its supporters say that it must be small and dark. It is, they say, unfortunately (or conveniently) at the other end of its elliptical orbit now, too far away to detect easily.
Even if a Death Star once existed, its orbit would be so distorted by close passages to the Sun that any periodicity would disappear after a few return visits. So even if the extinctions are periodic, Nemesis could not explain them. Altogether, the Nemesis idea, though vivid, was never satisfactory. That didn't prevent it from spawning lots of publicity, several TV programs, two covers of Time, and at least five books!
Problems with the Nemesis idea led almost immediately to the Planet X hypothesis. Planet X is a hypothetical planet supposedly orbiting somewhere outside Neptune. If Nemesis the Death Star cannot produce regular comet showers, perhaps X the Death Planet could shower the inner solar system with comets. Planet X must have substantial mass (as much as Earth, or more), yet IRAS, the infrared orbiting telescope, has not been able to find it. In any case, computer models show that Planet X cannot produce periodic showers of comets any more than Nemesis can.
While all the astrophysical arguments were going on, several statisticians argued that the extinction data, and all the other data on physical events in Earth history, contain no cyclic periodicity at all. The so-called cycles are artifacts caused by the wrong methods of statistical analysis, or by undue reliance on absolute dates that have been suggested for geological stages, or both. Most paleontologists are not convinced that the 26 m.y. cycles are real, and a recent analysis found no periodicity at all in a new compendium of data from the fossil record.
The Nemesis idea and its various offshoots are and always were weak scientific hypotheses. They were proposed to explain periodic extinctions that turn out not to have been periodic and therefore did not need special explanation in the first place. Each has relied on the supposed influence of astronomical objects for which there has never been any evidence. We do need to examine major extinction events carefully for evidence of impacts, but we should be very careful about ascribing those impacts to the effects of hypothetical stars, planets, or comets.
So far there is only enough evidence to fix a major impact at one of the mass extinctions in the fossil record, the K-T extinction, and perhaps to suggest another at the FF boundary. Likewise, there is only enough evidence to connect giant flood basalt eruptions with two mass extinctions, at the P-Tr and the K-T boundary extinctions. It is clear, however, that the largest known impact and the largest known eruption coincide with undoubted mass extinctions. It would be amazing if that was a coincidence. These questions are still open!

Further Reading

  • Benton, M. J. 1993. Late Triassic extinctions and the origin of the dinosaurs. Science 260: 769-770. Status of the Triassic-Jurassic boundary question.
  • Benton, M. J. 1995. Diversification and extinction in the history of life. Science 268: 52-58. No sign of periodic extinctions in a new data set.
  • Bowring, S. A., et al. 1998. U/Pb zircon geochronology and tempo of the end-Permian mass extinction. Science 280: 1039-1045.
  • Bottomley, R. et al. 1997. The age of the Popigai impact event and its relation to events at the Eocene/Oligocene boundary. Nature 388, 365-368.
  • Erwin, D. H. 1993. The Great Paleozoic Crisis: Life and Death in the Permian. New York: Columbia University Press. Excellent: Erwin clearly labels his thoughts and inferences so that we can participate along with him as co-thinkers. Contains more geology and paleontology than the title suggests.
  • Farley, K. A., et al. 1998. Geochemical evidence for a comet shower in the Late Eocene. Science 280: 1250-1253.
  • Hut, P., et al. 1987. Comet showers as a cause of mass extinction. Nature 329: 118-126. If there's no evidence for asteroid impact, blame comet showers.
  • Isozaki, Y. 1997. Permo-Triassic superanoxia and stratified superocean: records from lost deep sea. Science 276: 235-238.
  • Quinlan, G. D. 1993. Planet X: a myth exposed. Nature 363: 18-19.
  • Raup, D. M. 1991. Extinction. Bad Genes or Bad Luck? New York: W. W. Norton.
  • Raup, D. M., and J. J. Sepkoski. 1986. Periodic extinction of families and genera. Science 231: 833-836.
  • Renne, P. R., et al. 1995. Synchrony and causal relations between Permian-Triassic boundary crises and Siberian flood volcanism. Science 269: 1413-1416. Volcanic scenario.
  • Retallack, G. 1995. Permian-Triassic life crisis on land. Science 267: 77-80.
  • Spray, J. G., et al. 1998. Evidence for a late Triassic multiple impact event on Earth. Nature 392: 171-173. However, there are serious doubts about this one.
  • Stothers, R. B. 1984. The great Tambora eruption of 1815 and its aftermath. Science 234: 1191-1198.
  • Turco, R. P. et al. 1990. Climate and smoke: an appraisal of nuclear winter. Science 247: 166-176. Revised version of their original 1983 suggestion.
  • Clymer, A. K. et al. 1996. Shocked quartz from the late Eocene: Impact evidence from Massignano, Italy. Geology 24: 483-486.
  • Erwin, D. H. 1988. The end and the beginning: recoveries from mass extinctions. Trends in Ecology & Evolution 13: 344-349.
  • Goldsmith, D. 1986. Nemesis: The Death-Star and Other Theories of Mass Extinction. New York: Walker.
  • McGhee, G. R. 1996. The Late Devonian Mass Extinction: the Frasnian/Famennian Crisis. New York: Columbia University Press. McGhee covers all serious hypotheses, and favors multiple medium-sized impacts.
  • McLaren, D. J., and W. D. Goodfellow. 1990. Geological and biological consequences of giant impacts. Annual Reviews of Earth & Planetary Sciences 18: 123-171.
  • Molina, E., et al. 1993. The Eocene-Oligocene planktic foraminiferal transition: extinctions, impacts and hiatuses. Geological Magazine 130: 483-499.
  • Muller, R. A. 1988. Nemesis: The Death Star. New York: Weidenfeld and Nicolson. A pop book by a notable member of the Berkeley group.
  • Rampino, M. R., et al. 1988. Volcanic winters. Annual Reviews of Earth and Planetary Science 16: 73-99.
  • Retallack, G. J., et al. 1998. Search for evidence of impact at the Permian-Triassic boundary in Antarctica and Australia. Geology 26: 979-982. They did not find any.
  • Retallack, G. J., et al. 1999. Postapocalyptic greenhouse paleoclimate revealed by earliest Triassic paleosols in the Sydney Basin, Australia. Bulletin of the Geological Society of America 111: 52-70.
  • Rose, W. I. and C. A. Chesner. 1990. Worldwide dispersal of ash and gases from earth's largest known eruption: Toba, Sumatra, 75 ka. Global and Planetary Change 3: 269-275.
  • Stanley, S. M. 1991. Delayed recovery and the spacing of major extinctions. Paleobiology 16: 401-414.
  • Visscher, H., et al. 1996. The terminal Paleozoic fungal event: evidence of terrestrial ecosystem destabilization and collapse. Proceedings of the National Academy of Sciences 93: 2155-2158.
  • Wang, K., et al. 1991. Geochemical evidence for a catastrophic biotic event at the Frasnian/Famennian boundary in south China. Geology 19: 776-779.
  • Wang, K. et al. 1997. Carbon and sulfur isotope anomalies across the Frasnian-Famennian extinction boundary, Alberta, Canada. Geology 24, 187-190. An impact occurred when ecosystems were already stressed.
  • Warme, J. E., and Sandberg, C. A. 1996. Alamo megabreccia: record of a Late Devonian impact in southern Nevada. GSA Today 6: 1-7.

RC, June 1999.

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