The bust is a mass extinction event, caused by events such as an ice age or an asteroid hitting the earth. The boom, as Miller has found, is that evolutionary groups have a significantly better longevity in the geologic record if they first appear right after a mass extinction event. Miller writes about these findings in the Nov. 7 issue of Science, with co-author Michael Foote of the University of Chicago. The study examined longevity trends throughout the last 540 million years (the Phanerozoic eon), using a database of marine fossil genera compiled by the late J. John Sepkoski.
Miller and Foote find that groups of genera first appearing following mass extinctions survived for substantially longer periods of time, on average, than those that first appeared at other times. While mass extinction events wipe out a lot of life, they also present new opportunities.
"As bad as a mass extinction is, it does open new possibilities," says Miller, head of the Department of Geology in the UC McMicken College of Arts and Sciences. "It's sort of like politics. It's hard to unseat an incumbent candidate, and in evolution it is also hard to unseat incumbent genera. Mass extinctions are good at removing incumbents. A lot of genera and species will become extinct during the event, but it also opens the opportunity for other types of genera and species to arise and thrive afterwards."
His study has significant ramifications for conservation biology, if one believes that the earth is currently headed for a human-induced mass extinction. "While we can't look at a species and say that this species would survive and this would not, we can expect that those that would survive the present-day extinction would likely be widespread," he says. Miller and Foote's findings suggest that the same may be true of species that originate in the future to take the places of species that are lost. These species may be of the "weedy" type -- "those typically regarded as pests, capable of wide dispersal," he says, adding that species that are endemic, or localized, would be less likely to survive for long periods of time.
Miller and Foote's longevity findings apply only to the Mesozoic and Cenozoic, the last two of the three eras that make up the Phanerozoic eon. The pattern does not appear in earlier, Paleozoic mass extinctions during the Cambrian, Ordovician and Devonian periods. "The way genera responded in the Paleozoic era seems to have been different," Miller says.
Miller and Foote divided the Phanerozoic into 156 "bins" or substages. Examining the average longevity in each bin, they found that significant peaks in mean longevities occurred in the substages that followed three major mass extinctions in the Late Permian, Late Triassic and Late Cretaceous, and following one lesser extinction at the end of the Jurassic.
Miller continues to work with geologists worldwide to build an online database of marine genera throughout the Phanerozoic eon that contains information on the geographic and environmental occurrences of the genera. This will permit him to investigate directly whether the longer-lived genera that originated after mass extinctions really were more widespread, as predicted, than their shorter-lived counterparts.
His work is supported by NASA's Program in Exobiology and the National Science Foundation's Program in Biocomplexity.