"What we're seeing is a real and very strong signal that the history of life on our planet has been shaped by a 62 million year cycle, but nothing in present evolutionary theory accounts for it," said Richard Muller, a physicist who holds joint appointments with Berkeley Lab's Physics Division, and UC Berkeley's Physics Department. "While this signal has a huge presence in biodiversity, it can also be seen in both extinctions and originations."
Muller, and his grad student, Robert Rohde, presented their findings in the March 10, 2005 issue of the journal Nature. In a commentary on this research in that same issue of Nature, UC Berkeley professor of earth and planetary sciences, James Kirchner, stated, "It is often said that the best discoveries in science are those that raise more questions than they answer, and that is certainly the case here."
Muller and Rohde discovered the 62 million year fossil diversity cycle after creating a computerized version of an exhaustive database compiled by the late University of Chicago paleontologist Jack Sepkoski. Entitled Compendium of Fossil Marine Animal Genera, Sepkoski's posthumously published database is the most complete reference available for the study of biodiversity and extinctions. It covers the Phanerozoic eon, the past half billion years during which multicellular organisms left abundant fossil records in rocks; uses genera, the level above species in taxonomy, because genera classifications are more manageable and less often revised than species classifications; and includes only marine fossils because the records are longer and better preserved than records of land fossils.
For their study, Muller and Rohde defined fossil diversity as the number of distinct genera alive at any given time. This yielded a total of 36,380 genera, whose history the Berkeley scientists tracked over time, using the International Commission on Stratigraphy's 2004 time scale.
"Putting the timescale in a chronologic format was critical to our findings, because there were no specific years assigned to the geologic timescale used by Sepkoski," Muller said. "We are the first to reconstruct diversity from the final version of Sepkoski's Compendium, and the first to use the 2004 geochronology time scale. In a sense, our work has verified the new time scale."
Muller and Rohde have been working on this study for nearly two years, and first discovered the 62 million year biodiversity cycle in November, 2003. They spent the next year trying to either knock it down or explain it. Despite examining 14 possible geophysical and astronomical causes of the cycles, no clear explanation emerged. Muller and Rohde each has his own favorite guess.
Muller suspects there is an astrophysical driving mechanism behind the 62 million year periodicity.
"Comets could be perturbed from the Oort cloud by the periodic passage of the solar system through molecular clouds, Galactic arms, or some other structure with strong gravitational influence," Muller said. "But there is no evidence even suggesting that such a structure exists."
Rohde prefers a geophysical driver, possibly massive volcanic eruptions triggered by the rise of plumes to the earth's surface. Plumes are upwellings of hot material from near the earth's core that some scientists believe have the potential to reoccur on a periodic basis.
"My hunch, far from proven," Rohde said, "is that every 62 million years the earth is releasing a burst of heat in the form of a plume formation event, and that when those plumes reach the surface they result in a major episode of flood volcanism. Such volcanism certainly has the potential to cause extinctions, but, right now there isn't enough geologic evidence to know whether flood basalts or plumes have been recurring at the right frequency."
In examining their results, Muller and Rohde found that the fossil diversity cycle is most evident when only short-lived genera (those that survived less than 45 million years) are considered. They also found that some organisms seem to be immune to the cycle, while others are exceptionally sensitive. For example, corals, sponges, arthropods and trilobites follow the cycle, but fish, squid and snails do not. In general, longer-lived genera that are more diverse and widespread stand a better chance of resisting the 62 million year cycle.
Muller and Rohde also found a second, less pronounced diversity cycle of 140 million years.
"The 140 million year cycle is also strong, but we see only four oscillations in our 542 million year record," Muller said. "This means there is some chance that it could be accidental, rather than driven by some external mechanism."
If it is real, the 140 million year fossil diversity cycle could be tied to a reported 140 million year cycle in Ice Ages. Said Rohde, "It is also possible that this 140 million year fossil diversity cycle is driven by passage through the arms of the Milky Way galaxy".
Muller and Rohde are continuing their examination of the data for both cycles, and their search for explanations. This research was supported, in part, by the Ann and Gordon Getty Foundation, the Folger Foundation, and the Larsen Fund.
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