Natural Science Forum / Biology / Paleontology / March 2005

http://www.lbl.gov/Science-Articles/Archive/Phys-fossil-biodiversity.html

BERKELEY, CA - A detailed and extensive new analysis of the fossil records of
marine animals over the past 542 million years has yielded a stunning surprise.
Biodiversity appears to rise and fall in mysterious cycles of 62 million years
for which science has no satisfactory explanation. The analysis, performed by
researchers with the U.S. Department of Energy's Lawrence Berkeley National
Laboratory (Berkeley Lab) and the University of California at Berkeley, has
withstood thorough testing so that confidence in the results is above
99-percent.

"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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