Putting Limits on the Diversity of Life
(Fragments of
this review with links to the complete commentary and article of the
original publisher)
Taken from: Richard A. Kerr
Science 2001 May 25; 292: 1481 (in News
Focus)
A new way to analyze
the fossil record is suggesting that life… long ago hit a limit to its
diversity
Abstract:
“In a paper
published… in The Proceedings of the National Academy of Sciences, 25
paleontologists demonstrate a fresh approach to extracting a history of life
from an imperfect fossil record imperfectly sampled by paleontologists for 180
years. Preliminary results contradict previous studies that showed biodiversity
increasing over time. Although far from the last word, the method marks a
turning point in the study of paleontological databases, the researchers say.”
Review:
“Life only gets bigger and better, or so one might
infer from the history of marine life that paleontologists have drawn up in the
past few decades. The number of distinctly different kinds of organisms living
in the sea may have taken a hit now and again, but over the long haul ocean
life has only become more and more diverse, the data seemed to show. But now a recount of fossils is giving paleontologists second thoughts
about that ever-upward trend.”
“In a new record of
life published this week in The Proceedings of the National Academy of
Sciences (PNAS)1, 25 paleontologists demonstrate a fresh approach
to extracting a history of life from an imperfect fossil record imperfectly
sampled by paleontologists for 180 years. And the group's first, preliminary results suggest
that previous
studies may indeed have overstated life's penchant for diversification. Although far from the last
word, the method marks a turning point in the study of paleontological
databases, says co-author Richard Bambach of Virginia Polytechnic Institute and
State University in Blacksburg. "This isn't the end of the story, but it's a step in
the direction we want to go," he says.”
“Paleontologists
wouldn't need a new record of diversity if their predecessors had done it right
the first time--laboriously sampling and documenting all the accessible
exposures of fossil-bearing rock around the world. Instead, earlier researchers
searched most intensively nearest to home--mostly Europe and North America--and
often pursued paleontological novelty rather than systematic surveying. Making the best of a bad
situation, one paleontologist spent 20 years compiling a now-classic diversity
record from the hodgepodge of published and unpublished data. John
"Jack" Sepkoski of the University of Chicago, who died in 1999 at the
age of 50, combed reports of marine fossil finds and created a "phone book"
that listed each fossil genus by name, the first time it appeared in the fossil
record, and the last time it appeared. From that database he plotted a
diversity curve (see left figure) that rose sharply… as life burgeoned in
the…."explosion," plateaued…, and then rose steeply and steadily… to
the present day. Read literally, Sepkoski's curve implied that the great… mass
extinction… somehow reignited life's drive to diversify. Life in the seas
became more active, more predatory, and continually more varied as the
so-called modern fauna found new ways to thrive.”
Upward or onward?
Where
Sepkowski's numbers (left) told of steadily increasing
marine diversity, a recount (right)
shows a… plateau.
CREDITS: (LEFT) MARK
NEWMAN/SANTA FE INSTITUTE/PNAS; (RIGHT) ALROY ET AL./PNAS
“A nice story. But has
diversity really tripled…? "We may have been misled for 20 years," says paleontologist
Douglas Erwin of the National Museum of Natural History in Washington, D.C.
"There may be some real problems." The biggest may be that the harder
paleontologists look at a particular rock outcrop or at outcrops from the same
slice of geologic time, the more kinds of fossils turn up. With the meager
information in his phone book-style compilation, Sepkoski had no way of correcting
for the wide range of sampling intensities from time to time or place to place
or for other potential biases.”
“To fill the gap,
paleontologists John Alroy of the University of California, Santa Barbara
(UCSB), Charles Marshall of Harvard University, 23 colleagues, and dozens more
contributors are assembling the Paleobiology Database. Housed at the National
Center for Ecological Analysis and Synthesis at UCSB, the still-growing
database records a range of information beyond first and last appearance.
Perhaps most important, it contains the reported occurrences of a genus through
time and space.”
“Drawing on the
compiled information, Alroy and his colleagues sliced geologic history into…
intervals and used statistics to try to even out the effects of varying
sampling intensity in each interval. To bracket as many ways of correcting
for bias as possible, the group tried four ways to standardize the data and two
ways to count fossil occurrences--a total of eight different statistical
recipes.”
“All eight
approaches had much the same effect on the diversity curve (see right figure). "What's surprising
is that diversity… doesn't shoot up as extremely as was originally found in
Jack's data set," says co-author David Jablonski of the University of
Chicago. If diversity really hasn't risen much…, then all of life's
innovations in the oceans since the days of the trilobites--from new predators
to armored snails and burrowing clams--have been unable to break through some
set ceiling on diversity.”
“This paper
represents a real step forward," says ecologist Michael Rosenzweig of the University of Arizona
in Tucson. "For the first time, a large group of people is saying
paleobiology has been making a mistake, that it's very important to deal with
sampling issues. And when you try to get rid of the biases, the diversity curve
looks a lot flatter.”
“Rosenzweig, like
Alroy and his colleagues, regards the paper as a progress report on the road to a
larger and more thoroughly analyzed paleo data set. But even the
most sophisticated data mining, the scientists warn, might not extract the true
history of diversity. Paleontologist Jeremy Jackson of Scripps Institution of Oceanography
in La Jolla, California, thinks existing data are still too biased to
do the trick.
It may be "the information isn't there to begin with," he says. The
compilers of the Paleobiology Database intend to find out.”
References:
1.
Mark
Newman
From the Cover: A new
picture of life's history on Earth
Proc. Natl. Acad. Sci. USA, Vol. 98, Issue 11, 5955-5956, May 22, 2001
http://www.pnas.org/cgi/content/full/98/11/5955
2.
J. Alroy,
C. R. Marshall, R. K. Bambach, K. Bezusko, M. Foote, F. T. Fürsich, T. A.
Hansen, S. M. Holland, L. C. Ivany, D. Jablonski, D. K. Jacobs, D. C. Jones, M.
A. Kosnik, S. Lidgard, S. Low, A. I. Miller, P. M. Novack-Gottshall, T. D.
Olszewski, M. E. Patzkowsky, D. M. Raup, K. Roy, J. J. Sepkoski Jr., M. G.
Sommers, P. J. Wagner, and A. Webber
From the Cover: Effects
of sampling standardization on estimates of Phanerozoic marine
diversification
PNAS 2001 98: 6261-6266; published online before print May 15, 2001
http://www.pnas.org/cgi/content/full/98/11/6261
Tasters of the Word (YouTube), videos recientes: "Astronomía y Nacimiento de Jesucristo: Once de Septiembre Año Tres A.C.", "Estudio sobre Sanidades" (en 20 episodios), "Jesus Christ, Son or God?" and "We've the Power to Heal":
Tasters of the Word (the blog, with: "Astronomy and the Birth of Jesus Christ"):