After the most devastating mass extinction swept the planet 250 million years ago, the earth witnessed a nearly unabated increase in the variety of living organisms leading to unparalleled heights of diversity — or so paleontologists have long thought.
Now a new study in The Proceedings of the National Academy of Sciences suggests a radically different picture that, if correct, will require a large-scale rewriting of the history of life.
In the first results from a huge new database of fossil records being assembled on the World Wide Web by an international team of scientists, researchers report findings that suggest there may have been no such relentless increase in diversity. In fact, the new results suggest the possibility that diversity levels quickly hit a plateau and stayed put and that the real peak of life's diversity may have come and gone more than 400 million years ago.
But scientists, including the authors, cautioned that the results should be viewed as preliminary. Even though the new study addresses what some feel are critical flaws in the previous work showing steep increases in diversity, scientists note the results are a first look at a complex new database.
The current study focuses on fossil marine organisms, but scientists plan to include records of fossils of all kinds from the span of the history of life on earth in the database (flatpebble.nceas.ucsb.edu/public) at the National Center for Ecological Analysis and Synthesis, which supported much of the work.
But while adopting a wait-and-see attitude, scientists outside the study acknowledged that if true, the findings would require a radical rethinking of not only when diversity levels rose and fell, but why.
"This calls into question longstanding views of the diversity of life," said Dr. Douglas Erwin, a paleobiologist at the Smithsonian Institution, adding it was too soon to draw firm conclusions. But he said, "It's very exciting for paleontology."
For example, the many theoretical explanations for the steady increase in diversity would become irrelevant and a whole new series of questions about the relative constancy in levels of diversity would emerge.
"This is a very important enterprise," said Dr. Jeremy Jackson, paleobiologist at Scripps Institution of Oceanography, though he was highly skeptical of the new work. "If it were true, the implication would be that there was some sort of ceiling on diversity, something limiting it, which would be fascinating."
It was the pioneering work of Dr. J. John Sepkoski, a paleobiologist at the University of Chicago, who died in 1999, that provided the first evidence that diversity had been on a steady climb (with occasional minor setbacks like the extinction that took the dinosaurs) ever since the mother of all extinctions, known as the Permo- Triassic. Similar studies of plants, insects and other animals followed and showed the same unending rise in the variety of forms of life.
But there have always been nagging doubts. As it turns out, counting up what different kinds of fossil organisms lived in any particular time period is not as simple as it may sound. Because researchers draw on the many published studies of fossils done over the years to fill their databases, among the thorny problems is the fact that scientists have studied some areas with much greater intensity than others.
In one example of true paleontological zeal, Dr. G. Arthur Cooper, a paleobiologist at the Smithsonian Institution, led a 40-year effort collecting more than 100,000 fossil specimens from the Glass and Guadalupe Mountains in West Texas. The fossils, which are naturally made of glass, include the clamlike marine organisms known as brachiopods.
Dr. Cooper, who died recently, "wanted to find every last species of brachiopod," Dr. Erwin said.
Yet information from such exhaustive surveys from one time and place would be compared with information from much more superficial surveys, which would have missed most of the rarer species, from other time periods around the world.
Even more problematic is the fact that newer rocks and fossils are easier to get at and have been much more intensively studied by paleontologists. So as scientists tally the organisms discovered in more than a century of paleontological research, they will necessarily find a much greater diversity of newer organisms than older ones, whether they were actually more abundant in reality or not.
But even with consideration of the acknowledged problems, scientists said the consensus remained that the increase in diversity was real.
Dr. John Alroy, paleobiologist at the National Center for Ecological Analysis and Synthesis at the University of California at Santa Barbara, and Dr. Charles Marshall, paleobiologist at Harvard, are the first two authors on the 25-author paper in which researchers tried to get around some of the problems by making a variety of statistical attempts to sample fossil species equally from the different periods of time. The paper is dedicated to Dr. Sepkoski, who is also an author on the study.
The result is a variety of possible chains of events for how diversity waxed and waned, all different from the standard view, but all indicating that the steep increase in diversity seen in previous studies may have been influenced by the bias in information about newer fossils.
Scientists note also that the new study has its own biases. For example, the study focuses on marine organisms from around North America and Europe, and a different pattern could emerge as the database gains more geographic breadth.
"The data's got a lot of dimensions, and there are a whole range of things to be done," said Dr. Marshall, who said confidence versus wariness over the new results varied greatly even among the 25 authors. He added, "We ain't there yet."
And even as paleontologists applauded the Herculean efforts to work on this bigger, better database, some suggest the approach itself may be flawed.
Some including Dr. Jackson say researchers may be better served by getting out of the library and into the field, doing new fossil surveys. Rather than fighting biases in data collected 100 years ago for other purposes, they could collect the most appropriate data in a uniform and comparable way.
Likely to continue to spur as much criticism as support, the analyses of the new database go on.
"I hope they're right," Dr. Erwin said of the new findings, "because life will be more interesting for the next 10 or 20 years if they are. We'll have to re-examine a lot of assumptions."
File Date: 5.22.01