A statistical analysis of the rates of extinction and origination in the fossil record shows that life seldom rebounds rapidly from an extinction. The results imply that the diversification of life obeys "speed limits" set by evolutionary processes, said study author James Kirchner, professor of earth and planetary science at UC Berkeley.

"There seem to be biological mechanisms that limit diversification of new organisms and control which ones become successful enough to persist," he said. "Biodiversity is slow to recover after an extinction."

This apparent speed limit on the rate at which surviving organisms evolve and diversify has major implications for present-day extinctions.

"If we substantially diminish biodiversity on Earth, we can't expect the biosphere to just bounce back. It doesn't do that. The process of diversification is too slow," Kirchner said. "The planet would be biologically depleted for millions of years, with consequences extending not only beyond the lives of our children's children, but beyond the likely lifespan of the entire human species."

The paper by Kirchner appears in the Jan. 3, 2002, issue of the journal Nature.

Kirchner has been mining a fossil database created by the late University of Chicago paleontologist Jack Sepkoski, who catalogued the genera and families of fossil marine animals over the past 530 million years, from the Cambrian to the present. Using a technique called spectral analysis, Kirchner has looked for patterns in the rates at which new organisms appear or disappear.

Last year Kirchner and colleague Anne Weil reported that the Earth needs, on average, about 10 million years to recover from global extinctions, whether they involve the loss of most life on Earth or wipe out far fewer species. This was much longer than most scientists thought.

The new results come from asking a related question: How do rates of extinction and diversification vary, and how are they related? This is important because, if rapid diversification is possible, biodiversity might be able to rebound quickly from a global extinction.

Kirchner's analysis found that extinction rates and diversification rates are about equally variable over long spans of geological time. Over shorter periods, however, diversification rates vary much less than extinction rates do. That means that evolution doesn't accelerate quickly in response to rapid bursts of extinction.

One possible explanation for why diversification takes so long to rev up after an extinction is that extinction eliminates not merely species or groups of species, but takes away ecological niches. It eliminates both organisms and the roles those organisms played in the ecosystem. Recovery thus becomes more complicated.

"This shows that extinction is not like knocking chess pieces off a chessboard, with the empty squares ready for you to plunk down new pieces," Kirchner said. "Extinction is more like knocking down a house of cards. You only have places to put new cards as you rebuild the structure of the house."

It's as if the ecosystem must bootstrap its way to the level it was at before.

"For a new kind of organism to evolve and survive long enough for us to notice it - for it to become common enough to leave a fossil record - requires that it have an evolutionary niche. The organism has to have some role in order to succeed in its ecosystem," he said. "As a result, the ecosystem must first increase in complexity so there are niches for new organisms to fill, which is probably a very complicated process.

"At a fundamental biological level it takes time to build niches, evolve new organisms and filter out unsuccessful ones, although it's not yet clear what all the limiting factors are."