Evolution's Slow Recovery
The biosphere bounces back from mass extinctions with the origin of new species.
Two years ago, Earth scientist James W. Kirchner and paleontologist Anne Weil published what could be described as a good news-bad news story. The good news is that the biosphere bounces back from mass extinctions with the origin of new species, a process also called diversification. The bad news is that it takes ten million years.
|Earth scientist James W. Kirchner
This year, Kirchner followed up with more sobering news. Those peaks of diversification don't match the height of the extinction peaks that immediately precede them.
It's as though the extinction goes through with a bang and then ten million years later you hear the echo in diversification," Kirchner says. So the first paper was about: Those evolutionary echoes of extinction are delayed. This paper is about: Not only are those echoes delayed, they're also damped."
Kirchner, at the University of California, Berkeley, and Weil, at Duke, study a large database of extinctions and diversification of marine animals throughout the history of multicellular life, originally compiled by the late University of Chicago paleontologist Jack Sepkoski. Looking at a graph of these raw data two spiky lines like a graphic representation of sound waves at a construction site tall extinction spikes catch the eye. A closer look reveals shorter spikes in the second line, indicating the emergence of new families and genera (groups of related species). The same held true for the background ebb and flow of evolution, Kirchner says. We found that that time delay was still ten million years, even after you take all the big extinctions out of the running."
In the current work, published in the January 3, 2002, issue of Nature, Kirchner used spectral analysis, a mathematical method used widely in astrophysics, to determine the variability of the two wave forms. (Kirchner and Weil used the same analytical method in their March, 2000, Nature paper.) Is extinction intrinsically a faster, more spiky process than diversification?
Kirchner's results are clear, says Arnold I. Miller, a geologist at the University of Cincinnati. Based on Kirchner's analysis, it does seem that origination is slower to respond, if you will, or less responsive, on short time scales, than extinction. I think that this is interesting."
Miller's use of the term on short time scales" is important. If extinction is a hare, quick off the block, diversification is a tortoise, winning in the end. Over the long term, the Sepkoski data show the number of families and genera increasing. At some periods in the geological record, the extinction line remains below the diversification line, and at a few points, diversification peaks occur in the absence of extinction. But the definitions of short" and long" become crucial when the subject is geological history of the Earth. Kirchner found the lower variability of diversification in time spans of below 25 million years. At longer wavelengths Kirchner also measured wavelengths of 25-50 million years and above 50 million years separately both processes seem similarly variable.
The Sepkoski data set has been the gold standard for the geological record of extinction and diversification, says Douglas H. Erwin, chair of the paleobiology department at the Smithsonian Institution. It's been fundamental to most of what paleobiologists have done for the last 20 years. That doesn't mean there aren't problems with it. There have been a number of papers in the past year suggesting that the data may be biased by the amount of available rock." In the latest of those papers, March 28, 2002 in Nature, Shanan E. Peters and Michael Foote, both of the University of Chicago, argue that some of the spikiness of the extinction record may arise from the spotty availability of intact fossil-containing rock.
Kirchner's conclusions, though, apply to differences between the extinction data and the diversification data. If this were simply an artifact of preservation," he says, I would expect to see similar effects on diversification rates as on extinction rates. That is, any time extinction rates go up artificially because preservation is bad, I would expect diversification rates to go down artificially for the same reason."
|Paleontologist Anne Weil
However solid, the research raises the question, Why should diversification after an extinction take so long?" The answer lies partly in correcting a misunderstanding of how the biosphere works.
The place a species holds in an ecosystem, known as a niche, is more than just a physical location that can be quickly refilled by a new species.
The standard argument," Erwin says, has been that extinctions create new niches or open up new niches. I think that's probably wrong. That's probably what happens when you have a small-scale ecological crisis. But probably not during a mass extinction. In mass extinctions, what's probably happening is that the ecospace is collapsing. It has to be reconstructed rather than simply be refilled."
"Extinction is like knocking down a house of cards, Kirchner says. You can only put new cards into the house as you rebuild the entire house."
Today, humans are removing cards from the house with every acre of rain forest we clear and every road we build. We are causing the extinction of many species. I don't think there's any question about it," Miller asserts. But whether such anthropogenic extinctions will reach the levels seen in the fossil record remains an open question, still debated by paleontologists.
Kirchner doesn't venture an opinion on how massive current anthropogenic extinctions are. But, he says, his research tells us that if we ultimately impoverish global biodiversity, the consequences of that will outlast all of us as a species. It will be the longest-lasting legacy that we leave on this planet. And conversely, what we do to sustain biodiversity will also be a long-lasting legacy."
Kirchner and Miller agree that a major challenge for the future is to gather better data on past extinctions and evolutionary responses. This is a focus of both Erwin's and Miller's ongoing research. I think the biggest challenge now is to move beyond the statistics and to look in detail at these specific events in Earth history, who goes extinct and why, who comes into existence and why," Kirchner says.
The other thing we all very badly need is better paleontological data." Just such data is the aim of The Paleobiology Database a project overseen by John Alroy and Charles Marshall and involving Erwin, Miller and dozens of other paleobioloby researchers which Kirchner says may be years or even decades in the making.
What they're aiming to do is to build up a database that not only records the first and last occurrence of each species, but also catalogues all the places and all the points in geological time that those fossils are found, so that you can begin to put together the global patterns of who and where and when. And then, hopefully, from these patterns we'll learn something about the really big questions, the how' and the why.'"
Related Web Pages
Digital Zookeepers Take a Census
The Paleobiology Database
John Alroy's Research Interest page
Recovery after extinction