Earth's "Wobbles" Spurring Cycles of Evolution and Extinction?
for National Geographic News
|October 11, 2006|
Small changes in Earth's orbit and tilt may have regulated the cyclical rise and fall of many prehistoric mammal species, new research suggests.
Earth's orbital patterns are believed to drive long-term climate change.
Over millions of years these climatic shifts may have regularly spawned events that give rise to new mammal species.
They may have also caused the periodic extinctions that doomed other mammal lineages to oblivion, says a team of researchers led by paleontologist Jan van Dam of Utrecht University in the Netherlands.
"The question of climate's role in causing both evolution and extinction has been a big area of contention," said Tony Barnosky, a paleobiologist at the University of California, Berkeley.
Barnosky is not affiliated with van Dam's new research, which will appear in tomorrow's edition of the journal Nature.
The work is the first to document direct correlations between changes in Earth's orbit and patterns of speciation—the rise of new species—and extinction.
"I think they've nicely demonstrated a correlation between periodic climate events and what we'd recognize as normal turnover in fauna that would happen over and over again," Barnosky said.
Life and Death
Van Dam and colleagues combed through a fossil record from central Spain spanning some 22 million years—from 24.5 to 2.5 million years ago (map of Spain).
They studied about 80,000 fossils, predominately molars and premolars, to reveal when 132 different species of small rodents first arose—and when the animals disappeared.
The fossils, found scattered across more than 200 sites, appear to reveal a striking symmetry between the rise and fall of small mammal species and two cyclical "wobbles" in Earth's orbit. (Related: "Did Million-Year-Long Eruptions Cause Mass Extinction?" [May 2, 2006].)
The longer of the two cycles peaks about every 2.5 million years. This peak occurs when Earth's orbit around the sun most closely resembles a perfect circle. Most times Earth's orbit has a more oval shape.
A second cycle, of million-year peaks, occurs when the planet is tilted at a more extreme angle on its axis.
While Earth's tilt is currently 23.5 degrees, it cycles between 22 and 25 degrees. More tilt causes more severe seasonal differences.
Both of these regular "wobbles" are believed to significantly impact Earth's climate.
Analogous to the shorter Milankovitch cycles described in 1941, long climate cycles may result in periods of cooling, expanding ice sheets, and altered precipitation patterns.
Such long-term climate changes could explain the rise and subsequent fall of many species, van Dam's team says.
"During shorter time periods migration is the normal response to climate change," van Dam said. In these cases, mammals move to new habitats rather than adapt to changes in their current ranges.
"Apparently it requires more extreme change of longer duration for extinctions or to bring new species into existence."
"This is the first time someone has really shown the periodicity [of speciation, extinction, and climate change] on a timescale of several millions of years," said Elisabeth Vrba, a paleontologist at Yale University in New Haven, Connecticut.
"Mammal species do not respond to closely spaced cycles—they return to the status quo much too quickly," she said.
Vrba was not involved in the current research, but she is the creator of the "turnover pulse hypothesis." The concept suggests that major climactic changes spurred "pulses" of speciation, evolution, and extinction.
Mammalian species are typically believed to survive for an average of 2.5 million years, so van Dam and colleagues' work may present a striking explanation for previously recorded data.
"What's been well worked out is that there is an overall background rate of extinction during the past 60 million years," Berkeley's Barnosky said.
"What has not been worked out, except here, is a relation between these extinctions, migrations, and speciations and periodic climate cycles."
If orbit-driven climate change did cause species to appear and vanish, Yale's Vrba stresses that such events are quite different from catastrophic extinctions such as the Permian period die-off that eliminated some 70 percent of all land species and whose cause remains a mystery. Orbital climate change is more like geological business as usual.
"Mass extinctions, as far as we know, are not periodical," she said. "They are a very different kettle of fish.
"These [orbital cycles] are cycles of extinction and also speciation, where mammal species may be [arising] at the start of one of these cycles and bowing out at the other end," she said.
"It's part of the regular climactic heartbeat present throughout geological time."
Not everyone is convinced that the research is on the right track.
John Alroy is a research biologist at the National Center for Ecological Analysis and Synthesis in Santa Barbara, California.
In 2000 Alroy published a similar time series analyses of North American mammals (with co-authors Paul Koch and Jim Zachos from the University of California, Santa Cruz) that found no connection between climate changes and originations/extinctions.
He suggests that the new study used far too little data by identifying only 132 species over such a lengthy time period, which included many intervals of speciation and extinction.
He further suggests that the apparent periodicity of speciation and extinction is likely just a statistical artifact.
"At best, it suggests that you get more fossils in some climate regimes than others, which is not of biological interest," Alroy said.
"When you have certain climates, you have better rocks for preserving fossils, so you find more of them and then find more 'origination' and 'extinction' events that are in the wrong place," he continued.
"The stretches with low turnover are simply stretches with little sampling. The basic reason is that if you have no data, you can't show a species was present, so you can't show it originated or went extinct at that point."
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