Earth's "Wobbles" Spurring Cycles of Evolution and Extinction?

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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."

Novel Find

"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."

Alternate Explanations

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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