Ancient Mass Extinctions Caused by Cosmic Radiation, Scientists Say

Scott Norris
for National Geographic News
April 20, 2007

Cosmic rays produced at the edge of our galaxy have devastated life on Earth every 62 million years, researchers say.

The finding suggests that biodiversity has been strongly influenced by the motion of the solar system through the Milky Way and of the galaxy's movement through intergalactic space.

Mikhail Medvedev and Adrian Melott, both of the University of Kansas, presented their new theory at a meeting of the American Physical Society earlier this month.

The theory offers the first explanation for a mysterious pattern previously noted in the fossil record.

"There are 62-million-year ups and downs in the number of marine animals over the last 550 million years," Melott said.

Until now, however, even the scientists who first discovered the cyclical pattern had not been able to explain it. (Read related story: "Mystery Undersea Extinction Cycle Discovered" [March 9, 2005].)

A number of possible explanations had been considered—including volcanic activity, comet impacts, and changes in sea level—but none could account for the phenomenon's regularity.

The Kansas researchers discovered that high rates of extinction in the cycle coincide almost perfectly with periodic "excursions" of the solar system outside the central plane of the Milky Way galaxy.

"Excursions to galactic north coincide with drops in biodiversity," Melott said.

During these periods, which include some of the largest mass extinctions known from the fossil record, Earth is bombarded with high levels of cosmic radiation.

The radiation may harm biodiversity by causing mutations or by triggering climate change, the researchers said.

Richard Muller is the University of California, Berkeley, physicist who first discovered the 62-million-year cycle with his graduate student Robert Rohde.

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