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May 18, 2007
Contact: Brendan M. Lynch, University Relations, (785) 864-8855.

Looking to heavens, KU researchers solve puzzling extinctions on Earth

Our solar system, represented by the white dot, moves around the center of the galaxy (like planets around the sun). It also moves up and down around the mid-plane of the galaxy. The mid-plane is shown by the dashed white line. The solid green line represents the up-and-down motion of the solar system as it circumnavigates the galaxy.

LAWRENCE — In 2005, University of California-Berkeley researchers Robert Rohde and Richard Muller revealed that massive die-offs occur on Earth with startling regularity — about once every 62 million years. But scientists have struggled since to pinpoint a cause for the consistently recurring extinctions uncovered in the fossil record. Now, a team of University of Kansas professors has found the likely reason by looking to the heavens.

According to Adrian Melott, professor of physics and astronomy at KU, the motion of the solar system exposes Earth to an onslaught of cosmic rays on a schedule that is synchronized to the mass extinctions. Melott and colleague Mikhail Medvedev, associate professor of physics and astronomy, presented their idea in April at a meeting of the American Physical Society. Since then, it has gained worldwide attention.

“I spent quite a bit of time obsessing about what could possibly be going on,” said Melott. “When I heard about Rohde and Muller’s result on biodiversity and the 62 million-year period that they identified, I thought that it must have something to do with our galaxy.”

For Melott and Medvedev, the clockwork-like movements of the solar system through the Milky Way and the galaxy’s own rush through the universe are the keys to understanding the mysterious cycle of extinctions.

“The galaxy is a thin disc, like a Frisbee,” said Melott. “But the galaxy’s motion is not like that of a Frisbee, but like a pie in the face.”

The KU researchers hypothesize that the leading, north side of the Milky Way generates a shock wave as the galaxy plunges through the universe.

When the solar system periodically journeys up to the north boundary of the galaxy — about once every 64 million years — the galactic shock wave exposes Earth to a huge dose of high-energy radiation.

“I did notice that not only did these time scales appear to be almost the same, but the drops in biodiversity coincide with the times when the sun is on the north side of the galactic disc,” said Melott. “I already knew the north side of the galactic disc was the direction toward which the galaxy is falling.”

The 62-million-year estimate of the extinction cycle and the 64-million-year estimate of sun's motion match, because both calculations have a 3-million-year uncertainty. “In the scientist’s eyes, they’re the same number,” said Melott.

Melott said a bath of cosmic rays produced by the Milky Way’s shock wave could cut down numbers of Earth’s species in a variety of ways: boosting exposure to elementary particles called muons, creating a blanket of planet-cooling clouds and damaging the ozone layer so that increased radiation causes more mutations, cancers and cataracts.

According to Melott, radiation weakens the biosphere for prolonged periods so that when sudden events occur, such as meteor hits or volcanic eruptions, the disturbance to life on Earth is more severe. “It’s like having the flu and then getting shot,” Melott said.

But the news is not all bad for Earth’s inhabitants. Data shows that while overall diversity of Earth’s species drops during the 62-million-year cycle, it also rebounds every time, like a spring.

“Radiation could increase the number of mutations and also help new species arise,” said Melott. “During these times of lowered biodiversity a lot of new species come into existence, which ride the wave up to the new peak of biodiversity.”

As for the next die-off, there is plenty of time to prepare. “We’ve just passed the mid-plane of the galaxy,” said Melott. “We’re on the way up and we’ll reach a peak in about 10 or 12 million years. That’s when the radiation should start getting bad again — if our idea is right.”

Melott cautioned that in science most ideas are wrong and said it is too soon to know if the theory is a leap forward in understanding the mechanism of evolutionary biology.

However, the discoverer of the die-off cycle thinks Melott and Medvedev may be on the right track.

“They succeeded where I failed in coming up with a possible explanation for the effect that we observed,” said UC Berkeley’s Richard Muller. “It’s the most elegant solution that’s been uncovered.”

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