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2007

Meteorites May Have Fostered Life on Earth
Wired.com, December 21, 2007
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About 470 million years ago, back when our ancestors were jawless fishes and the land was ruled by insects, Earth was pounded by a series of enormous meteorites. The traces of that hammering still survive today in ancient rocks in southern Sweden and central China, where scientists have found exotic mineral grains found only in meteorites.

By measuring the amount of the grains in the rocks, the scientists calculated the rate of meteorite impacts jumped by a factor of 100 around 470 million years ago. A number of the impacts were big enough to leave 20-mile craters. The energy unleashed was 10 million times greater than the energy in the nuclear bomb dropped on Hiroshima.

At the same time, scientists have also been putting together a chronology of fossils from the same time, known as the Ordovician Period. They're recording when species first emerged in the fossil record, and when they disappeared as they became extinct. And this week in the journal Nature Geoscience, the scientists report that the impacts coincided with a drastic change in the world's biodiversity.

You might expect mass extinctions. The most famous of all impacts, a ten-mile-wide asteroid that hit the Earth at the end of the Cretaceous Period 65 million years ago, has been linked to mass extinctions that wiped out Tyrannosaurus rex, the other dinosaurs without wings, and about half of all other species on Earth.

But 470 million years ago, that's not what happened. Instead, the diversity of life took a sharp climb right after the meteorites started falling.

Scientists say the discovery is quite unusual. Nature Geoscience's press release declares, "These results are surprising as meteorite impacts are often more commonly associated with mass extinctions."

But actually, aside from the Cretaceous impact, no other case for impact-triggered mass extinctions has strong support today. Earth has regularly been pummeled by meteorites -- mostly small, but in some cases huge. Many of them have had little effect on life, and some may have actually fostered it. The new study of the meteor shower 470 million years ago is important because it's part of a trend, not an exception to the rule.

It may be hard to imagine that a huge rock from space could not have caused mass extinctions. But that idea is actually relatively new. In the late 1970s, Walter and Luis Alvarez, a father and son team of scientists, and their colleagues offered evidence that something very big hit the Earth right around the time the dinosaurs died, at the end of the Cretaceous Period. They claimed the impact was the kill mechanism. They were greeted with scoffs, but over the next couple decades other scientists found a wealth of supporting evidence, from shocked bits of quartz to the outline of the crater itself buried in Mexico.

The asteroid appears to have caused massive tidal waves and splashed molten rock far and wide, triggering gigantic forest fires. The soot from the fires blackened the sky, which was also loaded with other nasty compounds from the impact that may have caused a global blackout for months, acid rain, and severe global warming. These days, most scientists who study the impact don't think it's a coincidence that this environmental catastrophe came right around the time that 50 percent of the world's species became extinct. If not the sole killer, the impact was at the very least an accessory to the crime.

As years went by, the Cretaceous impact went from shocking hypothesis to icon. Who can count all the paintings that have been made of the crash -- often from a doomed dinosaur's perspective? The impact hypothesis had a big effect on scientists, too. It became a prime example of evolution's contingency, how its path can turn suddenly due to an abrupt change in the environment.

Some scientists began to argue that other impacts may have triggered other extinctions. They offered evidence of a rise in extinctions every 26 million years. They suggested that a star in our neighborhood periodically disturbed the cloud of comets surrounding our solar system, sending some towards our defenseless planet. Other researchers looked at big pulses of extinctions, such as the one at the end of the Permian Period, 250 million years ago, when over 90 percent of species disappeared. A few claimed to find traces of impacts that coincided with those major die-offs as well.

But these theories have lost a lot of their luster. The cycles of extinctions have blurred since scientists have learned how to make more precise estimates of the ages of rocks. It turns out some of the impacts linked to mass extinctions actually hit millions of years before or after the die-offs.

Meanwhile, geochemists and paleontologists are finding evidence that points to other culprits for the mass extinctions. Global warming appears to be behind many of the biggest. Volcanoes can unleash vast amounts of carbon dioxide, and heat-trapping methane can rise up from the sea floor. There's some evidence that these past episodes of global warming triggered a chain reaction of destruction, from the acidification of the oceans to the destruction of the ozone layer. It looks as if our planet can wreak its own destruction without the help of asteroids (or even people).

At the same time, impacts have proven surprisingly impotent. In one study, John Alroy of the National Center for Ecological Analysis and Synthesis, for example, surveyed all the impacts that occurred in the past 65 million years -- including some that left craters 50 miles wide or more. He then compared them to the well-documented fossil record of mammals in North America. He found no link whatsoever between impacts and changes in the extinction rate.

So how could an asteroid cause so much damage at the end of the Cretaceous, and so little at other times? It's always possible that there are extinction-triggering impacts yet to discover. It's also possible that impacts do cause extinctions, but not a lot. Earlier this year, for example, a team of scientists announced that a meteorite may have exploded over North America 12,900 years ago, killing off mastodons and other big mammals of the New World. But that's small potatoes compared to the major die-offs in Earth's history.

It's also possible that the impact at the end of the Cretaceous was a major fluke. It was unusually big, for one thing, forming the third biggest known crater on Earth. Its effects may have also been amplified because of where it hit the planet. Instead of plunging into the deep ocean, it smashed into a shallow shelf of calcium carbonate and other minerals. It lofted this deadly cocktail into the atmosphere, unleashing hell on life below. Smaller impacts that hit other places may not have been able to similarly break Earth's food network.

But how could meteorites drive up the diversity of life? The researchers aren't sure, but they suggest that the impacts may have disturbed old ecosystems, creating a new space in which new species could evolve. They're not the first scientists to argue that meteorites can be beneficial in the long term. A large impact can rework the ocean floor or a shoreline, or even scoop out a new lake. In these new habitats, new species can evolve.

It's particularly intriguing to compare the number of species in crater lakes to other lakes of the same size. Crater lakes are often much more diverse. It's possible the asteroids or comets that scoop out the craters also fertilize them with minerals and other nutrients, supporting more species.

This fertilization from space may have made life itself possible on Earth. Four billion years ago, comets and asteroids delivered water, organic molecules, and other raw materials necessary for life. Some of the early craters may have even served as natural beakers that synthesized new chemicals essential for life. Not every impact was so benign on the early Earth; judging from the craters still preserved on the moon, the biggest ones may have boiled off the oceans. But some researchers have also speculated that microbes could have been lofted into space, where they could have survived on meteorites, or perhaps on other planets, before reseeding Earth.

This new view of impacts doesn't mean that we shouldn't be worried about the asteroids that may cross our path in years to come. But what's bad for human civilization may not be so bad for life as a whole.

Copyright 2007 Carl Zimmer

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