BioEd Online

The explosion of a star in our cosmic neighbourhood may not sound like good news for life on Earth. But a team of US researchers says that just such a catastrophe could have showered our planet with fertilizer that helped plants to colonize the land about 440 million years ago¹.

Scientists have speculated for at least a decade that astrophysical explosions called gamma-ray (-ray) bursts could have caused some of the mass extinctions in Earth's distant past.

These explosions are thought to be either the by-product of a supernova, when an old star explodes, or the result of a collision between ultradense bodies called neutron stars. They release torrents of high-energy radiation (-rays) focused into twin 'lighthouse' beams.

All the bursts seen so far have occurred in distant galaxies, but in the past billion years at least one is likely to have occurred close enough to Earth to have had some dramatic effects on our planet. A nearby burst would have destroyed much of the ozone layer that protects the Earth's surface from the Sun's harmful ultraviolet rays, so that many species would have been fried.

Dark age

To make matters worse, the -rays would convert nitrogen and oxygen in the atmosphere into nitrogen dioxide, a brown, toxic gas that is released today in vehicle exhaust, causing urban smog. A '-ray burst smog' would have cast a shadow over the planet and could even have triggered an ice age.

Two years ago, Adrian Melott of the University of Kansas claimed that this might have happened at the end of the Ordovician period, about 440 million years ago. The geological record shows evidence at this point of mass extinctions and global cooling².

Melott has now collaborated with astrophysicists and atmospheric scientists to develop a computer model of the effects that a nearby -ray burst might have. The initial results, which they reported earlier this year, looked like an unrelenting litany of disaster³. Ozone depletions of about 35% globally (and much more in some spots), tripled intensity of ultraviolet light, widespread DNA damage to surface organisms, and massive formation of nitrogen oxides and consequent acid rain. It all looked very gloomy.

Fertile rain

But the researchers have now delved into another side-effect. Nitric acid produced from nitrogen oxides might indeed shower the Earth with corrosive rain, but this would subsequently enrich the land in nitrate, which is an essential nutrient for plants. Today, farmers add nitrate to their soil in fertilizers.

The geological record shows that it was precisely in the late Ordovician period that plants began to spread extensively over the land, in the crucial first step of colonization by life. So if a -ray burst did cause widespread extinctions, it may also have boosted plants' growth.

"There was very little life on land at the end of the Ordovician, essentially just algae," says Melott's Kansas colleague Brian Thomas. "Plant life began to really take hold after this period, and we think that the nitrate deposited after a -ray burst could have aided this transition."

"Nitrate is always a limiting factor for plant growth, and would have been much more so then, before there were nitrogen-fixing plants on land," says Melott. "The three to five years of extra nitrate that we predict could have boosted plants trying to get a foothold on land."