杏吧原创

FORGET the dinosaurs. To the true devotee of destruction there鈥檚 only one
mass extinction worth talking about. Four times as ancient as its overhyped
rival, the die-off that ended the Permian period was also much more devastating.
Two hundred and fifty million years ago, it wiped out up to 95 per cent of the
species living at the time. And no one knows how.

鈥淚magine standing in a forest, walking on a prairie or snorkelling on a coral
reef. Now imagine the same environment with most of the species gone,鈥� says
David Jablonski, a palaeontologist and extinction expert at the University of
Chicago. Human impacts don鈥檛 come near it. 鈥淥f course, there are parking lots in
the suburbs now where there used to be prairie, but the Permian wiped out almost
everything everywhere on the planet.鈥�

The Permian mass extinction sealed the fate of the ubiquitous trilobites. It
wiped out all but two lines of a wild and baroque array of sea urchins. On the
large and menacing end of the scale, it lowered the curtain on the toothy,
eight-foot-long reptiles known as gorgonopsids. It is also the only extinction
event ever to have come close to halting the long triumphal march of the
insects.

The doomsday device, whatever it was, killed off multitudes of species both
on land and in the ocean. It appears to have acted fast, probably in less than a
million years which is a blink of the eye in geological terms. And it left
behind a tantalising and unexplained anomaly, a sharp increase in the ratio of
carbon-12 to carbon-13 on land and in the sea. Clearly, scientists have some
explaining to do.

The search for the smoking gun has led investigators to the Siberian Traps, a
vast expanse of hardened lava spewed out by volcanoes in the late Permian.
Today, the Traps (the name in Swedish means 鈥渟tairs鈥�, which refers to the
stepped appearance of the eroded basalt) cover a chunk of Siberia as large as
the US and up to four kilometres thick. Geologists have discovered that the
million-year period during which the volcanoes were oozing and burping lava
coincides suspiciously with the time of the Permian extinction. What did the
deed almost certainly was not lava itself, but rather the gases that bubbled out
with it: sulphur, water vapour, methane and carbon dioxide.

Paul Renne, director of the Berkeley Geochronology Center and the experts鈥�
expert on the Siberian Traps, says there are two ways in which the volcanoes
could have caused such widespread extinctions. 鈥淎nd the funny thing is, they鈥檙e
opposite in effect,鈥� he says.

In Renne鈥檚 first scenario, the sulphur oxidises and forms aerosols floating
around in the atmosphere. The aerosols reflect sunlight away from Earth, causing
temperatures around the globe to plummet. Over tens of thousands of years, ice
builds up at the poles. The sea level falls, and ocean circulation changes.
That鈥檚 very disruptive, to say the least, to life in the oceans. Land-dwellers
suffer, too, as enormous quantities of sulphur wash out of the atmosphere in the
form of acid rain.

The other scenario centres on heat. Carbon dioxide, water and methane are
greenhouse gases. Expelled by the volcanoes, they trap solar radiation near the
Earth鈥檚 surface, causing global warming that bakes the biosphere.

鈥淢y favourite hypothesis is that it was caused by heat,鈥� says Peter Ward, a
geologist at the University of Washington in Seattle. Working in South Africa
with geologist Roger Smith of the South African Museum in Cape Town, Ward has
found evidence that the extinction of land animals occurred more slowly in high
latitudes, indicating that they were a refuge of some sort鈥攑erhaps from
the killer heat of the tropics. 鈥淭here鈥檚 no doubt that it was already really hot
in the Permian. There鈥檚 no evidence for vertebrate fossils anywhere in
equatorial regions at that time. I think vertebrate life was found at the
辫辞濒别蝉.鈥�

But Renne sees no need to choose. 鈥淚 think it鈥檚 possible you have both
effects, because they don鈥檛 cancel each other out,鈥� he says. Sulphur-driven
cooling is almost instantaneous, while greenhouse gases act more slowly. 鈥淪o you
have a quick cold snap with lowered sea level and acid rain followed by warming.
It could be the environmental stress of this pulse of extremes happening in
quick succession that causes the extinction. But what we have is tons of
conjecture and very little evidence.鈥�

The same can be said of that mysterious carbon-12 spike. Because plants take
up carbon-12 in preference to other carbon isotopes, an increase in the
carbon-12 to carbon-13 ratio might indicate that a lot of plants suddenly died
and released their carbon. By fire, perhaps, after volcanic lava set nearby
forests ablaze. Or perhaps the spike came from the mortal remains of plankton
and other marine creatures that perished en masse when whatever happened,
happened. On the other hand, volcanoes spew out plenty of carbon-12, too.

And some researchers think the Traps may not have been the only Doomsday
Machine at work. Douglas Erwin, a palaeontologist at the National Museum of
Natural History in Washington DC, believes that volcanic gases cannot fully
explain the mass extinction. He says that there is not enough evidence of a drop
in sea level or extensive glaciation. It鈥檚 not even clear that the Siberian
eruptions started before the extinctions did.

Instead, Erwin thinks that other factors may also have played a part鈥攁
comet or asteroid, for example. No one has linked the Permian extinction to an
impact crater yet. But a comet wouldn鈥檛 necessarily leave one, he points out. If
a large comet slammed into the Earth鈥檚 atmosphere it might vaporise, unleashing
deadly quantities of CO₂ and other chemicals. A 鈥渕elting gun鈥�,
perhaps?

It鈥檚 even possible that it took more than one disaster to send the Earth鈥檚
ecology into a tailspin. 鈥淚t could be `the world went to hell in a hand basket鈥�
model of extinctions,鈥� says Jablonski. If a string of catastrophes struck the
planet close together, their individual effects might be all but impossible to
tease out.

Perhaps the most remarkable thing about the Permian extinction is how quickly
life bounced back. A mere 80 million years after hitting rock bottom, living
creatures were more diverse than ever before. Although the menacing gorgonopsids
didn鈥檛 make it, the ancestors of the dinosaurs managed to squeak through.

As did some squat, stubby-legged carnivores called cynodonts. That鈥檚 a good
thing, because otherwise there might not be anyone around to wonder about what
happened on Earth 250 million years ago. The cynodonts were the ancestors of all
modern mammals, including, of course, us.