TOP 5 EXTINTIONS

Big 5 die-offs

Over the last half-billion years there have been five major wipe outsin which over half of living creatures disappearedwithin a geological blink of the eye

Most of the scientists agree that a "mass extinction" event is under way the earth's wildlife disappearing at a alarming rate, mainly due to human activity.

But this is not a first time: over the last half-billion years there have been five major wipe outs in which well over half of living creatures disappeared within a geological blink of an eye. All told, more than 90 percent of organisms that have ever strode, swam, soared or slithered on Earth are now gone.

Here are the biggest die-offs, each showing up in the fossil record at the boundary between two geological periods:

ORDOVICIAN EXTINCTION

When: about 445 million years ago

Species lost: 60-70 per cent

likely cause: short but intense ice cage

Most life at this time was in the oceans. It is thought that the rapid, planet-wide formation of glaciers froze much of the world's water, causing sea levels to fall sharply. Marine organisms such as sponges and algae, along with primitive snails, clams, cephalopods and jaw-less fish are called ostracoderms, all suffered as a consequence.

DEVONIAN EXTINCTION

When: about 375-360 million years ago
species lost: up to 70 per cent
likely cause: oxygen depletion in the ocean

Again, ocean organisms were hardest hit. Fluctuations in sea level, climate change, and asteroid strikes are all suspects. one theory holds that the massive expansion of plant life on land released compounds that caused oxygen depletion in shallow waters. Armored, bottom-dwelling marine creatures called trilobites were among the many victims, tough some species survived.

PERMIAN EXTINCTION

When: about 252 million years ago
species lost: 95 per cent
possible causes: asteroid impact, volcanic activity


The mother of all extinctions, the "great dying" devastated ocean and land life alike, and is the only event to have nearly wiped out insects as well. Some scientists say the die-offs occurred over millions of years, while others argue it was highly concentration in a 200,000-year period.

In the sea, trilobites that had survived the last two wipeouts finally succumbed, along with some sharks and bony fishes. On land, massive reptiles known as moschops met their demise. Asteroid impacts, methane release and sea level fluctuations have all been blamed for this extinction. 

TRIASSIC EXTINCTION

When: about 200 million years ago
species lost: 70-80 per cent
likely causes: multiple, still debated

The mysterious triassic die out eliminated a menagerie of large land animal, including most archosaurs, a diverse group that gave rise to dinosaurs, and whose living relatives today are birds and crocodiles. Most big amphibians were also eliminated.

One theory points to massive lava eruptions during the breakup of the super-continent Pangea, which might have released huge amounts of carbon dioxide, causing runaway global warming. Other suspect asteroid strikes are to blame, but matching craters have yet to be found.

CRETACEOUS EXTINCTION

When: about 66 million years ago
species lost: 75 per cent
likely cause: Asteroid strike

An space rock impact is suspected No. 1 for the extinction event that wiped out the world's non-avian dinosaurs, from the T-Rex to the three horned Tricera tops. A huge crater off mexico's Yucatan Peninsula supports asteroid hypothesis.
But most mammals, turtles,crocodiles and frogs survived, along with birds as well as most sea life, including sharks, star-fish and sea urchins. With dinosaurs out of the way, mammals flourished, eventually giving rise to the species-Homo sapiens-that has sparked the sixth mass extinction.