The human impact on fragile eco systems could spawn a deadly scenario of species becoming extinct one after the other, leading to the possible extinction of life on Earth, according to scientists.

If vulnerable species keep disappearing from the face of Earth, and the current rate of extinctions goes on unabated, the possibility of amphibians, mammals, plants and fish getting extinct is quite high, the San Jose Mercury News reported, quoting scientists at UC Berkeley.

Life on Earth is hurtling toward extinction levels comparable with those after the dinosaur-deleting asteroid impact of 65 million years ago, propelled forward by human activities, according to scientists from UC Berkeley, the report says.

Once you lose species, you don't get them back. It takes millions of years to rebound from a mass extinction event, Mercury News quoted Nicholas Matzke, a UC Berkeley graduate student who authored a study on doom-gloom scenario, as saying.

The scientists are talking about the possibility of Earth losing about three quarters of its species in a span of three hundred years which, in cosmic terms, is just the time you take for a wink.

The UC Berkeley paper basically compares current day trends in species extinction to the vicissitudes surrounding the many mass extinctions that happened on Earth during its long life.

It has been generally accepted that Earth has witnessed five mass extinctions (called the Big Five) in its history, the severest being the one that occurred 65 million years ago which resulted in the extinction of dinosaurs.

It is said that when 75 percent of all species disappear from Earth within a geologically short time frame, the phenomenon can be called a mass extinction.

In the past 540 million years there have been five major events when over 50% of animal species died, a Wikipedia entry says. The entry adds that several major mass extinctions have significantly exceeded the background extinction rate.

The UC Berkeley scientists compared recent rates with species die-offs during the Big Five, taking into account presently endangered species, in order to assess if current species disappearances could tantamount to mass extinction rates.

They found that extinction rates are dramatically higher than expected, though the percentage of vanishing species is not elevated as yet. Berkeley paleobiologist and lead study author Anthony Barnosky said things are clearly going extinct too fast today, according to the Mercury News. The article also quotes a Stanford University biologist as saying that evidence of the sixth extinction is all around.'

Wikipedia says mass extinctions happen when a long-term stress is compounded by a short term shock. According to it the most commonly suggested causes of mass extinctions include sea level falls, asteroid or comet impacts, sustained and significant global cooling, sustained and significant global warming, hydrogen sulfide emissions from the seas, movement of the continents and a nova, supernova or gamma ray burst.