There were five Major mass extinctions in
Earth's
history, (not counting the current ongoing human caused mass extinction).
Late Ordovician
The Late Ordovician extinction occurred about 440 million years ago, it killed 60 percent of all marine species.
Possible cause; fluctuation in sea levels.
Late Devonian
The Late Devonian extinction occurred about 365 million years ago, killing about 50 percent of all marine species.
It is belived to have been caused by global cooling and loss of oxygen in the atmosphere.
Permian-Triassic
The Permian-Triassic extinction occurred about 250 million years ago, it killed 84 percent of all marine species and 70 percent of all land species.
Possible causes are; asteroid or comet impact, severe volcanism, and fluctuations in climate or sea level.
Late Triassic
The Late Triassic extinction occurred about 200 million years ago, and it killed about 50 percent of all marine species. It was belived to be caused by volcanism and global warming.
Cretaceous-Tertiary
The Cretaceous-Tertiary extinction, also known as the K-T extinction, which wiped out the dinosaurs, killed 18 percent of all land species, and 75 percent of marine species. It is belived to have been caused by an asteroid or comet impact.
Cletus the Foetus points out that:
"x% of whatever kind of species" does not mean "x% of life."
"Since some species are more represented, a given percentage of species could be much more, or much less, than that percentage of life"
and asks:
"...what percentage of land and marine species are being lost due to humans."
The Human Extinction
Timeframe: ongoing.
A majority of the worlds biologists belive the earth is in the midst of the fastest worldwide extinction in its 4.5 billion year history. The current species loss is greater and faster than that durring the K-T extinction. One-fifth of all living species could disappear within 30 years.
In a poll of 400 scientists commissioned by New York's American Museum of Natural History this problem was "ranked as one of the planet's gravest environmental worries, surpassing pollution, global warming and the thinning of the ozone layer..."
Sources:
http://www.enn.com/enn-features-archive/1998/09/091698/fea0916_23526.asp
http://www.well.com/user/davidu/extinction.html