This first planetwide extinction occurred at the close of the Cambrian about 100 million years after the first plantewide explosion of sea life.
Evolution was a speed demon back in the early Cambrian. All kinds of sea life-in the forms of crablike trilobites, bivalve brachiopods, and other critters-expanded and filled the shallows. This is beautifully described in the 1989 award-winning science book "Wonderful Life" by the late geo scientist Stephen Jay Gould.
As far as we know, no living thing yet occupied the Earth's barren supercontinents of Gondwanaland and Laurentia (although it's suspected microbes were already breaking down terrestrial shield rocks).
So what caused this great explosion of "Wonderful Life"? Well, paleontolgists guess that environment, oxygen and climate played big roles. Ocean temperatures and oxygenation were ideal for life. But then something mysterious occurred and crashed the system.
There's some evidence that a global ice age at the end of the Cambrian may have chilled the seas and reduced the oxygen content of the water. But what caused the ice age? Here are a few suggestions offered: cosmic impact (asteroid, comet or NEO swarm), climate change, supervolcanism, gamma ray bursts, plate tectonics-or maybe a combination of some, or all, of the above.
Not satisfied with such vagueness in the physical sciences? Well, think about today's climate change/global warming tussle; there's just no consensus regarding the Cambrian-Ordovician extinction.
•Extinction 2-known as Ordovician-Silurian Extinction-occurred about 444 million years B.C. This extinction may actually have been a series of events. No matter, it was the second greatest of the five extinction events on Earth. Again, sea life was affected, and again the event(s) ushered in a deep ice age. Gone was the "greenhouse" Earth of the warm Ordovician Period. Atmospheric CO2 crashed, too, and with it went many species. Cosmic or terrestrial agent to blame? It's still a mystery.