Total # of species that have existed: common estimate 30 billion, some as high as 4 trillion. Regardless, 99.99% of all species ever are extinct - "To a first approximation, all species are extinct" [David Raup of Univ of Chicago].
Avg lifespan of a species, four million years.
1. amphibians and reptiles, ex Dimetrodon [sail-back], which was a synapsid. Four kinds of early reptiles, distinguished by number/location of small holes in the sides of their skulls: synapsids, with one hole in lower temple [Dimetrodon falls here], diapsids with two holes, euryapsids with a single hole higher up, and anapsids [no hole?? ... anapsids were precursors of turtles, which were actually poised to dominate the world at a point here; lurch left them to "settle for durability rather than dominance"]
2. Synapsids had divided into four four streams, only one of which survived beyond the Permian, and it evolved into a family of protomammals, called therapsids. These therapsids were megadynasty 2.
3. "Age of Dinosaurs" Under the therapsids, however, the diapsids had been evolving along, and one brance of the diapsids were to become the dinosaurs. So now under the dinosaurs, the therapsids almost entirely died out, leaving only small, burrowing creatures - little mammals.
4. "Age of Mammals" during the previous megadynasty, the largest mammals were no bigger than housecats, and most no more so than mice. But these, our ancestors, managed to survive the extinction that wiped out the dinosaurs.
Five major extinctions:
Ordovican - 440 mya, 80-85% wiped out
Devonian - 365 mya, 80-85% wiped out
Permian - 245 mya, >95% wiped out, incl a third of insects, only extinction known to affect them, end of trilobites [which had lasted for 310my), 52% of families [above genus and below order]
Triassic - 210 mya, 70-75% wiped out
Cretaceous - 65 mya, 70-75% wiped out
Lots of minor extinctions [not so bad to total species #s, but often horrible on specific populations]:
Hemphillian - nearly wiped out grazing animals; horses were down to a single species, which appears so sporadically in fossil record to seem like it was on brink of extinction for a while afterwards
approx 12 others
KT meteor: landed in shallow sea 10m deep at good angle, oxygen 10% higher than now so everything more combustible, floor of sea landed on was sulfur-rich rock [turned area size of Belgium into sulfuric acid, so for months afterward rain was acidic enough to burn skin], enormous explosion [100million megatons, meaning give everyone on planet a Hiroshima-sized bomb, you're still roughly a billion short of the KT impact]. Regional variation: New Zealand seemed to have very few problems, south hemi much less devastated than north. Some animals did well, including the turtles again - according to Flannery, period immediately following dinosaur extinction could be called Age of Turtles [16 species in north america survived, 3 more appeared almost immediately]. Almost 90% of land-based stuff went, only about 10% of water-based.
A timeline of life's history on earth
exterrestial origins, chemical evolutions, spontaneous development
early atmosphere: CH4/methane gas, NH3/ammonia, N2/nitrogen, CO2/carbon dioxide, H2S/hydrogen sulfate, H2O/water
CH2O -> urea -> amino acids -> adenine -> DNA & RNA
Protein & lipid "bubbles" -> microspheres -> proto-cells -> prok. cells -> euk. cells
Bill Bryson's A Short History of Nearly Everything, pages 342-347
A handout and notes from Mrs. Allen's Biology class