Well, there are several bursts in species diversity I can think of. The Cambrian and Ordov

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Well, there are several bursts in species diversity I can think of. The Cambrian and Ordovician explosions resulted in a vast increase in animal diversity. Likewise, after the one-two punch of the Permian and Triassic extinctions, the number of marine animals rose steadily (**) to an all-time high (*) just prior to the spread of humans. (**) biggest exception being the K/T (bye bye dinos) extinction (*) about 800 families Also, plants arose from green algae and colonized the land in succesive sweeps. Mosses colonized very wet environments first, ferns (who had evolved vascular tissues) took over more territory when they evolved (1). These were eventually (mostly) replaced by gymnosperms (pines and the like) (2) and then (mostly) displaced by angiosperms (flowering plants -- now the dominant plant group on the planet(3). Fungi also radiated greatly with the invasion of the land. (1) around the carboniferous (up to about 200 families) (2) around the triassic (up to maybe 250 families) (3) starting in the cretaceous (rising to about 600 families currently) It's unclear (to me at least) what the max equilibrium number of species the earth can hold (***) and if it has ever hit this in the past. It could be (warning: speculation alert) that diversity has never reached a peak because mass extinctions happen often enough to keep the total number down. (***) This would depend a great deal on how fragmented specific ecosystems were. See Cowen's book "History of Life" for a not-too-technical run-down on, well, like the title sez, the history of life. Or see, Wilson's "Diversity of Life" for a view centered more on current ecology -- this is (IMHO) the best popular biology book of (what the hell, I'll say it) all time. Chris Colby --- email: colby@bu-bio.bu.edu --- "'My boy,' he said, 'you are descended from a long line of determined, resourceful, microscopic tadpoles--champions every one.'" --Kurt Vonnegut from "Galapagos"


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