Chance and Repeatability The "big five" mass extinction events decimated a large fraction of the organisms alive at the time. Yet many of these extinctions occurred randomly. Besides being wide-spread, there is no clear attribute that protected a group of organisms from extinction. Stephen J. Gould argues that chance has played a defining role in evolution. He asks us to re-run the tape of life and imagine possible alternative worlds: What if eukaryotes had not evolved? "Run the tape again, and the first step from prokaryotic to eukaryotic cell might take twelve billion instead of two billion years - and stromatolites, never awarded the time needed to move on, might be the highest mute witnesses to Armageddon." What if a different collection of metazoans had survived the Cambrian? A large number of unusual, high-level taxa evolved during the Cambrian, & most went extinct. Run the tape again, and primitive chordates bite the dust and some other oddball creature sneaks through. What if an asteroid had not hit the earth at the end of the Cretaceous? Would dinosaurs have gone extinct? Would mammals have proliferated? "Mammals spent their first hundred million years two-thirds of their total history -- as small creatures living in the nooks and crannies of a dinosaur's world." "The divine tape player holds a million scenarios, each perfectly sensible. Little quirks at the outset, occurring for no particular reason, unleash cascades of consequences that make a particular future seem inevitable in retrospect... The end results are so different, the initial perturbations so apparently trivial." Most evolutionary biologists would agree that chance plays a role in the direction of evolution (drift is all about the effects that chance events have on evolution!) The power of the neo-Darwinian view of evolution is in its ability to understand the responses of living organisms to the environment & to chance events. If we played the tape of life again, we might not arrive at exactly the same place today. But it's likely that some striking similarities would appear.