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Edited on Mon Feb-16-09 05:30 PM by Occam Bandage
considering that the Earth had to undergo plenty of scrap-everything-and-start-over mass extinction events, yet few enough that each evolutionary flourishing could come into its own. Had there not been the most recent extinction event, it's likely that supermassive reptilians would still dominate Earth.
What I consider to be the beyond-no-return point of intelligence, creative language, has only been developed once in the history of the Earth, by one branch of overgrown squirrels. Heck, many of the prerequisites, like warm-bloodedness, may well have only developed once.
When you look at things that have developed frequently and independently, such as wings, teeth, and carnivorism, I would say that yes, it is somewhat likely we would find that on other planets, though I'd place no bets on the frequency life evolves beyond the bacterial stage. As for complex intelligent societies? I think that suggesting those are common is being a bit egotistical; our way of doing things seems far less stable than previous dominant species have managed, and stability wins in evolutionary history.
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