t that physicists and chemists tend to believe in intelligent extraterrestrial life； while biologists tend not to。 Many biologists feel the development of intelligent life on Earth required so many peculiar steps that it represents a unique event in the universe； that may never have occurred elsewhere。〃
〃Wouldn't intelligence arise again and again？〃 Barnes said。
〃Well； it barely arose on the Earth；〃 Beth said。 〃The Earth is 4。5 billion years old； and single…celled life appeared 3。9 billion years ago…almost immediately； geologically speaking。 But life remained single…celled for the next three billion ＇＇62＇＇ years。 Then in the Cambrian period； around six hundred million years ago； there was an explosion of sophisticated life forms。 Within a hundred million years； the ocean was full of fish。 Then the land became populated： Then the air。 But nobody knows why the explosion occurred in the first place。 And since it didn't occur for three billion years； there's the possibility that on some other planet； it might never occur at all。
〃And even after the Cambrian； the chain of events leading to man appears to be so special； so chancy； that biologists worry it might never have happened。 Just consider the fact that if the dinosaurs hadn't been wiped out sixty…five million years ago…by a et or whatever…then reptiles might still be the dominant form on Earth； and mammals would never have had a chance to take over。 No mammals； no primates。 No primates； no apes。 No apes； no man 。。。 There are a lot of random factors in evolution； a lot of luck。 That's why biologists think intelligent life might be a unique event in the universe； only occurring here。〃
〃Except now；〃 Ted said； 〃we know it's not a unique event。 Because there is a damn big spacecraft out there。〃