Oh yes; sorry; Lineweaver did say 1 billion, and that implies that one star in every 200-400 has an Earth-like planet. I suspect that number would include worlds which are quite earth-like but have never evolved a complex biosphere. It is quite an optimistic estimate, and means that there could be several Earth-like worlds within 50 light years (more, since the Earth-like worlds might be expected to congregate within the Galactic Habitable zone).
But the emergence of a complex biosphere, even on a planet which is quite Earthlike, may be a rare event; and every single instance of the emergence of life will follow its own evolutionary path (unless there are any cases of local panspermia, which may be rare but perhaps not vanishingly so).
We might expect some of those worlds to develop intelligent life; some instances of intelligent life would probably be bipedal, and some would have two arms and a head; some would have two eyes and a mouth. There may be billions of such races in the visible universe; even hundreds of billions. But how many of them would have human-like noses? But how many of them would have human-like jaws? But how many of them would have human-like skin? How many of them would have human-like backbones? How many of them would have human-like hands, feet, internal organs? It seems very unlikely that any alien species out of the billions we are considering would be similar enough to human to pass as such.
Parallel, or convergent, evoloution produces similar creatures on our world mostly because the creatures concerned are all closely related. Parallels between marsupials and placentals occur within the single class Mammalia; parallels between sharks and dolphins within the subphylum Vertebrata.
There are no Vertebrata on any other planets outside the Earth, no mammals anywhere else in the universe. Any phyla that may be found out there on extrasolar planets that resemble vertebrates or other terrestrial biota will surely be given a separate classification to indicate that they have evolved separately; they will not share any genetic material with Earthly organisms (once again, barring panspermia of some sort) so they will be accurately described as separate taxa.
Strictly speakiing there wont even be any animals,plants, fungi, archaea or eubacteria out there either, although some of the simplest forms may resemble our own simplest forms superficially. The more complex an extraterrestrial organism gets, the less likely it is to have an Earthly analogue.
Last edited by eburacum45 : 13-April-2008 at 01:52 PM.
Reason: spelling
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