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Originally Posted by eburacum45
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).
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It doesn't strike me as overly optimistic, given the increasing evidence that planets may be quite common. But even if the number is high by a factor of 2, or 5, or even 10, I don't think that it would change things significantly. Our estimate of the number of planets on which sentient life would evolve could be pessimistic by several orders of magnitude.
It's interesting that you mentioned the GHZ, since Lineweaver coauthored one of the few papers that I've been able to find on the subject. [Charles H. Lineweaver, Yeshe Fenner and Brad K. Gibson (January 2004). "The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way". Science 303 (5654): 59–62.] I believe the full text of the paper is still available online. I used it as a reference for a Wikipedia article on the GHZ, which has since been merged into the Habitable Zone article.
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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).
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I agree that species may evolve along quite a different path than that which occurred on Earth, although I still maintain that the necessity of adapting to the environment will require many physiological structures that we might find familiar.
I'm probably even less optimistic than you regarding the possibility of panspermia resulting in similar life forms, at least further up the evolutionary ladder; I'm not a huge fan of the idea to start with, and it seems to me that even if planets were seeded with the same simple organisms, the resulting creatures may still look quite different in a couple of billion years.
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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.
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I agree that the odds of another species very similar to ours evolving elsewhere are remote, but the reasons that I think it's possible are that a) we already know that creatures resembling humans
can evolve, and b) we're talking about a huge number of planets - 3.5 x 10^20, using our original numbers.
Let's look at it another way - out of billions or hundreds of billions of sentient life forms, what are the chances that no two would be strikingly similar? Of those that would be similar, how likely is it that one of them would be human?
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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.
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While all of that is true, I still believe that, given a huge number of similar environments, we can expect to see at least a few similar life forms evolve, even at the top of the food chain.
It may be that I'm stretching the principle of plenitude past its breaking point. And I'm certain that we will never know for sure. But I can't help thinking that, given the immense number of opportunities, if it happened once it could happen again.