Space: What's out there

Because we are products of billions of years of evolution on Earth. Aliens would be products of evolution elsewhere. It's highly unlikely that conditions on another planet would be exactly the same as they have been on Earth.

Indeed, even if you could "wind back the clock" and start the evolution of life all over again on Earth it's highly unlikely humans would turn out the same the second time around.

Quote:

[Q.] The late Stephen Jay Gould has written that if we were to rewind the "tape" of evolutionary history and play it again, the results would not be the same [S.J.Gould, Wonderful Life, 1989]. Why?

Douglas Futuyma: Well, it almost certainly would not be the same. I don't think anyone can say how different it would be, whether it would be along the same general lines or utterly, unimaginably different.

Of course, it wouldn't be the same, because first of all, random processes are involved in the evolutionary process. For example, the origin of new mutations: a lot of evolution is dependent on particular mutational changes in genes that were very, very rare or unlikely, but that just happened at the right time, in the right species, in the right environment, but it need not happen that way. So, there's this unpredictability.

In addition, the particular sequence of environmental changes that the Earth underwent and that organisms were exposed to over billions of years has left a long-term imprint on species as they are today. If the sequence of environmental changes were different, you would have a different evolutionary history, leading to entirely different organisms over time.

[url=http://www.actionbioscience.org/evolution/futuyma.html]Source[/url]

Thanks, MS. I hadn't thought to realize evolution elsewhere would be quite different from our own.