Believe it or not, this is a question that I’ve encounter many times before in my life, either through my blog, or because of my inherent love of science fiction. And certainly, I’m no expert in xenobiology or anything of that nature, I do have a unique perspective on it, which I feel warrants attention. I’m certain everyone else has a similar opinion, and if you have some logical conclusion that you think I have missed feel free to throw it at me.
Firstly, let me start by saying, that I am not saying extraterrestrial life will look like us, I’m just saying that there is probably a significantly strong chance that life will have characteristics in common with us. Based on what, you say?
Evolution. We can assume that traits that we’ve evolved here on Earth will take hold on an Earth-like planet, no matter where it is in the cosmos, as long as the fundamental laws of science still hold sway. For example, let’s take the act of reproduction. Long ago, in some primordial soup, two bacteria were swimming around. Both were dying, but one budded off and created two others. The other one died (because it failed to reproduce), therefore all future bacteria in this soup would reproduce.
You can take this example, and apply it to any significantly Earth-Like planet. Mammalian creatures are better to survive an ice age, thus mammals prosper simply because they adapt. A creature walking around with opposable thumbs is too weak to thrive, but too strong to die. It picks up a stick, hits something with it, and instantly starts using tools.
And while many evolutionary facets from non-Earth Like planets will be different (vastly different) some fundamental things about life itself will always remain. I think the fundamental definition of anything to live, is its ability to reproduce, under whatever circumstance.