Quote:
Originally Posted by Hom3r
We as carbon is the most common material, on would assume that other life forms would be carbon based.
Silicon based is another possibility.
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We have evolved to suit our environment. Our atmosphere is air which is a mostly Nitrogen (~78%) and Oxygen (~20%). Nitrogen is pretty much inert and its hard to bond with and to isolate from compounds. So few organisms can use it baring some special types of bacteria that act on nitrates. Off the top of my head I can't remember anything (from A-Level Biology and Chemistry) that acts on N2 gas. So we use the next most prevalent gas which is a lot more reactive: Oxygen. Although its only 20% but its more than enough for us as we expel the vast majority of Oxygen we breath in (IIRC we use about less 10% we breath in). We breath out Carbon Dioxide which is handy as plants can use this to convert back to Oxygen. Hence we have a sustainable source of ambient Oxygen.
I have seen or heard of nothing that says organisms on a planet with a different atmosphere to ours can't adapt to whatever is available. Alien life forms don't have to be reliant on Oxygen and water. I'd be suprised if they did.
Given the near infinite amount of genetic mutations over billions of years, the chances of any complex life forms being even remote similar to us is infantessimal IMO. Professor Hawking acknowleges this by saying they might be yellow or lizard-like but why can't their internal chemistry be different as well as their appearance?
So given that they'll probably find out planet inhospitable, hostile and the fact they'd be hyper-intelligent and hyper-evolved and therefore more peace loving (look out peaceful we've become over a mere 2000 years), I don't think we'll be invaded and enslaved by aliens.