&gt; How about life without oxygen? I always wonder why alien life or any other life cannot happen without it.<p>If by oxygen you mean molecular oxygen, O2 -- rather than atomic oxygen as part of some other compound -- we know life can happen without it, since there is life on earth that lives without it. And, IIRC, we have strong reason to believe that life that could live without O2 predates life that needs O2, and, in fact, it is life that made free O2 available, not free O2 that made life possible.