&gt; How about life without oxygen?<p>It&#x27;s thought that life began here in the absence of oxygen, and only over time did oxygen begin to play a part in cellular life. As another poster points out, there are still some cellular organisms here that do without oxygen, indeed they would find it toxic.<p>&gt; I always wonder why alien life or any other life cannot happen without it.<p>But that&#x27;s not true, not even here. Oxygen is regarded as a sign of life, but its absence doesn&#x27;t suggest the absence of life, only the kind of life that requires it.