[tsimionescu]

That only makes sense if we imagine life appearing in a primordial pool. But if abiogenesis were a common event on Earth, life should have formed all around, at distances far too large to immediately consume other proto-life forms. I also find it hard to imagine that, had this been a common event, a single strain of life should have resisted all the way to today.

Of course, until we manage to reproduce abiogenesis or find other examples of life, we won't know for sure. But I would say the theory with the least amount of assumptions right now is that life only appeared once in Earth's history or at least in one single relatively small place.