Every generation is so sure of its grasp of science only for later generations to learn how to shine a light into previously dark rooms.
The universe is so vast and stranger than we can imagine that's it's folly to assume we already have definitive answers. I suspect we don't even know we're overlooking something, perhaps something crucial, in trying to answer this question.
I recommend reading:
Also, we're talking about biochemistry in conditions we can recreate, not about massive black holes or other such extreme phenomena.
Our theoretical/empirical knowledge in biochemistry is pretty good.