The real ELI5 is: “we don’t know” but that is of course not very satisfying, is it?
There are a couple of theories about it. I personally like the idea that the thermal vents on the sea floor played a crucial role in the process. These have everything a living organism needs: energy – both in heat and chemical, the actual chemicals needed to build a simple cell, and a porous substance which creates small chambers where simple self-replicating mechanisms may form spontaneously.
But the real answer is: we don’t know.
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