We don't have a clear idea, because of the lack of evidence. The farthest back we can extrapolate from today's life is something called LUCA, the last universal common ancestor. Details here:
Last universal common ancestor - Wikipedia
This would have been a very primitive single-celled organism. If you read the article, you can see the discussion is all about the biochemistry it would have had, because that is where you end up, as you go right back towards the start. Much further back and you are into abiogenesis, i.e. what biochemistry went on before there was a stable form of reproduction involving inheritance. Before LUCA, we just don't know. All we have are hypotheses.