"on earth there are organisms in the deep sea that have completely different biology": Not sure what you mean by that. There are bacteria that metabolize diffe

"on earth there are organisms in the deep sea that have completely different biology": Not sure what you mean by that. There are bacteria that metabolize different things than what the bacteria up here metabolize, but they (and we) all use DNA, and with a couple exceptions they use the same codons to encode the same amino acids and start/stop. As far as anyone knows, that coding is completely arbitrary, i.e. the codons could have been entirely different. The fact that they are not entirely different implies a common origin. Lots of other biochemical similarities, too.