which makes it so you have to go back to the very first life form (if there was such a thing), if you take it to the logical conclusion. There are some things that all life shares. We are not so much interested in that stuff, because it does not tell us anything we don't already know (that all life appears to have come from a common origin). We are interested in when the things that make us different came into existence, when we "separated" from the other life that is not like us.