The tree of life is not a tree but shaped like this. Read this article which puts archaeons on the right hand branch, bacteria on the left, and us with all other eukaryotes as a result from a unexpected joining of a bacterium into an archaeon.
Lovelock mentions this in Novacene, where he basically uses this to support his thesis that on the known universe’s timescale there is too little time for alien species to have evolved. We are alone he says.
(found via Jeanie McGeehan)