Actually calling it a 'tree' of life is a simplification when you get down to the level of bacteria etc. We now know that horizontal as well as vertical (treelike) gene transfers occur which join up some of the branches and it gets to be spaghetti at the bottom of the trunk e.g.
More here:
Horizontal gene transfer - Wikipedia
Edited by Tangle, : No reason given.
Life, don't talk to me about life - Marvin the Paranoid Android