Evolution: All Present-day Life Arose From A Single Origin

"Because microorganisms of different species often swap genes, some scientists have proposed that multiple primordial life forms could have tossed their genetic material into life's mix, creating a web, rather than a tree of life.  

To determine which hypothesis is more likely correct, Theobald put various evolutionary ancestry models through rigorous statistical tests. The results, published in the May 13 Nature, come down overwhelmingly on the side of a single ancestor."

http://www.sciencenews.org/view/generic/id/59133/title/All_present-day_life_arose_from_a_single_origin