A new study uses statistics to test whether life on Earth can be traced back to a common ancestor (example shown in a) or multiple primordial life forms (b). 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. A universal common ancestor is at least 102,860 times more probable than having multiple ancestors, Theobald calculates.