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One isn’t such a lonely number. All life on Earth shares a single common ancestor, a new statistical analysis confirms. The idea that life-forms share a common ancestor is “a central pillar of evolutionary theory,” says Douglas Theobald, a biochemist at Brandeis University in Waltham, Mass. “But recently there has been some mumbling, especially from microbiologists, that it may not be so cut-and-dried.” 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. A universal common ancestor is at least 102,860 times more probable than having multiple ancestors, Theobald calculates.
Theobald’s study does not address how many times life may have arisen on Earth. Life could have originated many times, but the study suggests that only one of those primordial events yielded the array of organisms living today. “It doesn’t tell you where the deep ancestor was,” Penny says. “But what it does say is that there was one common ancestor among all those little beasties.”