All life shares a common ancestor. This is a standard notion in biology, but one that has so far not really received a good test. After all, living things swap genetic material all of the time, so it might seem that life today must have come from more than one primordial organism.
To test the idea, Douglas Theobald of Brandeis University in Waltham, Massachusetts, selected 23 common proteins with structures that vary in details from species to species. Looking at them in 12 species – four each from the three "branches of life": archea, bacteria and eukaryotes – he found that a model with a single ancestor and single gene swapping was a remarkable 103 489 times more likely than the best multiancestor model considered.
Deep down, it looks as though all life does come from a common start.