Peterson, a molecular palaeobiologist at Dartmouth College in Hanover, New Hampshire, and his colleagues compared the sequences of seven genes from 42 species of animal, and then used sophisticated computer programs to assemble the species into the most likely evolutionary tree. Sure enough, sponges branch out near the base – but Peterson’s tree differs from most others in a subtle but hugely significant way. Instead of putting sponges on a single side branch, Peterson’s tree has sponges on both sides of the trunk.
In other words, the ancestor of all complex animals not only resembled a sponge, it actually was a sponge. “If you had a time machine and brought back the last common ancestor of all living animals, and you gave it to an invertebrate zoology class, they’d call it a bath sponge,” says Peterson.