A molecular palaeobiologist at nearby Dartmouth College, Peterson has been reshaping phylogenetic trees for the past few years, ever since he pioneered a technique that uses short molecules called microRNAs to work out evolutionary branchings. He has now sketched out a radically different diagram for mammals: one that aligns humans more closely with elephants than with rodents.

“I've looked at thousands of microRNA genes, and I can't find a single example that would support the traditional tree,” he says. The technique “just changes everything about our understanding of mammal evolution”.

Peterson didn't set out to rewrite textbooks. A mild-mannered but straight-talking Montanan, Peterson had made a quiet career studying how bilateral body plans originated more than 500 million years ago. He has a particular interest in marine invertebrates and had intended to stick with that relatively obscure branch of the animal tree. But a chance investigation of microRNAs in microscopic creatures called rotifers led him to examine these regulatory molecules in everything from insects to sea urchins. And as he continues to look, he keeps uncovering problems, from the base of the animal tree all the way up to its crown.

That has won him many critics, but also some strong supporters. “Peterson and his colleagues have demonstrated that microRNAs are a powerful tool in determining the relationships of major animal groups,” says Derek Briggs, director of the Yale Peabody Museum of Natural History in New Haven, Connecticut.

Now, together with his colleagues around the world, Peterson is putting it all on the line with mammals. “If we get this wrong, all faith that anyone has in microRNAs [for phylogenetics] will be lost,” says Philip Donoghue, a palaeobiologist at the University of Bristol, UK, who has teamed up with Peterson. And there is more at stake than just the technique. “It could well be the end of all our careers,” he says.