Our ancestors branched off from those of chimpanzees some six million years ago. Since then, our lineage became human–and distinctly unlike other apes. Figuring out how that difference evolved is one of the grand challenges of biology. Until now, scientists have gotten most of their clues by looking at the fossils of extinct hominids. These fragments of bones only preserve a little information, but it’s not a random smattering of data. It’s more like a scaffolding on which other clues can be fixed, so that a picture of how we became human can gradually emerge. That’s because the changes documented in the fossil record were ultimately created through the evolution of our genome.

The power of combining fossils and genes was