Two remarkably preserved fossil finds near Yellowstone National Park show how primates first diverged from the rest of the mammals. The 56-million-year-old bones, which were discovered by a team led by Florida Museum of Natural History paleontologist Jonathan Bloch, belong to our earliest primate relatives—the plesiadapiforms.

Plesiadapiforms were as small as mice, lived in trees, and ate fruit. Despite genetic evidence, some scientists thought plesiadapiforms weren’t primates at all. Previous fossils hinted that they were ancestors of colugos, non-primates that glide like flying squirrels. But the fossils in this study don’t have evidence of webbed hands or gliding flaps. Previous plesiadapiform fossils also showed hands that could grasp and manipulate food—a defining trait of primates.

Identifying plesiadapiforms as primates helps scientists chart the evolutionary rise of our ancestors, relatives, and our own species.