In 2011, a team of palaeontologists led by Nancy Stevens, unearthed a single molar in Tanzania’s Rukwa Rift Basin. It was a tiny fossil, but its distinctive crests, cusps and clefts told Stevens that it belonged to a new species. What’s more, it belonged to the oldest known Old World monkey—the group that includes modern baboons, macaques and more. They called it Nsungwepithecus.

A year later, and 15 kilometres away, the team struck palaeontological gold again. They found another jawbone fragment, this one containing four teeth. Again, a new species. And again, an old and distinctive one. The teeth represent the oldest fossils of any hominoid or ‘ape’. They called it Rukwapithecus.

Together, these two new species fill in an important