For scientists who study human evolution, the last few months have been a whirlwind. Every couple of weeks, it seems, another team pulls back the curtain on newly discovered bones or stone tools, prompting researchers to rethink what we know about early human history.

On Wednesday, it happened again. Yohannes Haile-Selassie of the Cleveland Museum of Natural History and his colleagues reported finding a jaw in Ethiopia that belonged to an ancient human relative that lived sometime between 3.3 and 3.5 million years ago. They argue that the jaw belongs to an entirely new species, which they named Australopithecus deyiremeda.

While some experts agree, skeptics argue that the jaw belongs to a familiar hominid species, known as Australopithecus afarensis, that existed about 3.9 to 3 million years ago.

Studies like this one are adding fresh fuel to the debate over the pace of human evolution. Some researchers now believe the human family tree bore exuberant branches early on.