Earth's Earliest Primates Used Mobile Ankles to Swing from Trees

Researchers are taking a look back at some of our earliest ancestors. They've gotten a closer look at the ankles of Earth's first primates and have discovered a bit more about how they lived.

A small mammal that lived on a diet of fruit of insects, Purgatorius was one of the first primates on our planet. It's actually part of an extinct group of primates called plesiadapiforms, which first appears in the fossil record shortly after the extinction of non-avian dinosaurs. Scientists have long speculated that primitive plesiadapiforms may have been terrestrial rather than arboreal-that is, they walked on the ground rather than staying in trees. Now, it seems as if scientists have proof that this wasn't the case.

Before now, researchers have had only the animal's teeth and jaws to examine. This, in turn, left much of its appearance and behavior a mystery. Now, they've uncovered the ankle bones of Purgatorius, which show that this creature likely lived in trees.

"The ankle bones have diagnostic features for mobility that are only present in those of primates and their close relatives today," said Stephen Chester, one of the researchers, in a news release. "These unique features would have allowed an animal such as Purgatorius to rotate and adjust its feet accordingly to grab branches while moving through trees. In contrast, ground-dwelling mammals lack these features and are better suited for propelling themselves forward in a more restricted, fore-and-aft motion."

The findings reveal how arboreality played a key role in primate evolution. In fact, it implies that the divergence of primates from other mammals wasn't a dramatic event; instead, primates developed subtle changes that made for easier navigation and better access to food in trees.

"The textbook that I am currently using in my biological anthropology courses still has an illustration of Purgatorius walking on the ground," said Chester. "Hopefully this study will change what students are learning about earliest primate evolution and will place Purgatorius in the trees where it rightfully belongs."

The findings are published in the journal Proceedings of the National Academy of Sciences.

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