News in Science

First human ancestor looked like a squirrel

Oldest primate The world's oldest and most primitive known primate looked like a squirrel, according to a study of newly discovered fossilised bones.

The fossils, described at the recent Society of Vertebrate Paleontology's 72nd Annual Meeting in Raleigh, North Carolina, are the first known below-the-head bones for Purgatorius. Previously, only teeth revealed its existence.

"The ankle bones show that it had a mobile ankle joint like primates today that live in trees," says co-author Stephen Chester, a Yale University vertebrate palaeontologist.

This mobility would have allowed for rotating the foot in different directions as it adjusted to different angles presented by tree trunks and branches."

"It also shows that the first primates did not have elongate ankles that you see in many living primates today that are thought to be related to leaping behaviours," adds Chester.

He conducted the study with colleagues Dr Jonathan Bloch of the Florida Museum of Natural History and Dr William Clemens, a professor emeritus at the University of California at Berkeley and a curator for the university's Museum of Paleontology.

After analysing the fossils, the researchers believe that the specialised ankle bones of Purgatorius played a key role in the evolutionary success of early primates.

"These new fossils support the idea that the first 10 million years of primate evolution happened in the context of an intense period of similar diversification in flowering plants, including the ability to climb in branches and collect fruits and other products of the trees at the very beginning," says Bloch.

Unanswered questions

While many questions remain unanswered about Purgatorius, this and other studies are shedding more light on the animal. Its name comes from Purgatory Hill in eastern Montana where it was first discovered.

Purgatorius lived during the Palaeocene, shortly after the extinction of non-avian dinosaurs. Given the end of the Age of Dinosaurs, the new era began the mammal-dominated era, which we are still in.

This mammal is generally believed to have been small and brown, and had a bushy tail. The researchers liken it to another early primate, Dryomomys, for which more fossil material is available. Based on that and the newly found bones, Purgatorius weighed about 37 grams, making it roughly the size of the smallest living primates: the mouse lemurs of Madagascar.

The mammal had a lot of teeth, including relatively low-crowned molars, which were specialised for eating fruit, although it probably ate other things too.

Tree living served this and other primates well, such that all but a few existing species remain at least partly arboreal. Humans are part of the rare exceptions, since our more recent ancestors left the trees some 60 million years after Purgatorius' lifetime.

Dr John Fleagle, a professor in the Department of Anatomical Sciences at Stony Brook University, says "arboreality in Purgatorius is no great surprise," given that early other mammals, such as flying lemurs and tree shrews, had hand proportions suitable for tree navigation.

Purgatorius and similar mammals "seem to have been squirrel-like arboreal animals with large claws and often bushy tails," says Fleagle. These animals, along with tree shrews and flying lemurs, are all related to primates, he continued, but which is closest has been difficult to determine.

Genetic analyses can be used for evaluating relationships among living groups, Fleagle noted, but that is not possible for earlier animals like Purgatorius, which are known only from fossils.

Future work will hopefully reveal more information about these still enigmatic animals, but the latest finds are promising. In a separate talk at the SVP meeting, Clemens documented the oldest occurrence yet known of Purgatorius. Those remains date to 65-66 million years ago.