The discovery of a 56-million-year-old skeleton shows the first primates were small as mice, lived in trees, and ate fruit, according to an article in the Proceedings of the National Academy of Sciences.

The fossil is more than 10 million years older than what was thought to be the earliest member of the primate family and has caused scientists to reconstruct the base of the family tree that has more than 85 modern and extinct species.

The work suggests that study of these early primates is critical to understanding "the earliest phases of human evolution," coauthor Mary Silcox, professor of anthropology at the University of Winnipeg, said in a statement.

The discovery in Bighorn Basin, outside Yellowstone National Park in Wyoming, gives evidence that plesiadapiforms, mammals once thought to be related to flying lemurs, are the most primitive primates, said the lead researcher, University of Florida paleontologist Jonathan Bloch.

The fossil dates plesiadapiforms to the Paleocene era, about 65 million to 55 million years ago, the study said. That places these primitive animals between the extinction of the dinosaurs and the first appearance of undisputed ancestors of modern mammals.

Primates are characterized by relatively large brains, enhanced vision and eyes that face forward, a specialized ability to leap, nails instead of claws on at least the first toes, and specialized grasping hands and feet.

Although plesiadapiforms do not have all the traits, researchers believe the mammals acquired them to meet changing environmental conditions over 10 million years, the study said.

"This is the first study to bring it all together," coauthor Eric Sargis, associate professor of anthropology at Yale University, said in a statement.

"The extensive data set, the number and type of characteristics we were able to compare, and the availability of full skeletons, let us test far more than any previous study."