This is Notharctus, suspended in mid-leap. It is a prodigious leap, not only taking us across forty-nine million years, but also bringing us to our own branch of the evolutionary tree. Notharctus is one of the first modern primates, which makes it one of our oldest relatives. "Modern," in this case, means that Notharctus shared some significant features with primates that are living today—features that were not found in earlier primates.

One such feature is a larger brain, indicated by the skull size of Notharctus. Increased brain size was one evolutionary step toward modern apes, which include orangutans, chimpanzees, gorillas, and humans. Other features of modern primates that are apparent on Notharctus include grasping hands and feet, and eyes set forward in the skull for stereoscopic vision. (Primates are visually-oriented, relying far less on their sense of smell than most other animals.)

Notharctus also retained more primitive primate features. It had not yet evolved into an upright posture, for example, and it had a long tail. The living primates to which Notharctus is most closely related are lemurs—tree-dwellers found mostly in the old-world tropics.

This skeleton is a cast of a nearly complete Notharctus fossil that was found in Wyoming. The earliest known primates have been found in North America, raising the possibility that they may have originated on this continent more than fifty million years ago.