An early branch in our family tree

A partial skeleton of a female, known as "Ardi", combines human and other primate traits. Ardi moved in the trees using a grasping big toe, yet her pelvis was shorter and broader than an ape's—indicating that she could walk bipedally.

Nicknamed "Ardi," ARA-VP-6/500 is a 4.4 million year old female partial skeleton. So far, she is one of the most complete early human skeletons scientists have ever found, and only one of six known early human partial skeletons over 1 million years old.

When discovered, Ardi’s bones were so fragile they crumbled when the team touched them. Because the team couldn’t excavate the crumbling bones in situ, White’s team had to remove whole blocks of stone and earth to the National Museum of Ethiopia and excavate there. Her bones were then analyzed and reconstructed using micro-computed tomography, or CT scans.

Ardi’s skeleton includes most of her skull and teeth, as well as her hands, feet, and pelvis. While she has a small brain (300–350 cubic centimeters), her face is small with thin cheeks and incisors incapable of chewing tough foods. Her face does not project as far as a chimpanzee’s, and her brow ridges weren’t as thick.  She also has a short (probably down-turned) skull that helped to balance her head above her neck during upright walking.

Ardi stood almost four feet tall and weighed 110 lbs, making her similar in size to a chimpanzee. She walked upright, but could move through the trees using all four limbs equally. Her feet had opposable big toes that could grasp branches, while the rest of her toes were more rigid and helped with bipedal walking. Her pelvis was short and broad, which helped her keep her balance while upright, and her spine was long and curved like a modern human’s. In fact, some scientists believe she is so well adapted to bipedal walking, that her species must have been walking on two legs for a long time before her; however, with a large pelvis and opposable big toes, she probably did not walk like later humans.

She probably spent much of her time in the trees, though she probably wasn’t swinging from branches. Ardi’s hand can bend backwards at the wrist, which is unlike chimpanzees or gorillas who have stiff wrists designed for knuckle-walking. Ardi’s flexible hands allowed her to walk with four limbs carefully on top of branches­ (called palmigrady).