A surprising discovery in the desert
In 1994, a team of scientists led by Tim D. White, Gen Suwa, and Yohannes Haile-Selassie made a discovery at Aramis in the Middle Awash valley of Ethiopia. They found the fossilized remains of an early hominin species, Ardipithecus ramidus, dated to 4.4 million years ago. The most important find was a partial skeleton, ARA-VP-6/500, nicknamed "Ardi". This skeleton, which is about 45% complete, includes most of the skull, teeth, pelvis, hands, and feet. The discovery was remarkable because the bones were incredibly fragile; they crumbled upon touch, requiring researchers to excavate entire blocks of earth for careful extraction at the National Museum of Ethiopia.
The site where Ardi was found has yielded over 125 fossilized pieces of this one individual. In total, more than 100 specimens representing Ardipithecus ramidus have been found in the area. Analysis of associated animal and plant fossils revealed that Ardi lived in a woodland environment, not the open savanna long thought to be the setting for the evolution of upright walking. This discovery changed established ideas about the pressures that led to bipedalism.
Anatomy of a hand
The hand bones of Ardipithecus ramidus show that the human lineage did not pass through a knuckle-walking phase, contrary to a long-held hypothesis. Modern knuckle-walkers, like chimpanzees and gorillas, have stiff wrists and specialized hand bones to support their body weight. Ardi's hands were different. The wrist was flexible, and the hand bones lacked the ridges and features associated with knuckle-walking. This anatomy suggests that Ardi moved through trees by walking on its palms along branches, a form of locomotion called palmigrade quadrupedalism.
The finger bones, or phalanges, of Ardipithecus were long, and the thumb was robust, allowing for a powerful grasp. This hand structure was well-suited for climbing. At the same time, Ardi's pelvis and feet show clear adaptations for walking upright on two legs. The upper part of the pelvis is short and broad, which helps to balance the body during bipedal walking. This combination of traits, a hand for climbing and a pelvis for upright walking, indicates a creature comfortable in both the trees and on the ground. The foot also shows this dual adaptation, with a grasping, opposable big toe for arboreal life alongside more rigid outer toes for pushing off during bipedal locomotion.