Early Humans: Evolution of Australopithecines

Nearly five million years ago in Africa, an apelike species evolved with two important traits that distinguished it from the apes. This species had small canine teeth (next to the four front teeth), and it was bipedal , meaning it could walk on two legs instead of four. Scientists refer to these earliest human species as australopithecines, or australopith for short.

The fossil record shows that there is not an orderly sequence leading from one form to another. Several groups lived at the same time and characteristics developed at different rates; therefore the human family tree suggests a long and complex past.

Fossils from several early australopith species that lived between four million and two million years ago clearly demonstrate a variety of adaptations that mark the transition between ape to human. Prior to four million years ago, fossil remains are scarce and incomplete; where available, however, they do show a primitive combination of ape and human features.

Most of the key characteristics that stand out as distinctly human are related to their bipedal stance. The australopiths had an S-shaped spine that allowed for balance when standing. The opening through which the spinal cord attached to the brain was positioned more forward, allowing for the head to be balanced over the upright spine. The pelvic bone was shorter and broader than in apes, giving the pelvis a bowl shape that supported the internal organs when standing or walking upright. The upper legs angled inward allowing the knees to support the body while standing or walking. Shorter and less flexible toes functioned as rigid levers for pushing off the ground with each step.

Most early species had small canine teeth, a projecting face, and a small brain. They weighed between 22 and 37 kilograms (60 to 100 pounds), and were 0.9 to 1.5 meters (3 to 5 feet) tall. Males were generally larger than females. Both had curved fingers and long thumbs with a wide range of movement. The apes, in comparison, have longer, more curved, and stronger fingers that make them well adapted for hanging and swinging from branches. Apes also have short thumbs, which limits their ability to manipulate small objects.