Background Essay: Evolving Ideas: Did Humans Evolve?

In 1974, a team of paleontologists led by Donald Johanson discovered the fossilized skeleton of "Lucy," a 3.2-million-year-old human ancestor now known as Australopithecus afarensis. Lucy's skeleton was clearly different from other primates. Her knees could lock, her femur slanted inward, and her large toe aligned with her other toes, all of which allowed her to walk upright. The discovery of Lucy surprised paleontologists because although she was unquestionably bipedal -- able to walk upright on two legs -- she was remarkably apelike, with a brain about the size of a chimpanzee's.

Bipedalism is a tremendous adaptation for humans and a distinguishing characteristic between humans and other primates. There are many hypotheses about the advantages of bipedalism, including the ability to carry large loads from place to place, to walk long distances efficiently, to use tools while walking, and to see greater distances while traveling. Any or all of these hypotheses may be correct and are being explored by anthropologists today.

A second major adaptive advantage that appeared later in human evolution was an increase in brain size. Fossil evidence enables us to trace the development of the brain as it increased threefold over the last 3 million years. Early hominids (humanlike species) such as Lucy and other species in the genus Australopithecus had brains the size of modern apes (400 to 500 cubic centimeters [cc]). Homo habilis (2.3 to 1.6 million years old) had a brain size of about 650 cc and was probably the first hominid to make and use stone tools. As brain size increased, new capabilities evolved, improving the ability of hominids to adapt to and modify their environment.

Another early hominid, Homo erectus (with a brain size of approximately 900 cc), was the first to develop humanlike culture. This species, which lived until about 300,000 years ago, used tools, made fires, was probably also the first to have spread from Africa into Asia. Modern humans, or Homo sapiens, originated about 100,000 years ago and had brains ranging from 1200 to 1600 cc, the same size as humans living today.