The Step to Human
SEVENTY FOOTPRINTS in volcanic ash--two individuals of unequal size walking side by side and a third following in the bigger one's steps--were left to petrify for 3.5 million years in what is now the and Laetoli area of northern Tanzania until they were uncovered in 1977 by Mary Leakey, of the famous Kenyan family of fossil hunters.1 These ancient traces bear witness to the existence, in those remote times and in that part of the world, of creatures that walked erect on feet resembling ours. Such creatures must have wandered over much of East Africa at that time. The most famous is a young female named Lucy--after the Beatles song "Lucy in the Sky with Diamonds"--who made the headlines in 1974 when her amazingly complete remains--almost half a skeleton--were found in the Afar region of Ethiopia by Donald Johanson, the founder of the Institute of Human Origins in Berkeley, California.2 Lucy is about the same age as the Laetoli walkers. Her pelvic anatomy indicates that she too walked on two legs. So did the owner of a knee joint, dated 3.9 million years, likewise found by Johanson in the Afar region. These early hominids (prehumans) are now known under the name Australopithecus afarensis.
The name Australopithecus, meaning southern (australis in Latin) ape (pithêkos in Greek), was originally coined, with the epithet africanus, by Australian-born Raymond Dart, after his 1924 discovery in a cave at Taung, South Africa, of a fossil skull belonging to an immature apelike primate, now universally known as the "Taung child." On the strength of the position of the hole in the skull through which the spinal cord connects with the brain, Dart claimed that the Taung child walked erect and was an intermediate between ape and human. This claim was strongly resisted by the paleoanthropological establishment at the time. It is no longer in dispute now that it is known that the Taung child, which is "only" two million years old, was preceded by erect-walking apes that lived almost two million years earlier.
According to comparative molecular sequencing, our closest extant relatives are the chimpanzees of Central Africa, and the last ancestor we have in common with them lived about six million years ago. It is thus not surprising that the first traces