(3 of 7)
The crucial piece of evidence came in 1974 with the discovery of the long- sought "missing link" between apes and humans. An expedition to Ethiopia led by Donald Johanson, now president of IHO, painstakingly pieced together a remarkable ancient primate skeleton. Although about 60% of the bones, including much of the skull, were missing, the scientists could tell that the animal stood 3 ft. 6 in. tall. That seemed too short for a hominid, but the animal had an all important human characteristic: unlike any species of primate known to have come before, this creature walked fully upright. How did the researchers know? The knee joint was built in such a way that the animal could fully straighten its legs. That would have freed it from the inefficient, bowlegged stride that keeps today's chimps and gorillas from extended periods of two-legged walking. Presuming that this diminutive hominid was a female, Johanson named her Lucy. (While he was examining the first fossils in his tent, the Beatles' Lucy in the Sky with Diamonds was playing on his tape recorder.)
Since scientific names don't come from pop songs, Lucy was given the tongue- challenging classification Australopithecus afarensis. Many more remains of the species have turned up, including beautifully preserved footprints found in the mid-1970s in Tanzania by a team led by the famed archaeologist Mary Leakey. Set in solidified volcanic ash, the footprints confirmed that Lucy and her kin walked like humans. Some of the A. afarensis specimens date back about 3.9 million years B.P. (before the present), making them the oldest known hominid fossils.
The final clue that Lucy was the missing link came when Johanson's team assembled fossil fragments, like a prehistoric jigsaw puzzle, into a fairly complete A. afarensis skull. It turned out to be much more apelike than human, with a forward-thrust jaw and chimp-size braincase. These short creatures (males were under five feet tall) were probably no smarter than the average ape. Their upright stance and bipedal locomotion, however, may have given them an advantage by freeing their hands, making them more efficient food gatherers.
That's one theory at least. What matters under the laws of natural selection is that Lucy and her cousins thrived and passed their genes on to the next evolutionary generation. Between 3 million and 2 million years B.P., a healthy handful of descendants sprang from the A. afarensis line, upright primates that were similar to Lucy in overall body design but different in the details of bone structure. Australopithecus africanus, Paranthropus robustus, Paranthropus boisei -- all flourished in Africa. But in the evolutionary elimination tournament, the two Paranthropus species eventually lost out. Only A. africanus, most scientists believe, survived to give rise to the next character in the human drama.
This was a species called Homo habilis, or "handy man." Appearing about 2.5 million years B.P., the new hominid probably didn't look terribly different from its predecessors, but it had a somewhat larger brain. And, perhaps as a result of some mental connection other hominids were unable to make, H. habilis figured out for the first time how to make tools.