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Scientists have also unearthed new fossils of known species. This should help them trace the complex relations among our sundry ancestors. One remarkable skeleton, announced this past spring, suggests that modern humans and Neanderthals may even have mated successfully. And new evidence of stone-tool use, dating as far back as 2.5 million years, has provided tantalizing clues to how our forebears thought and behaved.
These discoveries not only further confirm that multiple hominid species are the rule but also bring us much closer to understanding the ultimate mysteries of human evolution: What were the changes that led to modern humans? When did these changes take place, and why? And perhaps most intriguing, will we continue to evolve, or has Homo sapiens (wise man) made evolution obsolete?
While all the answers won't be in for some time, experts have identified several key transitions in our evolutionary chronicle. The first, which happened around the time we diverged from the apes, between 6 million and 4 million years ago, was the development of bipedalism--two-legged walking rather than the kind of locomotion Tarzan learned from his adoptive ape family.
The second, which occurred perhaps 2.5 million years B.P., was the invention of toolmaking--the purposeful crafting of stone implements rather than just picking up handy rocks--and the transition to meat eating. Then, somewhere between 2 million and 1 million years ago, came the dramatic growth of the brain and our ancestors' first emergence from Africa. Finally, just a few tens of thousands of years ago, our own species learned to use that powerful organ for abstract thought, which quickly led to art, music, language and all the other skills that have enthroned humans as the unchallenged rulers of their planet.
SPLIT FROM THE APES
As recently as five years ago, all that scientists could really tell about our earliest ancestors was when they first appeared. Molecular biologists had measured the differences between human and chimpanzee DNA, then averaged the rate of genetic change over time. By calculating backward, they determined that great apes and hominids branched from a common ancestor between 6 million and 4 million years ago. But no fossils were on hand to support this scenario. The oldest hominid species known, Australopithecus afarensis (southern ape of the Afar), could be dated back only 3.6 million years. Its most famous member, Lucy, unearthed in Ethiopia's bleak Afar Triangle in 1974, is a mere 3.2 million years old.
Then, in 1994 and 1995, teams working in Ethiopia and Kenya announced that they had each found a new species of hominid. Both discoveries smashed the 4 million-year barrier. The first--and at 4.4 million years, the oldest--was dug up by an international team in the Middle Awash region of Ethiopia, about 50 miles south of where Lucy was discovered.