More than 75 years of digging in the ancient, arid sediments of East Africa has told scientists a great deal about the long evolutionary trail that led to modern human beings. They know about Lucy, the upright-walking proto-human australopithecine that strode the continent some 3.2 million years ago; about Homo habilis, the first known human species, which was making and using stone tools in the same region by 1.2 million years later; about Homo erectus, which emerged from Africa soon thereafter and spread across the world.
But while the broad outlines of this prehistoric genealogy have been well established, most of the crucial details still need to be filled in. Perhaps the most pivotal of them all: Precisely when, how, and most important, why did australopithecines like Lucy evolve into true humans? Murphy's Law, unfortunately, has arranged matters so that the fossil record is especially sparse between 2 million and 3 million years ago, just when the crucial transition took place.
That's why a series of discoveries presented last week in the journal Science has paleontologists in such a stir. An international expedition working in Ethiopia found a partial skull of a new species of human ancestor from 2.5 million years ago, right in the middle of the gap. They also discovered evidence that someone was using tools to butcher animals in the same location at approximately the same time. And they found fossil arm, leg and foot bones that will provide experts with important clues about how human ancestors were built in those days. Exclaims anatomist Alan Walker of Pennsylvania State University: "This is really exciting!"
That's not to say anyone knows what it all means yet. To start with, the researchers are not quite certain how the three discoveries relate to one another. The new species, for example, which the researchers call Australopithecus garhi (garhi means surprise in the Afar language), was identified on the basis of a fragmentary skull with a complete upper jaw full of unusually large teeth that was excavated from the arid, rocky ground of Ethiopia's Middle Awash region near the village of Bouri. When the paleontologists looked closely at the skull, they were shocked. The combination of teeth and bones clearly came from a species more primitive than the earliest humans yet more modern than known australopithecines. That means it could be the transitional species that led directly to the Homo lineage--or it could be a branch of the family tree that became an evolutionary dead end.
The arm, leg and foot bones were discovered in the same geological layer but some 900 ft. away from the skull and teeth. Without physical proximity to link the two finds, and without teeth for comparison, the paleontologists can't be sure that they are from the same species. But like the skull, these fossils show a mix of primitive and advanced traits. Australopithecus afarensis, which lived between 3.6 million and 2.9 million years ago, had forearms that were long compared with its legs, while Homo erectus, which appeared about 1.7 million years ago, had shortened forearms and longer legs, more like modern humans. The new fossils fall right in the middle, both chronologically and anatomically, suggesting that the leg bones lengthened at least a million years before the forearm bones shrank.