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.
In a way, though, the evidence of butchery is the most exciting discovery of all. It tells us not just what our putative ancestors looked like but also how they behaved. According to the report published in Science, the fossil jawbone of an antelope exhibits "unambiguous cut marks made by a sharp stone flake," which the scientists believe was probably used to remove the animal's tongue. A three-toed horse had been dismembered and the meat on its leg bone filleted. The leg bone of yet another animal is scarred by man-made cuts, chop marks and signs of hammering, presumably to get at the marrow inside. And while slightly older tools were found previously at a site about 60 miles away, this is the earliest evidence associating tools with carnivory, an important milestone in human evolution and perhaps a factor in increasing brain size.
The tool marks don't necessarily imply that these creatures were hunters. While this harsh desert was far less arid 2.5 million years ago than it is today, it was still primarily grassland with little cover. "It was not a particularly friendly habitat for bipeds," says paleontologist Tim White of the University of California, Berkeley, who is co-leader of the expedition. Because it would not have been easy for the tool users to hide in ambush, White, like many other paleoanthropologists, believes that they were probably scavengers.