JANUARY 17, 2000 VOL. 155 NO. 2

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THE EARLIEST HUMANS

Given their 2 million-year-plus life-span, the australopithecines were surely one of evolution's better experiments. But nature is an inveterate tinkerer, even with successful species. Between 3 million and 1.9 million years B.P., several variations on the Australopithecus theme popped up in eastern and southern Africa, including A. africanus, A. aethiopicus, A. robustus and A. boisei. (Just to complicate matters, the last three are assigned by some experts to an entirely different genus, Paranthropus.)

ALSO IN TIME

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But figuring out how they arose, how they were related and what they evolved into--those that weren't evolutionary dead ends--has proved elusive. Not only is the fossil record full of holes, but the hominid species from eastern Africa haven't shown up in southern Africa, and vice versa. A remarkably preserved skeleton found in South Africa's Sterkfontein cave could change all that. Believed to be at least 3.3 million years old, the bones may belong to A. afarensis, making it the first of Lucy's species uncovered in that area. But the skeleton hasn't been fully excavated yet, and its discoverer, Ron Clarke of the University of the Witwatersrand, in Johannesburg, thinks it may represent yet another previously unknown species.

Whatever the evolutionary relationships between these prehuman species, paleoanthropologists know that at some point a second major shift took place.

One of Lucy's descendants gave rise to a new kind of creature, the first of the genus Homo. Yet none of the known variants of Australopithecus seemed anatomically close enough to the Homo line to qualify.

Then, last spring, Asfaw and White's team made another dramatic announcement. A fragmentary skull found near Bouri, an Ethiopian village in the Middle Awash region northeast of Addis Ababa, could well be from the missing australopithecine that sired the human race (see cover photo). Excavated in 1997, its jutting face and upper jaw filled with large teeth clearly belong to a species more advanced than A. afarensis yet more primitive than the earliest humans.

The mix of characteristics wasn't precisely what the experts expected--they were looking to see smaller, more specialized teeth and a larger braincase. So they named their hominid Australopithecus garhi (garhi means surprise in Afar). But the skull's intermediate anatomy and its age--about 2.5 million years--put it midway in both time and form between the most recent A. afarensis and the oldest known fossils of our own genus.

That alone would make A. garhi a prime candidate for the long-sought evolutionary link between Lucy's species and the first humans. But the researchers also found that nearby animal bones dating from the same period had been butchered with stone implements. Cut marks on one antelope jawbone suggest that the hominids used a sharp stone flake to remove the animal's tongue. The leg bone of another animal is scarred by cuts, chop marks and signs of hammering, evidence that it was scraped clean of meat and bashed open to expose the nutritious marrow.

Earlier discoveries at Gona, an Ethiopian site about 95 km north of Bouri, had already shown that someone was using carefully manufactured stone tools in the area at about that time. Now Asfaw and White's team could make a circumstantial case that their species, A. garhi, was the gifted toolmaker. If so, this was a crucial bit of scientific sleuthing. In the 2 million years since the first human ancestor began to walk upright, nothing much had changed. Now something had. Rather than just using sticks and stones to leverage innate abilities--something done by plenty of animals, from chimps to otters to finches--someone had deliberately selected and modified specific raw materials in a sophisticated and consistent way, and with careful intent.

This wasn't just tool use; it was technology. Explains archaeologist Sileshi Semaw, a postdoctoral researcher at Indiana University in Bloomington, who helped find a huge cache of 2.6 million-year-old tools at Gona in the early 1990s: "The Gona hominids [carefully] selected workable raw materials." Since there are no local sources of such materials at Bouri, where the A. garhi fossils were found, the hominids must have carried their tools with them when they traveled there.

Did A. garhi make both the tools Semaw found and the ones used to butcher animals at Bouri? "If it wasn't garhi," asks White, "what would it have been?" Semaw is more cautious. "Australopithecus garhi is the best candidate thus far," he concedes, but he doesn't rule out the possibility that another species, yet undiscovered, deserves the credit.

Whoever did it, the creation of technology gave its inventors an astonishing advantage over other hominid species. Stone hammers and blades let them exploit carcasses left behind by other predators and permitted them to shift to an energy-rich, high-fat diet. "That," asserts Asfaw, "leads to all kinds of evolutionary consequences."

One of these, White suggests, was the ability to exploit a broader range of habitats, eventually enabling our ancestors to leave Africa and colonize most of the globe. But even more important was the expansion of our brain, with all the potential that went with it. Explains Meave Leakey: "The brain is a very expensive organ in terms of metabolism." It can grow larger only in a species that's routinely consuming high-energy food. One impetus for such growth--and in particular, the growth of the cognitive areas that distinguish ours from other large brains--could have come from our increasingly creative use of tools. Still, the ultimate use to which those big, sophisticated brains would be put would not appear for many hundreds of thousands of years.

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