The fist-size chunks of rock that Sileshi Semaw has collected from Gona, Ethiopia, over the past three years aren’t much to look at--at least not to an untrained observer. But Semaw, a paleoanthropologist at Rutgers, possesses a highly trained eye, and as soon as he uncovered the first of his rocks he knew he had found something unique. The rocks--he has discovered about 3,000 of them in Gona--were shaped by the hands of human ancestors. And dating from more than two and a half million years ago, they are the most ancient stone tools ever found.
Primitive hominids banged the rocks together to break off small, sharp-edged flakes for use as cutting and piercing tools. The resulting patterns on the cores, as the larger rocks are called, are unlike anything produced by natural erosion. Dating stone tools of such antiquity is often problematic--their age usually can’t be easily determined from surrounding layers of sediments. But Semaw got lucky at the Gona site. Lying on top of the tools was a 2.52-million-year-old layer of volcanic ash, which was reliably dated by measuring the decay of its radioactive elements. And magnetic grains in the rock layer underneath the tools recorded a flip in Earth’s magnetic field, called the Gauss-Matuyama transition, known to have occurred 2.6 million years ago. The tools, says Semaw, lay closer to the 2.6-million-year-old layer.
The simple flakes and cores Semaw found, made without much thought as to their final shape, are called Oldowan tools. They’re named after Olduvai Gorge in Tanzania, where Mary Leakey discovered 1.8-million- year-old stone tools in the 1970s. Oldowan tools have since been found throughout East Africa, including a recent discovery of some 2.3-million- year-old tools in Hadar, not far from Gona. The tools Semaw discovered, though they push back the origins of the Oldowan industry by some 250,000 years, are practically indistinguishable from those at Olduvai-- demonstrating that toolmaking technology remained unchanged for nearly one million years.
Quite a few archeologists believed that ancestral humans who lived prior to 2 million years ago were not as capable as those who lived at Olduvai Gorge, says Semaw. But the evidence at Gona shows they were capable of making Oldowan tools. And the fact that we found thousands of tools around 2.6 million years ago implies that the hominids who made them weren’t novices to toolmaking.
But as to who those hominids were, Gona has yet to offer any clues. Semaw hasn’t found any hominid remains at the site. Lucy and other members of her species, Australopithecus afarensis, roamed around nearby Hadar 3 million years ago. Researchers have recently discovered a 2.3- million-year-old jawbone at Hadar, which they attribute to an as yet unspecified species of our genus, Homo. But not much is known about what came after afarensis and before the sketchy early Homo. The Gona tools lie smack in the middle of a 700,000-year stretch in the Ethiopian fossil record marked by a paucity of evidence.
Some researchers are nevertheless willing to hazard a guess as to who might have fashioned the tools. Bernard Wood of the University of Liverpool suggests that Paranthropus aethiopicus (also known as Australopithecus boisei or A. aethiopicus, depending on whom you’re talking to) lived at the right time and in roughly the right place. But Semaw thinks that the tools were fashioned by some undiscovered early Homo species.
Aethiopicus has really heavy jaws, built for chewing or crunching almost anything, says Semaw. But the early Homo specimens we have are very lightly built compared with aethiopicus. Massive aethiopicus, sometimes affectionately referred to as the Nutcracker, would not have needed tools to break open seeds or bones. But the more slender, delicate early Homo, Semaw suggests, would have been motivated to find a better way to process food. Until someone finds hominid remains at Gona, the creators of the world’s oldest tools will remain anonymous.