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A human forceful precision grip grasping a bone at the base of the thumb from <i>Australopithecus africanus</i>.

A human forceful precision grip grasping a bone at the base of the thumb from Australopithecus africanus.

Tracy Kivell & Matthew Skinner

How human ancestors got a grip

Squeeze a baseball or pen between your thumb and the tips of your fingers: You are using what researchers call a precision grip, a highly evolved adaptation thought to be unique to modern humans and our most recent ancestors. Chimpanzees, for example, have thumbs too short to allow them to grip objects so precisely. But a new study suggests that human ancestors in South Africa had a good grip perhaps as early as 3 million years ago—and so may have wielded stone tools earlier than expected.

Human ancestors have been handy since at least 2.6 million years ago, when the earliest reliably identified stone tools appear in the fossil record at Gona, Ethiopia. But the identity of the first toolmaker has been mysterious, because more than one type of hominin was alive 2 million to 3 million years ago: our genus Homo, plus several species of Australopithecus, including A. africanus from South Africa and A. afarensis—the species of the famous skeleton of Lucy, who lived near Gona. Stone tools have been found at sites with Australopithecus fossils, as well as bones with possible cut marks dating back to 3.2 million to 3.4 million years ago, but in the absence of a fossil hand gripping a tool, it has been impossible to prove that australopithecines made and used tools.

To complicate matters further, complete hand fossils—full of tiny fragile bones—are extremely rare. Instead, paleoanthropologists have had to rely on clues from traits in isolated finger bones, such as a report in 2013 that our direct ancestor Homo erectus used a precision grip 1.7 million years ago. And in 1997, a landmark study of a composite hand skeleton of Lucy’s species, A. afarensis, suggested that Lucy lacked a full precision grip 3.1 million years ago, but that her species had developed several but not all of the traits in its hand bones that are associated with the precision grip required for habitual toolmaking. This hinted that the species was beginning to use its hands more like us, perhaps for tool use.

A new method for analyzing CT scans of fossils, however, is giving paleoanthropologists a new, more detailed look at the internal structure of hand bones, revealing how their habitual handiwork shaped their hands during life. In the report published online today in Science, husband-and-wife team Matthew Skinner and Tracy Kivell, both paleoanthropologists at the University of Kent and the Max Planck Institute for Evolutionary Anthropology, and their colleagues used CT scanners to examine the pattern of fossilized spongy bone tissue within finger and hand bones of hominins that lived 2 million to 3 million years ago.

When modern humans use a forceful precision grip frequently during childhood, their bones adapt: Tiny spicules, or filaments, of bony tissue called trabeculae form and act as struts to provide more bone density—and strength—where the forces are greatest. Gripping a tool distributes forces asymmetrically along the base of the thumb and the side of the knucklebones, and modern hands show a corresponding pattern of trabecular density. By contrast, when chimps grip a branch, the forces align across the thumb more evenly, leaving a symmetrical pattern in the trabecular bone.

When the team scanned hand bones from four members of A. africanus that lived in South Africa between 2 million and 3 million years ago, they found that the pattern of the trabecular bone was asymmetrical, as in modern humans and Neandertals that use tools frequently (as they also show in their study). This suggests that A. africanus was using a “human-like” precision grip “much earlier and more frequently than previously considered,” the authors write. They stop short of saying that A. africanus was using and making stone tools and acknowledge that these grips could have been used for a number of different activities with tools. However, the similarity of the pattern with modern humans suggests that hominins had the capacity for stone tool use more than half a million years before such tools are securely dated in the fossil record. They also have scanned hand bones of other members of Australopithecus, including Lucy’s species, A. afarensis, but the pattern of use was not preserved in that species’s trabeculae.

The new method provides “solid evidence that A. africanus was likely using forceful precision pinch grips,” says ancient hand expert Matthew Tocheri of Lakehead University in Thunder Bay, Canada, and the Smithsonian Institution’s Human Origins Program in Washington, D.C. But he notes that the evidence for tool use is still indirect. To show that, researchers will need to find stone tools at many early Australopithecus sites. Other activities that the hominins engaged in frequently during development, such as digging tubers or climbing, might also explain the signs of stress, warns paleoanthropologist C. Owen Lovejoy of Kent State University in Ohio.

Future studies will be needed to see if this pattern is found just in A. africanus or in other members of Australopithecus as well, says paleoanthropologist Brian Richmond of the American Museum of Natural History in New York City. Regardless of the precise cause, Tocheri says: “It provides further support for the hypothesis that australopithecines … actually used their hands in more humanlike ways.”