Getting a grip on prehistoric tool makers.

A small, unassuming bone runs halfway up the thumb from its base. That same bone offers scientists a surefire way to tell which ancient members of the human evolutionary family, known as hominids, possessed hands capable of making stone tools, according to a report in the Sept. 9 Science.

Moreover, an analysis of thumb fossils indicates that an extinct line of small-brained hominids called Paranthropus (or robust australopithecines) proved as anatomically prepared to fashion such implements as Homo erectus, a direct human ancestor, asserts Randall L. Susman, an anthropologist at the State University of New York at Stony Brook.

"I'm offering a new way to diagnose tool behavior," Susman says. "So far, it looks like no hominids were capable of tool making before 2.5 million years ago, and after that time all hominids were capable of tool making."

The new study supports Susman's prior elevation of Paranthropus to the status of tool maker, an assessment that challenged the widespread view that the Homo lineage held a monopoly on chipping useful devices out of stone (SN: 5/28/88, p.344).

The Stony Brook scientist examined lower-thumb bones of 12 pygmy chimpanzees, 49 common chimpanzees, and 41 modern humans, as well as single thumb fossils from Australopithecus afarensis (dating to about 3 million years ago), Paranthropus robustus (found at a 1.8-million-year-old South African cave), H. erectus (from the same cave), and a Neandertal dating to around 50,000 years ago.

Modern human specimens display a broad thumb head (where the joint forms) in relation to thumb length. Chimps' thumb bones show much narrower heads relative to their length.

The wider portion of the human bone allows for the insertion of three additional muscles that add strength and refined motor control to thumb movements, Susman argues. As a result, human hands can generate the force needed to manufacture and wield stone tools, he contends.

A chimplike thumb occurs in A. afarensis, but human thumb proportions characterize the remaining hominids, Susman holds.

"Susman has given us an apparently foolproof way of determining which of our early ancestors had hands that functioned in a way similar to our own," writes Leslie C. Aiello, an anthropologist at University College London, in an accompanying comment. But the thumb bone Susman identifies as P. robustus shows enough similarity to the corresponding H. erectus bone to raise the possibility that both fossils belong to the latter species, Aiello argues.

"We simply can't tell whether these bones belonged to Paranthropus or Homo," holds Erik Trinkaus, an anthropologist at the University of New Mexico in Albuquerque.

But in an unpublished comparison to modern primate thumbs, the two South African fossils show contrasts in size and shape that place them in different species, Susman responds.

He emphasizes that the first tool makers needed powerful hands, not large brains. Still, reorganization of motor and spatial regions in the brain must have made this behavior possible, Trinkaus argues.