For ancient hominids, thumbs up on precision grip

ALBUQUERQUE — A tiny fossil thumb bone provides a gripping look at the early evolution of human hands, according to a study presented April 16 at the annual meeting of the American Association of Physical Anthropologists.

An upright gait and a relatively sophisticated ability to manipulate objects apparently evolved in tandem among the earliest hominids at least 6 million years ago, said Sergio Almécija of the Autonomous University of Barcelona. That’s well before the earliest evidence of stone toolmaking, about 2.6 million years ago, arguing against the idea that fine motor skills for toolmaking drove the evolution of opposable thumbs.

Almécija and his colleagues studied a bone from the tip of a thumb belonging to Orrorin tugenensis. At an estimated 6 million years old, Orrorin is the second oldest hominid genus. A more recently identified hominid genus and species, Sahelanthropus tchadensis, may have lived 7 million years ago. Controversy exists over whether fragmentary Sahelanthropus and Orrorin fossils can be used to identify new hominid genera.

Limb and jaw pieces, as well as teeth, from at least five Orrorin individuals were unearthed in Kenya in 2000.

The thumb fossil indicates that Orrorin had a long enough thumb to meet the tips of the other fingers, allowing for fine manipulation of objects.

“The Orrorin thumb bone is the most humanlike in the available fossil record, other than recent Homo species,” Almécija said.

By comparing Orrorin’s thumb with thumb bones from a variety of ancient apes and hominids, as well as from living people, Almécija uncovered a pattern that he says argues against the current notion that hominids first evolved handier hands as they learned to make stone tools. No Sahelanthropus thumb bones have been found.

In Almécija’s view, early hominids inherited hands capable of fine manipulation from small-bodied apes that lived in Africa and Europe between 25 million and 5 million years ago. Hands then assumed a more apelike, less dexterous structure in later hominids, including Ardipithecus and Australopithecus, before again evolving a precision grip in the Homo lineage.

Orrorin’s humanlike thumb calls into question a long-standing assumption that 1.8-million-year-old hand fossils from Homo habilis, unearthed in Africa more than 40 years ago, represent the earliest transition to a precision grip, he added.

Almécija’s group compared the Orrorin thumb bone to corresponding fossils from later Australopithecus and Homo species, as well as to ancient apes. Many extinct apes possessed short hands with long thumbs suitable for tightly grasping objects or tree limbs, Almécija said. That arrangement served as the foundation for the evolution of early hominid hands, he hypothesized.

Russell Tuttle of the University of Chicago had previously predicted that early hominids had a relatively sophisticated grip. Tuttle called the new analysis of Orrorin’s thumb “unsurprising.” But what Orrorin would use a precision grip for is unclear, Tuttle noted. A thorough comparison of the Orrorin fossil to thumb bones from various living primates is needed, in his view.

“A chimp could knock out a basic stone tool with its hands if it had the cognitive ability to try to do so,” Tuttle said.

Tuttle originally analyzed the H. habilis hand fossils cited by Almécija. The new evidence doesn’t alter the status of those finds as representative of a key transition in hand anatomy related to stone toolmaking, Tuttle argued.

Another meeting presentation, delivered just after Almécija’s, suggested that Orrorin’s humanlike thumb didn’t necessarily evolve to support toolmaking. Erin Marie Williams of George Washington University in Washington, D.C., described surprising evidence that even the thumbs of living people aren’t specially designed to enable the high-impact strikes of one stone on another needed to produce sharpened implements.

“This calls into question hypotheses linking modern human thumb anatomy specifically to stone tool production,” Williams said.

Williams and George Washington University biological anthropologist Brian Richmond attached special sensor devices to the fingers of six experienced stone toolmakers as they used a “hammer stone” with one hand to strike sharpened flakes from another stone.

Volunteers usually snapped their wrists as they hammered on stones, often delivering the most force and pressure to fingers other than the thumb, Richmond said. This argues against the thumb being especially important for this type of toolmaking.