Scientists say a bone, believed to be a segment of an ancient hominin finger, is the oldest-ever example of the anatomy of a modern human hand in a prehuman specimen.

The 2-million-year-old bone was found at Olduvai Gorge, a rich paleontological site in northern Tanzania. Scientists say the fossil belongs to an unidentified ancient hominin species.

Its discovery pushes the origin of the modern-human-like hand -- a hand ideal for tool use but ill-suited for tree-climbing -- back some 400,000 years.

The hand is both a measure and driver of human evolution. The dextrous digits of the modern human hand enabled man to explore and manipulate the world in new ways, and it was this interaction between hand and brain the spurred the evolution of human intelligence.

Researchers say the newly discovered bone marks the descent of pre-human hominins from the trees to the ground -- the hand of a transitionary species, bridging the gap between Homo sapiens and smaller, tree-dwelling species like Homo habilis and Paranthropus boisei.

"This bone belongs to somebody who's not spending any time in the trees at all," lead researcher Manuel Dominguez-Rodrigo, a scientist at Madrid's Institute of Evolution in Africa, told the New Scientist.

Dominguez-Rodrigo is the lead author of a new paper on the discovery, published in the journal Nature Communications.

"This provides good evidence supporting the hypothesis that, by about 2 million years ago, our early ancestors lost the anatomy linked to our tree-climbing past," added Brian Richmond, an anthropologist with the American Museum of Natural History in New York.

Based on the bone, researchers guess the species likely stood five foot, nine. Homo habilis only measured three feet tall.

"For human evolution, the new discovery shows the oldest hominin adapted to terrestrial [life] completely," Domínguez-Rodrigo told the Christian Science Monitor. "This implies a creature using tools more frequently. This modern morphology is also documented in a hominin that is bigger than the other hominins previously known."

The team of researchers is currently looking for additional bones to confirm their evolving theories, and further illuminate early man's transition from an arboreal life to one spent fashioning tools.