A fossil hand bone from almost 2 million years ago belonged to an early human ancestor who may have been much more like us than other prehumans existing then, researchers say.
The bone from the "pinky" finger of a left hand, found in Tanzania and dated to around 1.84 million years ago, suggests its owner was taller and larger than any other contemporary species, they say.
If the size of the bone had the same proportion to body size as in a modern human, he would have stood around 5 feet 9 inches tall, they estimate.
The discovery pushes the earliest known "modern" hand back by some 400,000 years, they report in the journal Nature Communications.
It has long been assumed human hands began to evolve for tool use, but the discovered fossil suggests they began to take on a modern configuration earlier than previously believed.
The change from earlier to more modern hand shapes may have been more the result of early hominins abandoning a life spent in trees and transitioning to terrestrial living, says study lead author Manuel Domínguez-Rodrigo, a paleoanthropologist at Complutense University of Madrid.
Ancient hominin hand fossils often display curved finger bones well-suited for climbing in trees and hanging from branches, whereas modern humans are the only living higher primates who have straight finger bones like that found in Tanzania.
"A modern-like hand in the past would tell us when humans became fully terrestrial and when and how efficiently our ancestors used tools," Domínguez-Rodrigo says.
The discovered finger bone belonged to a hominin lineage, as yet unidentified, similar to Homo erectus, the first species known to make and possess tools.
Other experts agree that the straight finger bone suggests a turning point in human evolution.
"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," says Brian Richmond of the American Museum of Natural History in New York.
It's likely a taller, more agile species had already made the transition to terrestrial living by 1.8 million years, while still coexisting with smaller, more tree-dwelling species such as Homo habilis and Paranthropus boisei, scientists say.
Previous studies have suggested hominins took to walking on two legs about 6 million years ago, but until about 2 million years ago, all known hominin hand fossils showed retention of a tree-climbing configuration.
That's what makes the latest discovery so important, Domínguez-Rodrigo says of the 1.84-million-year-old specimen.
"This bone belongs to somebody who's not spending any time in the trees at all," he says.