John Gurche / Smithsonian
A reconstructed head of Australopithecus afarensis for an exhibit at the Smithsonian's National Museum of Natural History.
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updated 1/28/2011 6:18:10 PM ET 2011-01-28T23:18:10

Early human ancestors stopped swinging in trees and started walking on the ground sometime between 4.2 million and 3.5 million years ago, according to a new study.

This key moment, when our ancestors became anatomically and behaviorally less ape-like, coincides with increased cooling, more defined seasonality, and a grassland growth spurt. All transformed former forest habitats into more varied ones, forcing our very early relatives to change their ways.

"With the trees being farther apart, it became energetically advantageous for hominids to cross the gaps bipedally," said Gabriele Macho, lead author of the study that was published in the latest issue of Folia Primatologica.

Macho, a paleoanthropologist at the Catalan Institute of Paleontology in Barcelona, and his colleagues made the determinations after analyzing wrist bones from two early hominid relatives: Australopithecus anamensis and Australopithecus afarensis (also known as the "Lucy" fossil). The former species is 600,000 years older than the latter and is believed to be its ancestor.

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The researchers performed high-resolution CT scans of the central wrist bones, called capitates, of a modern orangutan, gorilla, chimpanzee and person to see how these bones differ between arboreal animals and more terrestrial ones.

They found that full-time tree swingers and dwellers load more force on the little finger side of their hands while terrestrial individuals load the thumb side more.

"Try it out yourself," Macho said. "Hold on to a pole or tree with a medium-sized diameter and observe where on the hand and wrist the greatest pressure is. You'll feel that the thumb side doesn't assume a great role."

The scientists observed that the Australopithecus anamensis wrist bones exhibited pressure loads associated with modern arboreal animals. The analyzed Australopithecus afarensis bones conversely showed stress loads comparable to those of more terrestrial species, including modern humans.

The researchers concluded that the important shift in early hominid lifestyle happened around the time when A. afarensis first emerged.

It's likely that Australopithecus anamensis walked on the ground at times too, but Macho points out that "form follows function."

Other evidence from the early human fossil record supports that major changes took place at about 4.2 million years ago.

Macho explained, "We know from cranio-dental remains that they also broadened their dietary niches and were no longer soft fruit eaters, as the last common ancestor is assumed to have been."

Carol Ward, a professor of integrative biology at the University of Missouri, thinks the new study is "important and highlights the need for more careful and detailed analysis of the functional anatomy of A. anamensis."

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Ward told Discovery News that for decades, the earliest known australopith was A. afarensis, but the more recently unearthed A. anamensis permits comparison studies to highlight differences between the likely related species.

Robin Crompton, a professor of musculoskeletal biology at the University of Liverpool, said the new study took a "refreshing approach," benefited by technology.

"The burgeoning power of computers allows us to model mechanical performance of fossil bones and whole skeletons, and effectively 'reverse-engineer' them," Crompton said.

He hopes the findings will inspire future research to solve puzzles these new findings pose. For example, Crompton asked, "Could it be that, like humans, the hand of the later species is actually more adapted for tool manufacture and use, while other features of the upper limb remain adapted to climbing?"

© 2012 Discovery Channel

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