CHAMPAIGN, Ill. -- During the past 100 years, scientists have tossed around a great many hypotheses about the evolutionary route to bipedalism, and what inspired our prehuman ancestors to stand up straight and amble off on two feet.
Now, after an extensive study of evolutionary, anatomical and fossil evidence, a team of paleoanthropologists has narrowed down the number of tenable hypotheses to explain the origin of bipedalism and our prehuman ancestors' method of navigating their world before they began walking upright.
The hypothesis they found the most support for regarding the origin of bipedalism is the one that argues our ancestors began walking upright largely in response to environmental changes -- in particular, to the growing incidence of open spaces and the way that changed the distribution of food.
In response to periods of cooling and drying, which thinned out dense forests and produced "mosaics" of forests, woodlands and grasslands, it seems likely that "some apes maintained a forest-oriented adaptation, while others may have begun to exploit forest margins and grassy woodlands," said paleoanthropologist Brian Richmond, lead author in the new study. The process of increasing commitment to bipediality probably involved "an extended and complex opening of habitats, rather than a single, abrupt transition from dense forest to open savanna," he said.
Richmond, from the University of Illinois at Urbana-Champaign, with paleoanthropologists David Begun from the University of Toronto and David Strait from the New York College of Osteopathic Medicine, describe their findings, which involved a comprehensive review and analysis of the five leading hypotheses on the origin of bipedalism, in a recent issue of the Yearbook of Physical Anthropology. Other hypotheses that remain viable, according to the team: "freeing" the hands for carrying or for some kind of tool use, and an increased emphasis on foraging from branches of small fruit trees, which is the context in which modern chimpanzees spend the most time on two legs.
For their study, the researchers combined data from biomechanics -- movement, posture and stesses in bones and joints -- and from bone growth and development. They found that our prehuman ancestors had terrestrial features in the hands and feet, climbing features throughout the skeleton, and knuckle-walking features in the wrist and hand; that finger bone curvature is responsive to changes in arboreal activity during growth, lending support to the hypothesis that many early hominid species, although bipedal, still climbed trees. Evidence from the wrist joint "suggests that the earliest humans evolved bipedalism from an ancestor adapted for knuckle-walking on the ground and climbing in trees."
The YPA article, according to Richmond, is "the first attempt in decades to bring together all of the available evidence for the argument that the earliest human biped evolved from ancestors that both knuckle-walked and climbed trees, rather than from ancestors living exclusively in trees and 'coming down from the trees,' or walking on the ground in ways similar to modern baboons."
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