NEW RESEARCH BY GW ANTHROPOLOGISTS SHEDS LIGHT ON HOW HUMAN ANCESTORS BEGAN WALKING UPRIGHT
Cover Article in Nature Helps to Narrow Theories of Human Evolution

WASHINGTON-- Bringing new scrutiny to bear on the fossilized wrist joints of some of the earliest known human ancestors, a pair of researchers at The George Washington University's Department of Anthropology has found evidence to support the theory that humans evolved not from tree dwellers, but from ancestors who were already adapted to walking on the ground. In the process, researchers Brian Richmond and David Strait shed new light on how our ancestors came to walk upright, the hallmark that sets hominids - members of the human family - apart from other primates. Their article on the subject will appear on the cover of the March 23 edition of Nature.

Richmond and Strait compared fossilized wrist joints from known human ancestors with the wrist joints of other primates. They essentially found that our ancestors' wrist structure resembles that of "knuckle walking", ground dwelling primates whose wrists are designed to support the compacted weight of the body as it rests on the ground.

"For over a century, scientists have hypothesized about how and why the first human ancestors began walking upright, but until now there has been very little evidence to help answer this question," Richmond said. "Our study is important because it demonstrates that our earliest ancestors did not simply come down from the trees. Rather, they evolved from an ape that was already adapted to life on the ground. This discovery changes our ideas about the origins of the human lineage."

Fossilized remains of two of the earliest known human ancestors, Australopithecus afarensis, best known by the skeleton nicknamed "Lucy", and A. anamensis, were discovered decades ago. However, up until now there has been very little attention paid to the wrist joints. The researchers found that the wrists of these early upright walkers show evidence that their ancestors walked on their knuckles in a way similar to how chimpanzees and gorillas knuckle-walk today. These primates have relatively long fingers that allow them to grasp branches and climb trees, but when they walk on the ground, they protect their fingers by curling them and walking on the backs of their knuckles. This requires a very stable wrist joint that doesn't extend back under the weight of the body.

Although the early human ancestors walked upright between 3 and 4.1 million years ago, their wrist anatomy resembles those of chimpanzees and gorillas in having features that buttress the wrist joint and help to lock the wrist into a stable position so the animal can support its weight on its fingers. The presence of the locking mechanism in the wrists of early hominids indicates that their ancestors were knuckle-walkers. It is not until the later species A. africanus, at 2 to 3 million years ago, that evidence of a more humanlike, flexible wrist joint exists.

"We are not certain why our human ancestors evolved a more mobile wrist joint, but it may have improved their abilities to make and use tools, or throw objects," said Richmond.

By showing that upright walking evolved from an ancestor that was already adapted to life on the ground, the study narrows down the theories on early human origins. For example, theories stating that human ancestors began walking upright because they were forced out of trees by shrinking forests no longer hold true. Instead, other theories now appear to make more sense, including theories that upright walking began with new ways of gathering food while standing upright or dedicating the hands to new uses, like carrying food or other objects.

"The evolution of upright walking required massive modifications in skeletal anatomy," said David Strait. "So there must have been a good reason for making those changes." The exact reasons, however, will remain a mystery until scientists better understand the anatomy, find new fossils from just before and just after the origin of upright walking, and gain a clearer picture of the environment at those times.