"Bipedalism probably does represent a fundamental first step in human evolution," Stringer said. "As Darwin recognized, walking on two legs frees up the arms and hands for tasks like carrying, tool making, and tool use. And much of what happened in human evolution later on stemmed from it."
Darwin postulated that tool use led to greater intelligence. Several theories exist to explain how and why bipedalism and upright posture emerged.
Toward the end of the Miocene epoch (23.8 to 5.3 million years ago), much of Africa's climate and landscape changed due to an extended drying period. The change in climate and landscape precipitated a change in food availability and distribution, necessitating a transition in how the animals lived.
Some scientists argue that the earliest hominids to walk with an upright posture were tree-dwelling primates forced to the ground as the African forests shrank and became more savanna-like. While many ape species probably became extinct during this period, the creatures who adapted to walking on two legs survived.
Others argue that the change in climate forced early forms of hominids to change to a diet based on insects, nuts, eggs, small reptiles, and fish found on the forest floor and at lake and stream edges, rather than foliage and hanging fruit.
The shrinking forests caused the tree-dwelling primates to leave the trees in the daytime in search of food. However, they returned to the trees at night, where they would be less exposed to predators on the ground. Walking on two legs would give them a distinct advantage in enabling them to carry the food they found back to the trees.
Owen Lovejoy, a biological anthropologist at Kent State University in Ohio, argues that bipedalism may have evolved to enhance the reproductive strategy of the species.
Since walking on two legs makes it possible to carry more food, bipedal families could practice a division of labor, according to Lovejoy's theory. One parent could go in search of food and return with a meal for the entire family. The other could remain "home," focusing more energy on child rearing.
This arrangement, Lovejoy suggests, would result in better survival rates for offspringgiven that the four-legged alternative is to forage together as a family, eating as you go.
"Both males and females would benefit from intensified cooperative care of offspring under such conditions, as this would reduce the burden on females and enhance infant survival rates," Lovejoy said. "Over time, females began to choose males who regularly offered food."
"What's amazing is that we can use the same equipment that doctors would use in a hospital if you had a hip problem," Eckhardt said. This equipment has been used "to identify structural adaptations to new behavior patterns in an intermediate creature that no longer exists."
CT scan analysis was conducted using a new and more advanced software program. The software revealed that the density of the fossil thigh bone, (known as BAR 1002'00 Orrorin tugenensis) closely matches the structure of human femurs. External qualities of the fossil also indicate an upright posture.
However, the scans, which were taken several years ago, could be made sharper by improved techniques. So while a consensus seems to be gathering that the owner of this bone was bipedal, some questions remain.
"I'm not sure the scans, which have some problems with them, are very convincing," Lovejoy said. "I am convinced, based on morphology and anatomy, that [the owner of] Orrorin was bipedal at least frequently, if not habitually. The question is to what extent."
Don't Miss a Discovery
Sign up for the free Inside National Geographic newsletter. Every two weeks we'll send you our top news stories by e-mail.
For more human origins stories, scroll to bottom.
SOURCES AND RELATED WEB SITES