What these ancestors looked like still remains a mystery, since few fossil records exist, Kramer said.
"There are lots of gaps in the fossil records. We always hope we'll find the record to help fill it in," she said.
The model could also provide more detail about the evolution of modern humans. (Explore a map of the human evolutionary highway.)
Over time, that awkward shuffle became a full-fledged walk, she said. The more the ancestors moved about bipedally for short distances, the more they began to develop variations in their anatomies that encouraged walking upright.
Those able to walk well passed the trait on to their offspring, Kramer said.
The ability to move on two feet also put into motion a new set of behaviors, which became precursors to modern humans' long legs and bigger brains.
"Bipedalism didn't cause [the changes], but maybe bipedalism allowed it to happen," Kramer said.
(Related: "6-Million-Year-Old Human Ancestor 1st to Walk Upright?" [March 20, 2008].)
Karen Steudel is a zoology professor at the University of Wisconsin in Madison and expert in bipedal movement in humans.
Kramer and colleagues' model could be expanded to incorporate new findings, such as what [early ancestors] ate, Steudel said.
"If we were able to find out more about what they were feeding on and if there was some way to extrapolate from modern relatives of these plants, then this model could be used to evaluate [what they ate]."
The theory that foraging spurred protohominds into bipedal movement is one of many. Other theories say they stood on two legs to see predators over tall grasses, or to carry things in their forelimbs. (Related: "Upright Walking Started in Trees, Ape Study Suggests" [May 31, 2007].)
"[The new study has] offered a new way of conceptualizing what had been previously a purely theoretical argument," Steudel said.
"They're putting some quantitative limits to the argument."
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