The use of a mechanical engineering technique more usually associated with industrial robotics than human evolution has led researchers at Liverpool University to conclude that if the earliest hominids walked for any length of time, then they walked upright.
The finding provides powerful backing for the theory that "Lucy", the famous fossilised hominid skeleton discovered in Ethiopia in 1974, walked similarly to us.
An opposing, and more widely accepted theory, suggests that Lucy, who lived about 3.6 million years ago, walked like a chimp - with flexed knees and bent back.
Robin Crompton, project leader at the university's department of human anatomy and cell biology, says that the Lucy skeleton is generally accepted to be a mix of some features which are chimp-like, suggesting an arboreal lifestyle, and some which are human-like, indicating terrestrial bipedalism.
Furthermore, Lucy's brain is chimp-size but the knee joint is very similar to our own. Some features of the skeleton are however unique.
Dr Crompton says that on the basis of the data his team has gathered, he is "virtually certain" that if Lucy walked, she did so without a flexed knee.
"That really is a non-starter," he says. A big unknown, though, is how much walking Lucy actually did as opposed to, for example, spending time in the trees.
The findings are based on the use of a technique called predictive dynamic modelling. It uses information on the length and mass distribution of segments, linked by joints, to predict the motion of the system as a whole.
Researchers kicked off by building mechanically accurate 3-D computer models describing the locomotion of living species - specifically humans and chimpanzees.
Having satisfied themselves that the models created on computer accurately reflected real behaviour, researchers then adapted them to represent the known or estimated measurement of the anatomy of fossil species. A "locomotor" model of Lucy is one of the outcomes.
Mapping a chimp-like mass distribution and chimp-like bipedal motion on to the Lucy model shows that it has no stability and falls over immediately when it tries to walk.
With a chimp-like mass distribution and the characteristics of a human imitating chimp gait (bent back, flexed knees) imposed on the model, considerable problems are experienced in regulating the heat that is generated internally.
But with a human-like mass distribution and human-like bipedal motion, the model is able to walk upright.