The evolution of walking on two legs in fossil humans can be detected by checking the foramen magnum — the place at which the spinal cord passes through the skull, researchers said.
Compared with other primates, foramen magnum in humans is shifted forward, they said.
Researchers, including those from Stony Brooke University in the US, have shown that a forward-shifted foramen magnum is found not just in humans and their bipedal fossil relatives, but is a shared feature of bipedal mammals more generally.
They compared the position and orientation of the foramen magnum in 77 mammal species including marsupials, rodents and primates.
Their findings indicate that bipedal mammals such as humans, kangaroos, springhares and jerboas have a more forward-positioned foramen magnum than their quadrupedal close relatives.
"This question of how bipedalism influences skull anatomy keeps coming up partly because it's difficult to test the various hypotheses if you only focus on primates," said Chris Kirk of University of Texas in the US.
"However, when you look at the full range of diversity across mammals, the evidence is compelling that bipedalism and a forward-shifted foramen magnum go hand-in-hand," Kirk added.
"We've now shown that the foramen magnum is forward-shifted across multiple bipedal mammalian clades using multiple metrics from the skull, which I think is convincing evidence that we're capturing a real phenomenon," said Gabrielle Russo of Stony Brook University.
The study was published in the Journal of Human Evolution.