The imaged skulls show the difference in position of the foramen magnum in a bipedal springhare, on the left, and its closest quadrupedal relative, the scaly-tailed squirrel, on the right. Photo by Russo and Kirk/Journal of Human Evolution
March 17 (UPI) -- New research by anthropologists at Stony Brook University and the University of Texas at Austin confirm the human skull and bipedalism co-evolved.
Scientists have previously linked bipedalism with the emergence of a key feature of the human skull, but the finding was contested. The latest analysis confirmed the link and suggests the correlation is found among all bipedal mammals.
The key feature is a forward-shifted foramen magnum, the hole in the bottom of the skull through which the spinal cord passes. Scientists argue the adaptation ensured the brain remained centered over the spine.
Researcher Raymond Dart first argued in favor of the evolutionary coupling in 1925 in his study of the 2.8 million-year-old fossil skull of an adolescent Australopithecus africanus specimen. Though many scientists have lent their support to Dart's original assertion, it continues to be a target of criticism.
The latest research -- detailed in the Journal of Human Evolution -- suggests the link between bipedalism and a forward-shifted foramen magnum isn't isolated to humans and their earliest bipedal relatives. Other bipedal mammals also developed a forward-shifted foramen magnum as they took to two feet.
"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," UT Austin anthropologist Chris Kirk said in a news release. "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 and his former colleague Gabrielle Russo, now an assistant professor at Stony Brook University, analyzed the position of the foramen magnum in 77 mammal species. The pair found kangaroos, springhares and jerboas all have foramen magnums shifted farther forward than their closes quadrupedal relatives.
"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," Russo said.