A Florida State University researcher and her colleagues say the findings suggest brain evolution was a result of a complex set of interrelated dynamics in childbirth among early, evolving bipeds.
Evolutionary anthropologist Dean Falk analyzed the Taung fossil, discovered in South Africa in 1924 and estimated to be about 2.2 million years old.
The fossil, believed to be that of a child 3 to 4 years old, is the "type specimen," or main model, of the genus Australopithecus africanus.
The first feature Falk and her colleagues found is a "persistent metopic suture," or unfused seam, in the frontal bone, which allows a baby's skull to be pliable during childbirth as it squeezes through the birth canal.
In great apes -- gorillas, orangutans and chimpanzees -- the suture closes shortly after birth, whereas in humans it does not fuse until about 2 years of age to accommodate rapid brain growth.
The second feature is the fossil's endocast, or imprint of the outside surface of the brain transferred to the inside of the skull, which allows researchers to examine the brain's form and structure.
Falk and her colleagues say their analysis suggests three important points: that the persistent metopic suture is an adaptation for giving birth to babies with larger brains, is related to the shift to a rapidly growing brain after birth and suggests a related expansion in the frontal lobes.
"These findings are significant because they provide a highly plausible explanation as to why the hominin brain might grow larger and more complex," Falk said in a university release Monday.