The Australopithecus child’s skull discovered in Taung in 1924 is an icon of human evolution. Of the neurocranium, the fossilized sediment filling has survived. The imprints of the original cerebral gyri on this rock core have fascinated paleoanthropologists from the outset and triggered much debate on the evolution of the Australopithecus brain.
Fossil cranial sutures cast in whole new light
The imprints of the cranial sutures that are also clearly visible on the rock core had long been forgotten. Now, anthropologists from the University of Zurich teamed up with researchers from Florida State University to examine their importance for brain growth in the Taung child. Sutures are bone growth fronts where the neurocranium can expand as the brain grows. Once the brain stops growing, the sutures ossify. The Taung child, who died at about four years of age, has something unusual: a suture between the two halves of the frontal bone. According to the research team’s analyses, this so-called metopic suture is already ossified in most chimpanzees of the Taung child’s age, but often is not in human children of the same age.
Typical brain development older than thought
As the researchers now demonstrate using computer-imaging comparisons of fossil crania, the late fusion of the metopic suture in the Taung child is not unique in fossils. It is also found in many other members of the species Australopithecus africanus, not to mention the earliest examples of our Homo genus. The three typical hallmarks of human brain development – a large neonate brain, rapid brain growth and large frontal lobes – therefore appeared much earlier in the hominin family tree than was originally thought.
Fast-growing brain behind late fusion
“The late fusion of the metopic suture in humans is linked to our special brain growth,” explains Marcia Ponce de León, a senior lecturer at the University of Zurich’s Anthropological Institute. A new-born human’s brain is as big as an adult chimpanzee’s. Accordingly, the cranium, which is deformed as it passes through the mother’s pelvis, is also large. This is only possible because all the cranial sutures are still wide open. After birth, the human brain grows extremely quickly, especially the large frontal lobes. “The late fusion of the metopic suture must be directly linked to this,” adds Ponce de León. In chimpanzees, these problems do not exist. Their neonates’ heads are comparatively small, their brain growth slows shortly after birth and the frontal lobes are not as pronounced. Consequently, the metopic suture also ossifies early.
Dean Falk, Christoph P. Zollikofer, Naoki Morimoto and Marcia S. Ponce de León. Metopic suture of Taung (Australopithecus africanus) and its implications for hominin brain evolution. PNAS, 7 May, 2012. doi/10.1073/pnas.1119752109