May 8, 2012
‘Taung Child’ Fossil Reveals Hominin Brain Evolution
Brett Smith for RedOrbit.com
The reexamination of an ancestral human fossil found almost 90 years ago indicates that evolutionary changes in human brain development started 2.5 million years ago, about the time these ancestors began to walk upright.
Florida State University researcher Dean Faulk and her colleagues analyzed the 2.2 million-year-old Taung fossil of a 3 to 4-year-old child and found several features that suggest a developed mechanism that allowed for the growth of a more complex brain. The fossil, nicknamed "Taung Child," was discovered in a lime mine in South Africa, and was the first specimen of this species of hominin.
An unfused seam, or “persistent metopic suture,” in the Taung skull´s fontal bone showed that the hominid was born with a pliable skull, which allowed it to squeeze through the mother´s birth canal. In great apes, the suture closes shortly after birth, while human children do not have a fully fused suture until 2 years old.
"The persistent metopic suture, an advanced trait, probably occurred in conjunction with refining the ability to walk on two legs," Falk said. "The ability to walk upright caused an obstretric dilemma. Childbirth became more difficult because the shape of the birth canal became constricted while the size of the brain increased. The persistent metopic suture contributes to an evolutionary solution to this dilemma."
The fossil´s endocast, or imprint of the outside surface of the brain transferred to the inside of the skull, also helped researchers to examine the size and structure of the ancient brain.
In addition to examining the Taung fossil skull, the researchers looked at hundreds of skulls and CT scans belonging to both apes and humans. The study concluded that the persistent metopic suture is an adaptation for giving birth to babies with larger brains.
The suture is related to the evolutionary shift to a rapidly growing brain after birth, and possibly–to the expansion in the frontal lobes, where higher brain functions occur. In humans, the frontal lobe has been found to have the ability to recognize future consequences, to make choices, and to determine similarities or differences between things or events.
"These findings are significant because they provide a highly plausible explanation as to why the hominin brain might grow larger and more complex," Faulk said.
The Taung Child was originally thought to be about six years old, but is now believed to have been 3 to 4 years old based on studies of the teeth´s enamel. Examinations of the fossil compared to that of an equivalent 9-year-old child suggest that the species had a growth rate more similar to modern apes than to that of modern Homo sapiens. However, intermediate species such as Homo erectus are thought to have growth rates between that of modern humans and apes.
The Florida State study comes on the heels of another human evolution study based on genetics. A research team recently uncovered a genetic variation that occurred in man´s ape-like ancestor about two or three million years ago. According to the study, this genetic variation helps drive development of the neocortex, which controls higher-order brain functions.
The findings were published in the May 7 edition of the Proceedings of the National Academy of Sciences (PNAS).
Marcia S. Ponce de Leon, Christoph P.E. Zollikofer and Naoki Morimoto of the Anthropological Institute and Museum at the University of Zurich in Switzerland contributed to the research.
Image 2 (below): Taung surrounded by a juvenile chimp skull and human skull, the latter having a fontanelle and metopic suture. The metopic suture is visible on the frontal lobe of Taung's endocast. Credit: CT-based images by M. Ponce de LeÃ³n and Ch. Zollikofer, University of Zurich.