A new analysis of the skull suggests that human brain evolution may have been shaped by changes in the female reproductive system that occurred when our ancestors stood upright.
At some point in evolution, our ancestors switched from walking on all four limbs to just two, and this transition to bipedalism led to what is referred to as the obstetric dilemma. The switch involved a major reconfiguration of the birth canal, which became significantly narrower because of a change in the structure of the pelvis. At around the same time, however, the brain had begun to expand.
One adaptation that evolved to work around the problem was the emergence of openings in the skull called fontanelles. The anterior fontanelle enables the two frontal bones of the skull to slide past each other, much like the tectonic plates that make up the Earth's crust. This compresses the head during birth, facilitating its passage through the birth canal. In humans, the anterior fontanelle remains open for the first few years of life, allowing for the massive increase in brain size, which occurs largely during early life. The opening gets gradually smaller as new bone is laid down, and is completely closed by about two years of age, at which time the frontal bones have fused to form a structure called the metopic suture. In chimpanzees and bononbos, by contrast, brain growth occurs mostly in the womb, and the anterior fontanelle is closed at around the time of birth.
When this growth pattern appeared is one of the many unanswered questions about human brain evolution. The new study, led by
Dean Falk of Florida State University, sought to address this. Working in collaboration with researchers from the
Anthropological Institute and Museum at the University of Zürich, Falk compared the skulls of humans, chimps and bonobos of various ages to the fossilized skull of the so-called Taung Child.
Taung Child was found in 1924 in a limestone quarry near Taung, South Africa, and was the first
Australopithecine specimen to be discovered. It belonged to an infant of three to four years of age, and is estimated to be approximately 2.5 million years old. The skull is incomplete, including the face, jaw and teeth, but it contains a complete cast of the brain case, which formed naturally from minerals that were deposited inside it and then solidified. "Most of Taung child's brain case is no longer present, but you see all kinds of interesting structures in the endocast, like the imprints of the cortical convolutions," says study co-author
Christoph Zollikofer. "We looked at the imprints of the sutures. These features are very well preserved, and have been known about for 50 years, but nobody paid attention to them."
In 1990, researchers from Washington University Medical School published a
three-dimensional CT scan of the Taung Child endocast, and Falk subsequently
reconstructed it again using more advanced computer technology. Comparison of this more recent reconstruction with scans of other species now reveal that the skull of Taung Child has a small, triangle-shaped remnant of the anterior fontanelle.
This suggests that Taung Child had a partially fused metopic suture at the time of death and, therefore, that the pattern of brain development in this Australopithecine species was similar to that of anatomically modern humans. Delayed fusion of the metopic suture indicates that fast brain growth in the period following birth came before the emergence of Homo, the genus that evolved from Australopithecines and eventually gave rise to our own species, Homo sapiens.