A recent article in PNAS looks at the metopic suture on the Taung endocast. Before discussing the article a few words are in order about the frontal bone.
The frontal bone forms part of the face and part of the calvaria. The frontal bone forms from two halves via intramembranous ossification from two primary centers, which are located near the supraciliary arches. The two halves are joined by the frontal, or metopic, suture, which fuse between the first and fourth year postnatal. Incomplete metopic suture closure varies with population and range as high as 10% in some populations. This condition has, in the past, been mistaken for a skull fracture. Premature fusion of the metopic suture leads to a condition called trigonocephaly. The anteriorposterior angles of the frontal meet the parietal and form the anterior fontanelle, 96% of anterior fontanelles are fused by the second year postnatal. Incidentally, according to Scheuer and Black (2000:118) the name fontanelle is derived from the following:
Fontanelle (French meaning ‘little fountain’) Medieval surgeons attempted to cure brain and eye diseases by placing a cautery at the site of the anterior fontanelle. The wound remained open by application of an irritant so that poisonous substances could supposedly escape. The welling of blood from the site was supposed to have looked like water bubbling from a spring.
Which brings us to the PNAS study. The paper, written by Dean Falk, Christoph P. E. Zollikofer, Naoki Morimotoc, and Marcia S. Ponce de Leónc, looked at metopic suture closure in 407 chimpanzees, 136 bonobos, 1,060 modern humans (plus an additional 360 from medieval Swiss graveyards), and 58 fossil hominins. Each sample was divided into dental age classes and scored on the state of metopic suture closure (based on direct examination of the skull, CT scans[where available], and endocasts [where available]). Suture fusion was scored as unfused, partially fused, or fully fused. Falk et al’s data suggest:
…that late MS fusion may have evolved in small-brained gracile hominins that lived between ~3.0 and 1.5 million y ago. Comparatively high frequencies of partially fused or unfused MS in Homo erectus from Asia and in the Neanderthals indicate that the trend toward late MS fusion continued in mid-to-late Pleistocene hominins…
That being the case, one would like to know what is behind this trend. Falk et al propose three different , but somewhat interrelated hypotheses. First, the obstetric dilemma. In humans there is a trade off between brain size, birth canal size, and bipedalism. As Falk et al point out:
The anterior fontanelle and patent metopic suture of human neonates facilitate parturition. During delivery, contractions of the birth canal cause the edges of the neonate’s frontal and parietal bones to overlap and glide together in the region of the anterior fontanelle, which compresses the head and facilitates expulsion of the neonate from the birth canal…
Second, high early postnatal brain growth (see here for more on growth rates). Basically human grains continue to grow at high rates, compared to chimpanzees, during the first year of life.
Third, reorganization of the frontal cortex. Unfused metopic sutures are associated with increased interorbital and frontal bone widths in modern humans. According to Scheuer and Black (2000) unfused metopic sutures are not related to either skull shape or cranial capacity. There is, however, a correlation with an increase in frontal curvature. They also point out that one interesting question regarding the metopic suture is why it closes much earlier than the sagittal suture. Something the Falk et al study doesn’t look but their data probably could address.
Falk et al (2012) Metopic suture of Taung (Australopithecus africanus) and its implications for hominin brain evolution. PNAS 2012; published ahead of print May 7, 2012
Scheuer and Black (2000) Developmental Juvenile Osteology