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Humans differ from primates and other mammals in a number of anatomies, including having big brains and big babies. The evolutionary origin and adaptive significance of a big brain and the consequent difficulty of giving birth to a big baby have long been discussed. Falk et al. (1), in PNAS, report on a small anatomical difference of the cranium between humans and chimpanzees, and document the presence of this trait in early human ancestors. This trait may have profound importance for understanding evolution of birth and brain development. Falk et al. (1) report on the metopic suture (MS) in modern humans, two species of chimpanzees, and our hominid ancestors, Australopithecus and earlier Homo. MS is the joint between the two frontal bones of the cranium (Fig. 1). In the human fetus and young child, the confluence of MS with the coronal and sagittal sutures is the anterior fontanelle. This is the “soft spot” in a baby’s cranium. Fusion of MS occurs in infancy (2⇓–4), whereas the anterior fontanelle closes by 2 y of age (5). Fusion of MS begins near the nose and progresses toward the anterior fontanelle. As the cranium forms around the developing brain, premature closure of MS causes atypical cranial development (6). Arrow indicates persistent MS in adult human (postmortem damage to nose and right orbit). Image courtesy of Dr. Clark Spencer Larsen. Based on large samples of humans and chimpanzees, Falk et al. (1) report that MS fuses at a later age in humans than chimpanzees, with age based on eruption of deciduous and permanent molars. In contrast with humans …

Small anatomical variant has profound implications for evolution of human birth and brain development