The Evolutionary Advantage of the Smooth Human Skull

When a child is born, their brain is only a fraction of the size it will eventually grow. A newborn’s skull is flexible, composed of bone and fibrous material called sutures—often referred to as “soft spots.” This smooth, flexible structure is a remarkable evolutionary adaptation, allowing the brain to grow and develop in a way unique to humans. Zeresenay Alemseged, a paleoanthropologist in the Department of Organismal Biology and Anatomy at the University of Chicago, explains that the human skull has provided a significant evolutionary advantage to our species. “Since we diverged from our cousins, the chimpanzees, around 7 million years ago, the hominid skull has changed dramatically,” Alemseged told Discover.

Why the Human Skull is Smooth

Over the course of human evolution, the braincase—the part of the skull surrounding the brain—has expanded to accommodate our growing brains. Simultaneously, the face has shortened, and the ridges and cranial structures that once anchored powerful chewing muscles have diminished as our diet shifted away from tough, fibrous foods. Attachments for strong neck muscles, crucial for our primate ancestors, have likewise significantly reduced over millions of years.

“In addition to the above-mentioned reasons, the human skull is smooth given that, during evolution, we lost many of the ridges and crests that served as attachments for heavy chewing muscles,” Alemseged explained. He emphasizes that the primary function of the modern human skull is to house and protect our exceptionally large brain, supported by the expanded braincase.

Protecting Our Brains

The structure of the human skull enables rapid brain growth after birth. At birth, the brain is approximately one-quarter to one-third of its adult size, doubling in size within the first year of life. By age five, it reaches around 90% of its adult size. The smoothness of the braincase may facilitate this postnatal brain growth.

Infants are born with unfused skull bones connected by “soft spots,” or fontanelles, which allow the skull to accommodate this rapid expansion. These sutures, according to the Cleveland Clinic, separate the major bones of the skull, providing space for growth. As the brain reaches its full size, the sutures fuse, creating a solid bone structure. Five major sutures form the skull: the coronal, lambdoid, metopic, sagittal, and squamous sutures.

Human Skull Development vs. Ape Skull Development

The development of an infant human skull differs significantly from that of apes. While the adult human skull comprises 22 bones, newborns have more separate elements, making it proportionally larger relative to the rest of the body. Adult apes and gorillas also have roughly the same number of skull bones, but the similarities largely end there.

“The human skull is a primate skull, and the overall configuration and components of the bones are similar to those seen in apes,” Alemseged stated. “But, the human skull differs from that of apes due to its exceptionally large brain, gracile jaws, and minor face, resulting in a globular shape. The human skull is strikingly different from that of other primates.”

Unlike humans, great apes possess a heavily built, projecting face with a balanced ratio between facial and braincase size. In contrast, the human face is positioned beneath the braincase and is significantly reduced in size. According to Alemseged, “we are almost face-less” when compared to great apes—at least in terms of size.