Why Do Babies Have Soft Spots?

Humans’ big, fast-growing brains and unique style of walking explain why it takes so long for infant skulls to develop

The soft spot and metopic suture
The soft spot and metopic suture are clearly visible on the skull of the young human (right) and absent in the young chimpanzee (left). Those features are present, although harder to see, in the fossil of a young Australopithecus (center). CT images by M. Ponce de León and C. Zollikofer, University of Zurich

Thinking about the soft spots on a baby’s head makes me squirm. I always envision the mushy parts of an overripe peach. Of course, soft spots are nothing like that. They’re strong, elastic patches of skull where bone hasn’t fully formed yet. Hominid babies have had soft spots for at least three million years, a new study finds. The researchers say soft spots evolved in response to our unique brains and unusual style of walking.

In the new study, a team led by Dean Falk, an anthropologist at Florida State University, considered a soft spot on the most famous hominid toddler, the Taung Child, an Australopithecus africanus tyke that lived about three million years ago. The Taung Child fossil consists of a fossilized brain, facial bones and a jaw. On top of the brain, right behind where the forehead ends, the team found the outline of a soft spot that had imprinted on the brain’s surface. They also found the trace of the metopic suture—a line that marks where the two sections of the frontal bone (the forehead) come together. It runs from the top of the nose to the soft spot behind the forehead.

As the two parts of the frontal bone fuse into one bone, the metopic suture disappears. In chimpanzees, this usually occurs in utero, before a chimp is born. In humans, this doesn’t happen until sometime between ages 1 and 2. And in some people, the metopic suture is still partially present in adulthood. In one study of more than 2,000 adult humans, 16 percent still had a partial metopic suture, but none of the study’s more than 350 adult African apes did. (Just to be clear, even if an adult has a metopic suture, that doesn’t mean his/her soft spot is still there. That would be fully hardened. And the suture is only something you’d only notice if you could peel back a person’s skin. For example, check out this skull with a metopic suture versus this one without the suture.)

The Taung Child was about 3 years old when it died, indicating hominids began experiencing a delay in the fusion of the metopic suture relative to other apes by at least three million years ago, the researchers report online today in the Proceedings of the National Academy of Sciences. The team also found metopic sutures in the skulls of Homo habilis and Homo erectus.

Why does it take so long for the hominid forehead to fully fuse?  Falk and her colleagues offer a few explanations. First, they highlight what’s known as the “obstetric dilemma.” The switch to upright walking changed the shape of the pelvis, and as a result, the size of the birth canal. As hominids evolved bigger brains—and therefore bigger heads—it became harder for hominid moms to give birth. Having a soft spot and a metopic suture helped alleviate the problem of pushing a big head through a small birth canal. As the researchers note:

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.

Falk and her colleagues also point out that human babies experience the same rapid brain growth during the first year of life that they experience as fetuses in utero. Saving some of the brain development until after birth probably helps with the problem of a small birth canal, and having a soft spot and unfused forehead allows the skull to keep up with the growing brain. In addition, the researchers note that the frontal cortex—the part of the brain that sits behind the forehead and is responsible for some of our advanced cognitive abilities—is different in humans than other apes. During the course of human evolution, some of the structures in this part of the brain changed size and shape. The changes in this area may explain why this part of the skull in particular stays pliable for longer in young humans, the researchers say.

It’s not clear when all of these factors came into play in hominid history. For example, australopithecines did have a reorganized pelvis, but their brains were no larger than chimps’, so it’s not certain that a big head going through a small birth canal was a significant problem for them. And there’s not enough fossil evidence to say whether they had rapid brain development as babies. However, features of their skulls do indicate that the frontal cortex may have been enlarged relative to earlier apes, so these changes might have contributed to the delayed development in the Taung Child’s skull.

Falk and her colleagues conclude that anthropologists need to start looking for soft spots and metopic sutures in even earlier hominids, such as Lucy’s species, Australopithecus afarensis, to see when such features first evolved in our lineage.

As for me, I’ve concluded soft spots aren’t as gross as I thought they were.

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