Human newborns have been helpless for 1 million years

If there is one thing modern humans share with our long-gone ancestors, it’s that caring for newborn babies is a daunting and exhausting task. Our species comes into this world much less developed than other great apes, with weak neck muscles, soft skulls, and still underdeveloped brains that rapidly grow after birth.

As a result, human parents must spend significantly more time holding and caring for their young to keep them alive or prevent deformational plagiocephaly, or flat head syndrome. It appears that this weakness has been the case for over one million years, according to a study published today in the journal Proceedings of the Royal Society B Biological Sciences.

One theory for this lag in development is called the obstetrical dilemma. The soft noggin and underdeveloped brain may be a trade-off for the large brains that develop later and our ability to walk upright (which affects the shape of the birth canal). Still, pinpointing when this helpless newborn phase arose within our evolutionary timeline is difficult. Fossils of our distant ancestors are mostly fragmented, and rarely include the remains of infants.

A new approach detailed in this study may be able to help. It can be applied to fossils of any age, and may provide some new pieces to the evolutionary puzzle.

“Our method looks for changes in the shape of the cranium (the top part of the skull) which occurred around or soon after birth and remained to some extent into adulthood,” Yousuke Kaifu, a study co-author and anthropologist from the University Museum at the University of Tokyo, said in a statement. “Nowadays, this is a common condition, called deformational plagiocephaly. The shape of the head is affected by the baby lying the same way frequently due to how they are held or laid down, and from the baby’s neck muscles being too weak to control the position of the head.”

In 2007, Kaifu was in Indonesia and saw a skeleton of Homo floresiensis, an archaic human species also called the “hobbit,” that lived more than 50,000 years ago. He was struck by the skull’s deformed shape, which appeared to have occurred after birth due to the structure of the bones. To understand why, he contacted a hospital clinician who identified the cause as deformational plagiocephaly or flat head syndrome. Since skull deformation is not caused by disease or after a body is buried, Kaifu then theorized that it may be an indicator of helplessness in infancy. A flat head on a skeleton implies a soft, underdeveloped skull and potential head immobility.

To test this theory, Kaifu and other researchers from Japan and Indonesia collected CT scans of 123 modern, healthy infants ranging from one day to 17 months old. They also gathered measurements from 385 human skulls from Japan dated from about 200 to 1,000 years old, 996 non-human great ape skulls, five fossilized Homo erectus skulls, and one fossilized Homo floresiensis skull.

Back views of the skulls of a historical human (left) and a Homo erectus from Ngawi, Java (right: undated), exhibiting severe transverse deformational plagiocephaly. Image: Kaifu Y et al. 2026 CC BY. — Back views of the skulls of a historical human (left) and a *Homo erectus* from Ngawi, Java (right: undated), exhibiting severe transverse deformational plagiocephaly. *Image: Kaifu Y et al. 2026 CC BY.*

They found that the chimpanzees, bonobos, gorillas, and orangutans, which are born more physically independent and capable, had a more narrow range of cranial deformity. All of the human samples (modern or fossil) had a much broader range of deformation and head shapes.

“This was exciting as we don’t know a lot about the behavior of Homo erectus. They lived between more than 1 million to 110,000 years ago, and we only have a tiny archaeological record from their stone tools,” Kaifu explained. “However, if they gave birth to physically helpless babies, then this tells us something about their behaviors, as they would need to have cared for the baby as diligently as we do.”

This level of infant care would have required a bit of social interaction, since it would have been difficult for new mothers to be the only one caring for and protecting a newborn from large predators and other dangers.

Another surprise was the level of head deformation in Homo floresiensis. The brain in this study was much smaller than expected, about the size of a chimpanzee’s brain. With this brain size, it is not clear why they didn’t see head formation that was more similar to other great apes.

“Even though their brain size was much smaller, their newborn stage was already following the same kind of difficult process as larger-brained modern human ancestors,” said Kaifu. “Rather than just being a trade-off for our large brains, the challenging responsibility of childcare and supporting mothers with their infants may actually be an integral part of our human history, at least since the time of the common ancestor of Homo erectus and Homo floresiensis more than 1 million years ago.”