By comparing skull deformation in fossils, modern infants and great apes, University of Tokyo researchers argue that human-like infant helplessness, and the childcare it demands, dates back more than a million years.

Human babies arrive remarkably unfinished. Compared with newborn chimpanzees or gorillas, they have weak neck muscles, soft skull bones and brains that keep growing rapidly long after birth. They cannot cling to a parent or hold up their own heads, and they depend on adults for almost everything.

A new study suggests that this kind of helplessness, and the demanding care it requires, is very old. Writing in Proceedings of the Royal Society B, a team led by Yousuke Kaifu of the University Museum at the University of Tokyo reports evidence of human-like infant helplessness in two archaic human species, Homo erectus and Homo floresiensis.

The evidence comes from a surprising source: the slightly lopsided skulls that can result when a baby spends too long lying in one position.

A question of timing

Why human infants are so helpless has long been debated. One influential idea, known as the obstetrical dilemma, holds that walking upright narrowed the birth canal while our ancestors’ brains grew larger, forcing babies to be born at an earlier stage of development. Other researchers have emphasized energy limits during pregnancy or the benefits of a long, socially rich childhood.

A central question is when this dependence began. That has been hard to answer because fossils of archaic humans are mostly fragmentary, and fossils of infants are rare. A method that could detect infant helplessness in skulls of any age would open up a much larger record.

Reading the shape of a skull

“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,” explained Kaifu.

Those changes arise when a baby with weak neck muscles repeatedly lies with its head in the same position, gradually flattening part of its soft skull. “Nowadays, this is a common condition, called deformational plagiocephaly,” Kaifu said. The condition became noticeably more common from the 1990s, when campaigns to reduce sudden infant death syndrome encouraged parents to put babies to sleep on their backs.

The idea traces back to 2007, when Kaifu examined the skull of Homo floresiensis, a fossil more than 50,000 years old, in Indonesia. The skull was distorted, and a clinician suggested that the shape resembled deformational plagiocephaly. That led Kaifu to propose that this kind of skull deformation could be used as a marker of infant helplessness.

Homo floresiensis, nicknamed the “hobbit,” was a small-bodied human species discovered at Liang Bua cave on the Indonesian island of Flores, and announced to the world in 2004.

Humans versus great apes

To test the idea, the team assembled a large comparison set. They analyzed CT scans of 123 healthy modern infants aged one day to 17 months, 385 historical Japanese skulls dating to roughly 200 to 1,000 years ago, and 996 great ape skulls from chimpanzees, bonobos, orangutans and gorillas. They also examined five Homo erectus fossils from the Javanese sites of Ngandong and Sambungmacan and the Homo floresiensis skull from Liang Bua.

The results drew a clear line. Great apes clustered within a narrow range of cranial deformation. Humans, including the fossil specimens, showed a much broader range, consistent with infants that spent long periods lying down without the strength to shift their heads.

Childcare deep in the past

Homo erectus lived from more than 1 million years ago until about 110,000 years ago, according to the release, and was the first human species known to spread widely beyond Africa. Yet much about its behavior remains unknown.

“This was exciting as we don’t know a lot about the behavior of Homo erectus,” Kaifu said. If they gave birth to physically helpless babies, he explained, “then this tells us something about their behaviors, as they would need to have cared for the baby as diligently as we do.”

That care would probably have required help. “This likely would have necessitated some social interactions, as it would be difficult for the mother to care for and protect the newborn in that environment, where there would have been other large predators, on her own,” Kaifu said.

That idea fits a broader view, championed by the anthropologist Sarah Blaffer Hrdy, that humans evolved as cooperative breeders, in which grandmothers, siblings, fathers and other group members help mothers raise their young. Few other primates share child care so widely.

The finding for Homo floresiensis is more puzzling. The species had a brain roughly the size of a chimpanzee’s, yet its skull showed the same kind of deformation, and the reason is not clear.

“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.

That observation sits awkwardly with explanations that tie infant helplessness only to big brains, and Kaifu draws a broader conclusion. “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.”

The study rests on six fossil skulls, a small number that reflects how rare well-preserved archaic crania are. Other factors can also distort fossil skulls, and future finds will be needed to test the pattern. The work was funded by JSPS KAKENHI grants and Child Health and Development Research funding.

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Notes

1. Kaifu, Y., Yano, W., Hikosaka, M., Shimizu, D., Nishimura, T., Kubo, D., Suriyanto, R. A., Jatmiko, Sutikna, T., Morimoto, N., and Kurniawan, I. “Cranial evidence for human-like helpless infancy in Homo erectus and Homo floresiensis.” Proceedings of the Royal Society B: Biological Sciences, 29 July 2026. DOI: 10.1098/rspb.2026.1055.
2. University of Tokyo. “Archaic human parents also had a hard time raising kids.” EurekAlert!, 28 July 2026. https://www.eurekalert.org/news-releases/1137243.

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