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If there’s one thing modern humans share with our long-lost 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 grow rapidly after birth.
As a result, human parents must spend much more time holding and caring for their offspring to keep them alive or prevent deformational plagiocephaly, or flat head syndrome. This weakness appears to have existed for more than a million years, according to a study published today in the journal Proceedings of the Royal Society B of Biological Sciences.
One theory for this developmental delay is called the obstetric dilemma. The soft head and underdeveloped brain may be a compensation for later-developing large brains and our ability to walk upright (which affects the shape of the birth canal). Still, it is difficult to determine when this helpless newborn phase emerged within our evolutionary timeline. The fossils of our distant ancestors are mostly fragmented and rarely include the remains of babies.
A new approach detailed in this study may 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 of the skull) that occurred around or shortly after birth and that remained to some extent into adulthood,” Yousuke Kaifu, study co-author and anthropologist at the University Museum of the University of Tokyo, said in a statement. “Today, this is a common condition, called deformational plagiocephaly. The shape of the head is affected because the baby frequently lies in the same way due to the way it is held or laid down, and because the baby’s neck muscles are too weak to control the position of the head.”
In 2007, Kaifu was in Indonesia and saw a skeleton of Homo floresiensisan archaic human species also called “hobbit”, who lived more than 50,000 years ago. He was struck by the deformed shape of the skull, which seemed to have occurred after birth due to the structure of the bones. To understand why, he contacted a doctor at the hospital 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 theorized that it may be an indicator of helplessness in childhood. A flat head on a skeleton implies a soft, underdeveloped skull and possible immobility of the head.
To test this theory, Kaifu and other researchers from Japan and Indonesia collected CT scans of 123 healthy modern babies between one day and 17 months old. They also collected measurements of 385 human skulls from Japan dating between 200 and 1,000 years old, 996 skulls of non-human great apes, five fossilized skulls homo erectus skulls and one fossilized Homo floresiensis skull.
They found that chimpanzees, bonobos, gorillas and orangutans, who are born more independent and physically capable, had a narrower range of cranial deformity. All human samples (modern or fossil) had a much wider range of deformations and head shapes.
“This was exciting since we don’t know much about the behavior of homo erectus. “They lived more than 1 million and 110,000 years ago, and we only have a small archaeological record of their stone tools,” Kaifu explained. “However, if they gave birth to physically helpless babies, this tells us something about their behaviors, as they would have had to care for the baby as diligently as we do.”
This level of infant care would have required some social interaction, as it would have been difficult for new mothers to be the only ones 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 brain. With this brain size, it’s unclear why they didn’t see a head formation more similar to that of other great apes.
“Although its brain size was much smaller, its newborn stage was already following the same kind of difficult process as modern human ancestors with larger brains,” Kaifu said. “Rather than simply being a compensation for our big brains, the challenging responsibility of caring for children and supporting mothers with their babies may actually be an integral part of our human history, at least since the time of the common ancestor of humans. homo erectus and Homo floresiensis more than 1 million years ago.”
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