Archaic human parents had to look after helpless infants like modern day parents do, according to new research. Human babies are born less developed than those of other great apes, and so are unable to move as freely. A common result of this is deformational plagiocephaly, also called flat head syndrome. Professor Yousuke Kaifu at the University of Tokyo led a team which studied head shapes from scans of living infants, historical Japanese skeletal remains, fossilized specimens of Homo erectus and Homo floresiensis, and compared these to measurements of dry skulls from other great apes. Human head shapes across the ages, as well as the fossilized samples, exhibited a far wider range of deformation. The team sees this as evidence that archaic humans birthed helpless infants, with implications for childcare practices and potentially cooperative social behaviors dating back hundreds of thousands of years.
Many parents understand the fear of taking a fragile newborn home. Compared to other great apes – large primates classified in the same family as humans – our children are born completely helpless, with weak neck muscles, soft cranial bones and still underdeveloped brains, which continue to grow rapidly after birth.
One theory for this, called the obstetrical dilemma, sees it as a trade-off for being able to walk upright (which affects the shape of the birth canal) and have large-brained offspring. But when in our evolutionary timeline did this helpless newborn phase – and the parental responsibilities which go with it – begin?
It is a difficult question to answer. Fossils of our archaic ancestors are mostly fragmented and rarely of infants. However, a new approach, which can be applied to fossils of any maturity, may provide the evidence researchers have been looking for.
“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 from the University Museum at the University of Tokyo. “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.”
Kaifu was inspired after visiting the skeleton of Homo floresiensis (who lived more than 50,000 years ago) in Indonesia in 2007. He was struck by the deformed shape of their skull which, from the structure of the bones, appeared to have occurred after birth. To understand why, he contacted a hospital clinician who identified the cause as deformational plagiocephaly.
This led Kaifu to theorize that skull deformation, which can be identified as not being caused by disease or post-burial, may be an indicator of helplessness in infancy, as it implies a soft, underdeveloped skull and head immobility.
To test this theory, he gathered a team including researchers from Japan and Indonesia to collect and compare CT scans of 123 modern, healthy infants aged from 1 day to 17 months old, measurements from historical human skulls from Japan dated from about 200 to 1,000 years old (385 people in total), great ape skulls (996 individuals), and fossilized human skulls of Homo erectus (five fossils) and Homo floresiensis (one fossil).
From the results, the team could see that other great apes – including chimpanzees, bonobos, gorillas and orangutans – which are born more physically independent and capable, were grouped within a narrower range of cranial deformity. The human samples – including fossilized specimens – exhibited 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. 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,” said Kaifu. “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.”
Another surprising outcome of the research was the level of head deformation in Homo floresiensis. Diminutive in stature (affectionately referred to as the “hobbit”), their brain was also much smaller, comparable in size to a chimpanzee, so it is unknown why their situation wasn’t 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.”
