Latest research: The "shoulder mystery" of human evolution solved

The human fetus's collarbone grows slowly before birth and then speeds up again during early childhood -- perhaps an evolutionary compromise that allows our relatively broad shoulders to pass through the pelvis during childbirth, according to a new study published in the Proceedings of the National Academy of Sciences on April 11.

Broad shoulders can help people maintain balance and enhance throwing ability, and even allow them to breathe more efficiently. But broad shoulders can cause problems during childbirth because the upright posture leads to a relatively narrow pelvis.

Naoki Morimoto of Kyoto University in Japan said that the newly discovered growth pattern of the human clavicle before and after birth - slow at first and then fast - seems to have solved the "shoulder mystery".

"Two things make childbirth difficult in humans: a big head and broad shoulders," Morimoto said. "Since [difficult childbirth] is dangerous, humans have evolved ways to deal with it."

Previous studies have shown that the human fetus' head grows rapidly in the womb and then slows down before birth, a trend also seen in other primates - although humans start to slow down their growth much later than in the latter.

To see if the shoulder grows in a similar way, Morimoto and colleagues analyzed CT scans of 81 humans, 64 chimpanzees, and 31 Japanese macaques. About half were fetuses at various stages of development, while the others were infants and adults.

The team measured the lengths of various bones in the skull, shoulder, upper arm, pelvis, thigh and spine. In general, growth of the spine is not subject to birth restrictions, so it serves as a good basis for comparing growth rates of other bones, Morimoto said.

The researchers confirmed that in all three species, the rate of skull growth slowed before birth. Other bones, such as the arms and pelvis, grew steadily in the womb but accelerated after birth.

As for the clavicle, Morimoto said that the growth rate of chimpanzees' clavicles is quite stable from before birth to after birth; the growth rate of macaque clavicles is stable before birth and slows down after birth. However, the human clavicle shows a special growth pattern - it slows down about two months before the fetus is born, and then accelerates again in the next five years, which forms what researchers call "growth inhibition", which coincides exactly with the time when the shoulder needs to pass through the pelvis during childbirth.

"At this point, we don't know why this particular pattern of the shoulder -- and not other approaches, such as slower, more steady growth -- was selected as a means of alleviating dystocia in humans," said Mikaze Kawada of Kyoto University, one of the paper's authors. "This requires further research."

Related paper information:

https://doi.org/10.1073/pnas.2114935119