Shoulder growth may slow during human development to make birth easier

CT scans of humans, chimpanzees and macaques reveal that human collarbones slow their growth rate in the final months of pregnancy, perhaps to make it easier for babies to squeeze through the pelvis

By Christa Lesté-Lasserre

The collarbones of a human fetus grow more slowly just before birth, with growth then speeding up again during early childhood – probably an evolutionary compromise that allows humans’ relatively wide shoulders to fit through the pelvis.

Broad shoulders may help us with our balance and our ability to throw, and might even help us breathe more effectively. But a fetus with broad shoulders poses a problem during childbirth, because our upright posture has led humans to develop a relatively narrow pelvis.

The newly discovered slow-down-then-catch-up-later growth pattern in human clavicles – collarbones – around the time of birth appears to resolve this “shoulder mystery”, says Naoki Morimoto at Kyoto University in Japan.

“There are two things that make human childbirth difficult: a big head and wide shoulders,” he says. “Since [difficult birth] is dangerous… it is sensible to think that humans evolved some ways to ease the problem.”

Previous studies have shown that the heads of human fetuses grow at fast rates in the uterus and then slow down just before birth, he says, which is a trend seen in other primates too – although human heads start to slow down their growth very late compared with other primates.

Curious to know whether the shoulders grow in a similar way, Morimoto and his colleagues examined CT scans of 81 humans (Homo sapiens), 64 chimpanzees (Pan troglodytes) and 31 Japanese macaques (Macaca fuscata). About half of these subjects were fetuses at various stages of development starting from about the beginning of the second trimester. The others were infants and adults.

The team measured the lengths of various bones in the skull, shoulders, upper arm, pelvis, thigh and vertebral column. Generally speaking, the vertebral column’s growth isn’t affected by birth constraints, so it serves as a good basis of comparison for the growth rates of the other bones, says Morimoto.

The researchers confirmed that the growth rate of the skull in all three species reduced just before birth, says Morimoto. Other bones, such as the arms and pelvis, had steady growth in the uterus, but then picked up speed after birth.

As for the collarbones, chimpanzees showed a fairly steady growth rate from before to after birth, he says. The macaques’ collarbones grew steadily before birth and then more slowly after birth.

The human collarbones, however, showed a standout growth pattern, he says. They slowed down about two months before birth and then sped up again over the next five years – creating what the researchers call a “growth depression” that lines up perfectly with when the shoulders need to fit through the pelvis.

“Currently, we simply do not know why this specific pattern in the shoulder – and not other ways like [a slower, steadier growth] – was selected in humans as a means to ease the difficult childbirth,” says co-author Mikaze Kawada, also at Kyoto University. “We need to wait for further studies.”

Journal reference: PNAS, DOI: 10.1073/pnas.2114935119

Sign up to Our Human Story, a free monthly newsletter on the revolution in archaeology and human evolution