Shoulder growth may slow during human development to make birth easier

By Christa Lest.

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 to fit through the pelvis.

Broad shoulders may help us with our balance and our ability to throw. A fetus with broad shoulders poses a problem during birth because of our upright posture.

The shoulder mystery appears to have been solved by the discovery of a slow-down-then-catch-up growth pattern in human clavicles.

There are two things that make it difficult to give birth.

He says that previous studies show that the heads of human fetuses grow at fast rates in the uterus and then slow down just before birth, which is a trend seen in other primate as well.

Read more: Are caesareans really making us evolve to have bigger babies?

Morimoto and his colleagues looked at scans of 81 humans, 64 Chimpanzees, and 31 Japanese macaques to see if they had the same shoulders. The majority of the subjects were fetuses at various stages of development. Adults and infants were the others.

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

The growth rate of the skull in all three species was confirmed by the researchers. The uterus had steady growth of the arms and pelvis after birth.

Chimpanzees had a steady growth rate from before to after birth. The collarbones of the macaques grew slowly after birth.

He says the human collarbones showed a growth pattern. 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.

We don't know why this specific pattern in the shoulder was selected in humans as a means to ease the difficult childbirth.

There is a journal reference in the PNAS.

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