The human head is not our only possession to be born immature and then blossom in early childhood: our shoulders too! According to Japanese works published in PNAS, the growth of our shoulders slows down during fetal life and then accelerates after birth. A biological trade-off facing the dilemma between facilitating childbirth and providing people with the broad shoulders that are likely needed for locomotion and breathing.
The shoulders sometimes remain locked during childbirth.
Each species has its own survival strategy. In humans, our expensive and bulky brains are our greatest asset. Unfortunately, the mothers’ pelvis could not keep up with the same growth, presumably so as not to affect their motor abilities or to better hold the abdominal organs and the fetus during pregnancy. For this reason, our newborns are born with small heads, very immature brains, and skulls that have not yet fused. As a result, the mass of the brain of a newborn human reaches only 30% of the mass of an adult, compared with 40% in chimpanzees and 60% in macaques.
But the skull would not be the only organ adapted to the mother’s birth canal. In any case, this is demonstrated by Japanese researchers who looked at the width of the shoulders. Several hypotheses explain why our shoulders should remain broad. They could serveto stabilize the body when walking upright and facilitate high-speed throws“, the researchers suggest. It is also likely that shoulder width is related to the size of the chest and, therefore, with good postnatal respiratory function.
Result, “prevalence of shoulder dystocia, i.e. locking of the shoulders of the fetus in the birth canal, relatively high in humans“Especially when the child is of considerable weight, they exhibit. However, maybe our shoulders have adapted for the best to stay narrow during birth. “We assume that in humans, intrauterine development of the shoulder joint is limited by obstetric reasons.‘, the researchers add.
The human head is not our only possession to be born immature and then blossom in early childhood: our shoulders too! According to Japanese works published in PNAS, the growth of our shoulders slows down during fetal life and then accelerates after birth. A biological trade-off facing the dilemma between facilitating childbirth and providing people with the broad shoulders that are likely needed for locomotion and breathing.
The shoulders sometimes remain locked during childbirth.
Each species has its own survival strategy. In humans, our expensive and bulky brains are our greatest asset. Unfortunately, the mothers’ pelvis could not keep up with the same growth, presumably so as not to affect their motor abilities or to better hold the abdominal organs and the fetus during pregnancy. For this reason, our newborns are born with small heads, very immature brains, and skulls that have not yet fused. As a result, the mass of the brain of a newborn human reaches only 30% of the mass of an adult, compared with 40% in chimpanzees and 60% in macaques.
But the skull would not be the only organ adapted to the mother’s birth canal. In any case, this is demonstrated by Japanese researchers who looked at the width of the shoulders. Several hypotheses explain why our shoulders should remain broad. They could serveto stabilize the body when walking upright and facilitate high-speed throws“, the researchers suggest. It is also likely that shoulder width is related to the size of the chest and, therefore, with good postnatal respiratory function.
Result, “prevalence of shoulder dystocia, i.e. locking of the shoulders of the fetus in the birth canal, relatively high in humans“Especially when the child is of considerable weight, they exhibit. However, maybe our shoulders have adapted for the best to stay narrow during birth. “We assume that in humans, intrauterine development of the shoulder joint is limited by obstetric reasons.‘, the researchers add.
Shoulder growth slows before birth, then accelerates
The scientists then looked at shoulder development over time, from embryonic to adult, in both humans and chimpanzees and Japanese macaques. To do this, they combine 3D skeletal models of 81 humans, 64 chimpanzees and 31 dead Japanese macaques. “The size and shape of the skeleton of cadaveric specimens remain the same as in the original specimens.thanks to computed tomography data. When scientists plot growth curves for the three kinds of arms, the result is surprisingly clear. Because if in chimpanzees and Japanese macaques the development of the shoulders is relatively regular, then in humans there is a clear contrast between before and after birth. “The growth of human shoulders slows down before birth, but accelerates after birth.“, the researchers write in the study. “We were very surprised that the depression of shoulder growth in a person is so clearly and so beautifully expressed in the perinatal period.”, report Mikaze Kawada and Naoki Morimoto, two authors of the study.
The ‘obstetrical dilemma’ between successful delivery and continued performance
For them, this development of the shoulders in two stages, slowing down / speeding up, indicates a mechanism for facilitating human birth. “This indicates that changes in fetal development were necessary during human evolution, not only in the head, but also in the shoulders, in order to compensate for obstetrical limitations.‘, the researchers note. This compensation may appear dysfunctional as shoulder dystocia continues to occur. For researchers, a person’s shoulder width is probably “faced with an obstetric dilemma” is similar to the head. Real “developing conflictbetween the evolutionary pressures of a broad head and shoulders, on the one hand, and the limited size of the mother’s birth canal, on the other.
Other observed primates have a different strategy: instead of being born immature and then accelerating development in the young, their growth is fairly even for both the skull and the shoulders. Therefore, humans are born at a much more mature stage and therefore less vulnerable than humans, but with more limited abilities. However, it remains to be determined whether other primates have adopted an intermediate or even human-like strategy. “It would be very interesting to study other primates, especially gibbons, because they have broad shoulders and relatively small birth canals like humans.‘, the authors conclude.