As every mother knows, giving birth is hard, painful work. "The evolution of human birth", an article by Karen Rosenberg and Wenda Trevathan in the November issue of Scientific American, explains why. The difficulties associated with human labour and birth, they suggest, prompted the custom of seeking help during delivery.
Humans are the only primates that look for assistance, a practice that may have started as early as five million years ago, when our ancestors first began to walk on two legs.
Anthropologists report that women in all cultures look for help with delivery. The Kung Bushmen of the Kalahari Desert, in southern Africa, consider solitary birth a cultural ideal, but Kung women normally give birth alone only after several assisted deliveries.
In labour, the large human skull and brain have to pass through a relatively narrow opening in the woman's pelvis, and the baby's head and shoulders must rotate, sometimes awkwardly, as they pass through the birth canal, which is shaped like a twisting oval tube. The baby has to turn repeatedly in order to keep its head and shoulders lined up with the widest part.
The average opening in the female pelvis measures 13 centimetres at its widest point and 10 centimetres at its narrowest. The average infant head measures 10 centimetres from front to back; the average shoulders are 12 centimetres wide. This makes labour difficult and somewhat risky, and explains why mothers like help.
If you look back far enough, you eventually reach a point at which birth was not so difficult. This may have been Proconsul, a primate ancestor that lived 25 million years ago and had a pelvic structure similar to that of a modern monkey. Although a monkey's head is typically almost as wide as its mother's birth canal, the infant makes its way into the world with more ease than does a human baby, because the birth canal does not twist, keeping the same shape from entrance to exit.
Also, the baby monkey emerges facing its mother, who usually squats on her hind legs. As the infant emerges, the mother reaches down to guide it out of the birth canal and wipe the mucus from the nose and mouth, to help breathing.
The evolutionary changes to the human pelvis that enabled hominids to walk upright cause most babies to be born facing away from the mother, making it difficult for her to guide the baby as it emerges. Even if she did, she would risk injuring the baby by bending its back against the natural curve of the spine.
Bipedalism, or walking on two legs, caused the first fundamental move away from the non-human-primate way of giving birth. Bipedalism emerged about four million years ago, in the early human ancestor Australopithecus, who stood no more than four feet tall and had a brain only slightly bigger than that of a modern chimpanzee. Fossils show that Australopithecus faced labour difficulties unlike those known of in any other primate.
Bipedalism constricts the maximum size of the pelvis and of the birth canal, whose shape required the rotation of the infant's shoulders, giving it a 50-50 chance of emerging facing backwards or, more easily, forwards. Mothers would have benefited from some kind of assistance with backwards-facing babies.
The expanding size of the human brain posed a further obstacle to easy birthing. One species of Australopithecus led to Homo, the human genus, about two million years ago. Homo's brain was about twice as large as that of Australopithecus, but only two-thirds the size of the modern human brain. Early Homo probably had a birth mechanism similar to that of Australopithecus.
The pelvic anatomy of early Homo limited the growth of the human brain until evolution expanded the birth canal enough to allow a larger head to pass through. These changes allowed the dramatic increase in human brain size that took place between two million and 100,000 years ago.
Human fossils spanning the last 300,000 years, when large-brained children have been born, indicate that babies had to rotate their heads and shoulders within the birth canal, emerging facing away from their mothers, as is the case in the modern human. Because these difficulties have such a long history, it seems very likely that natural selection long ago favoured the behaviour of seeking assistance during birth.
Our early female ancestors would have looked for help because of fear, pain and anxiety. It is therefore probable that natural selection also favoured the imprinting of these emotions onto the human make-up, as they provided an evolutionary advantage by ensuring that women were not alone at such a crucial time.
About 10 years ago I wrote with my wife, Dr Breda McLeavey, a psychologist at Cork University Hospital, a booklet outlining a psychological technique that mothers can use to alleviate the pain of labour. Published by Cow &Gate, it was distributed free for many years to expectant mothers at antenatal clinics in Ireland.
William Reville is a senior lecturer in biochemistry and director of microscopy at University College, Cork