Birth in humans is difficult compared to birth in most mammals. The limiting factor is the adjustment between size of baby and its head, and size of the bony pelvic inlet and outlet – the birth canal of the mother. In humans cephalopelvic disproportion is a frequent problem. Death often occurs in childbirth.
From the point of view of human evolution and increases in brain size, giving birth to a baby that starts off larger with a larger head is the direction of the trend. Thus, a major factor in human development is those ‘child-bearing hips.’
Human evolution shows an increase in pelvic size due to increase in stature. The first human species was only a metre high. I’m 1.7 metres tall and many women are taller than me. As overall height and size increase, pelvic size increases, so bigger babies can be born.
My thesis concerning human evolution is that human life style has been the main factor in changes to the body. Early human species such as Homo ignis / Homo ergaster were adapted with narrow bodies to run fast to escape predators and to hunt animals by running them down. Later species were far more sedentary. Species such as Homo praecursor heidelbergensis had wider bodies. Homo centralis / Neanderthal had medium stature and a well-built, wide body.
I see several ways of adopting a more sedentary way of life even in prehistory:
- The first way is to use a canoe for locomotion instead of walking and running.
- The second is to hunt using traps rather than pursue the quarry animal.
- The third is the sexual division of labour – with women remaining in and around the camp, harvesting vegetable crops, cooking and looking after young children. Thus, the distance that women would walk would be much less than for male hunting parties. Later on small animals were kept for food in the camp.
When women ceased to hunt, they became specialized in the gestation of large fetuses. A wide body is an advantage in childbirth.
An increasingly sedentary life is an on-going trend in human culture.
Therefore, human culture and ways of life have changed with human inventions and use of technology. The evolution of increased cranial capacity was dependent upon what human culture had to offer. This goes back to prehistory, at least 400 000 years ago, and it is a trend that continues today.