Human evolution shows an increase in cranial capacity – the question is, how did this come about?
The answer is:
- The development of the skull became delayed so that the fetal condition continued long after birth with fontanelles remaining open for longer.
- The skull bones became thinner in modern humans compared to most prehistoric species and this allows for moulding during birth.
- The female pelvis became bigger in wide-bodied populations so babies with larger heads could be born.
Compared to animals, human babies are born helpless and dependent on the parents for protection to a much higher degree. Human babies are born with holes in their skulls that have only cartilage and membranes covering the brain, with no bone. These holes are called fontanelles.
Fontanelles allow for two things: Firstly, they allow the baby’s head to be deformed during birth as it moulds during the descent down the birth canal. Secondly, the skull grows outwards from these holes and from sutures between cranial plates. The longer the fontanelles stay open, the more the skull can grow outwards from them.
This delayed pattern of growth – with the fetal condition being retained after birth – is the human pattern of growth. It is called secondary altriciality. By the time ape babies are born their fontanelles have almost closed up. In the human baby the fontanelles remain open for the first year and the infant’s brain grows at fetal rates during this year.
The human pattern of growth is found in Homo ignis / Homo erectus. We can infer that babies of this species were born with open fontanelles from two million years ago. This allowed cranial capacity to increase to 800 cc or more.
My own calculations, which will appear in a book I am currently editing, show that Homo primocreatus – Primordial Man had an intermediate degree of secondary altriciality compared to apes. There would be a small delay in development.
Homo ignis / Homo erectus had thick skull bones and well-developed browridges. He and she lived in a world where being clubbed over the head by an enemy was a real possibility. Survival was dependent on a reinforced skull.
Gracilization of the skull came with Homo aequalis / Homo sapiens 100 000 years ago – I think due to internal tribal peace and a new set of weapons used against enemies.
Thinner skull bones meant that the head of a baby could mould during childbirth, thus a child with a larger head survived the birth process. It also meant a more delicate infant requiring more protection from falls.
An alternative way of having babies with larger heads was for the female bony pelvis to become more ample. This seems to have been the way forward for Homo centralis / Neanderthal. These were wide-bodied people with ample pelvis for childbirth. The skulls of Neanderthals had thick bones so they must have achieved large cranial capacity by this alternative method.
In modern people all of the above traits are combined such that women give birth to infants whose skulls mould as they are born, and continue to develop at a high rate after birth. The combination of traits has increased cranial capacity over the course of human evolution to what we consider normal today.