The following is my response to a writing exercise I recently had to complete for a job application. Of several titles, I chose to address this one:
‘You’re born with 300 bones, but when you get to be an adult, you only have 206.’
Here’s my response (limited to 200 words):
This amusing fact actually provides a very interesting insight into our evolution.
Many factors have enabled us to become the dominant species of our planet. One of these is our disproportionately large brains. But, such emphasis on the brain has required a host of adaptations.
After nine-months, our skull is already as large as it can grow without our becoming trapped in the womb. The principle adaptations that allow us to grow our brains are: premature birth, menopause and mobile skull-plates.
Compared to other mammals we are born twelve-months premature. This concession to size leaves us defenceless and a burden at birth. To compensate for this, we evolved menopause (almost unique to humans) which provided the necessary carers.
The final adaptation is mechanical and the answer to our initial riddle.
The embryonic skull-case is not rigid like an adult’s but consists of five bones with flexible sutures. This flexibility allows the skull to be crushed as it passes through the birth-canal, return to shape after the trauma of birth, and to continue growth outside the womb. Fusion of the bones starts within months but is not complete until adulthood (around eighteen).
Thus, five bones become one. Similar fusion of other bones explains how 300 become 206.