cranial sutures

alex duncan (
18 Aug 1995 01:09:34 GMT

Well, this has actually turned out to be an interesting discussion. We
had a previous post suggesting that the unfused cranial sutures in human
infants were an adaptation to the requirements of travel down the birth
canal. I responded with the following:

> I'm a little baffled. If this is WHY human infants have unfused cranial
> bones (resulting in "softness"), why do the same features show up in all
> other mammals, and even birds? Do birds need a "soft" skull to get out
> of their eggs? I would think a hard skull would help them break out of
> the shell better.

> (That was sarcasm, above. I'd hate to think no one noticed.)

> There's just got to be an adaptive explanation for EVERYTHING.

I wasn't suggesting that there is no adaptive reason for cranial sutures,
but that they probably originated for some function other than allowing
large brained human infants to be born. I didn't spell it out, but the
"reason" for cranial sutures that I had in mind was that they allow brain
growth. Harry Erwin agreed w/ me:

>>Unfused sutures allow the brain to grow.
>Give me some credit, Harry. I'm aware that unfused sutures allow the
>brain to grow (an adaptation). I was satirizing the suggestion that we
>have unfused sutures to allow us to squeeze down the birth canal (an

And now, this from Seth (who apparently doesn't notice sarcasm even when
it's labeled as such):

>1) Not only does it allow the brain to grow (humans being the leader for
>postnatal brain growth) but birds do not squeeze out of egg; they peck
>out, hence no need for a soft spot.

My comment about sarcasm refers only to the above statement.

>2) Not everything needs to be "adaptive." Some elements just are. Read
>some of S.J. Gould's work on the panda's thumb.

This second point in Seth's post made me think I wasn't being primitive
enough. Cranial sutures are probably not an adaptation to allow
postnatal brain growth. More likely, they are present at birth simply
because the crania is still in an immature state, and the cranial bones
haven't gotten large enough to begin the fusion process. In this aspect,
they are similar to most of the bones in the mammalian body, which are
generally in a very immature state (e.g., unfused epiphyses) at birth.

This has turned out to be useful in larger brained creatures who complete
much of their brain growth postnatally, but it's not what unfused sutures
are FOR. The apparent adaptation is an epiphenomenon of ontogenetic
changes in cranial bone development.

Anyway, thanks to Seth for spurring further thought.

And BTW, I think S. Gould has made the same argument about cranial
sutures -- that they are adaptations to brain growth.

Alex Duncan
Dept. of Anthropology
University of Texas at Austin
Austin, TX 78712-1086