Sunday, June 27, 2010

Diet, Jaws and Human Evolution

It's been thought for some time that human diet might have something to do with evolution. That big brain is expensive and it needs to be paid for. One idea was proposed a few years ago. It turns out that the gene for salivary amylase, an enzyme that turns starch into sugar, has a variation in humans. We have multiple copies of that gene. This means we have significantly more amylase in our saliva which means significantly more starch is predigested. Hence, more calories for less effort.

One would expect it, then, to show up in older hominids by its absence; that is, by more robust jaws and teeth.

This was an interesting sidelight to the discovery of a new human species: Homo gautengensis. This hominid emerged about 2 MYears ago and died out about 600k years go. It is a robust species with large teeth and a small brain. It produced and used stone tools and could have made fire. Likely it was a close answer but not a direct descendant.

So, we think. Big teeth. Strong jaw. Maybe it had the amylase variations. Or did it have to digest with brute force.

Not so fast, I say.

It turns out those weak humans have a mighty bite-- better than chimps or orangutans. How? You may ask.

It turns out the big jaws and tough teeth of our ancestors aren't necessarily the best way to get a good bite. A new study suggests that though we are not so heavily muscled in our jaws are efficiency in biting is considerably greater than we might have otherwise considered.

A team in Australia found that the human skull does not have to be robust because it transmitted more of the muscle force to the teeth than our relatives. The teeth are highly adapted to a strong bite with a thick enamel-- which has been a bit of anomaly since we were considered wimpy biters.

One of the interesting features of great ape anatomy is the saggital crest. This ridge of bone across the center of the top of the skull is the place were big jaw muscles attach, the other end being the jaw. Big muscles mean more force to the teeth.

However, they may also restrict increase of cranial size since that increase would have to be at the cost of those muscles.

The new study suggests that humans found another way to apply that force, reducing the need for the big jaw muscles that covered the skull-- possibly another enabling change to allow increase in brain size.

You are what your ancestors ate.

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