Gene MYH16: A Tasty New Jawbreaker

kid_wonder writes "Jeremy Roenick take heart! Glass Joe take heart! Scientists discovered that humans owe their big brains to a single genetic mutation that weakened our jaw muscles about 2.4 million years ago. So I guess now we can call all those dopey muscle bound guys 'apes' with a clear conscience."

Re:Silly protozoa!

[Otter]

What? You say you're missing thousands of other necessary genes and you can't assign responsibility for such large changes on one single change?

It's not obvious to me that your spin is more correct than his, though. Does a single mutation take you from a chimp to a reality show contestant in one jump? Of course not. As you say, there are thousands of other changes involved.

But what's being proposed here is precisely that a single mutation radically changed primate head morphology and changed the selective constraints on all those other intelligence-enhancing mutations. Is it true? Who knows? But that does seem to be what's being argued.

Re:Uncertain cause and effect

[Ayaress]

It seems possible, and even likely, in this case, that our already advanced brains provided a large enough offset against the loss of powerful jaw muscles

Take a look at most simain brains. The jaw muscles of a chimp, gorrilla, or even an Australopithescene, attach at the very peak of the skull, and are very thick, comprising the bulk of the head.

There's just not room to expand the brain with ape-like jaw muscles. You're right on one thing, though: Weak jaws are a severe handicap without expanded brains.

There are three ways the two changes could have come: Brain, then jaw; jaw, then brain; both in parallel.
The brain can't expand against simian jaw muscles, so the first one's out.
Weak jaw and small brain are a severe handicap, and the remaining strong-jawed humans would have outcompeted their slackjawed relatives, and the weak-jawed strain would have been bred into extinction.

However, both simultaneously makes the transition profitable and possible, but you're ignoring something important: Related growth rates.
Just the act of lowering the point of connection of the jaw muscles (in the case of apes, this is a ridge on the very top of the skull - in the case of humans, it's the top of the temples, just behind the eyebrows) makes the braincase of the skull larger.

Re:Uncertain cause and effect

[Chilltowner]

Excellent points!

The problem I have, though, is that the article implies that the weak jaw is the result of a single mutation (or a small cluster of them). This would seem to point toward a very "punctuated" change in the strength of the jaw. The fact that it didn't do us any harm in the long run could indicate one of several things:

1) the environment we were living in didn't require us to chew the kinds of plants that simians did (and do). Weak jaws were not maladaptive.

2) it was better to be able to chew on plants that required a more simian jaw and dental arcade, but our brains carried us through. Weak jaw is potentially maladaptive, but less relevant.

3) weak jaws were maladaptive, but other parts of our morphology (e.g. the attachment point of the muscle, different kinds of teeth) soften the blow long enough for the benefit of a weak jaw, larger potential cranial capacity, to come to the fore.

I don't pretend to have the answer to any of this, but I think a lot these points support my main idea: the gene is not the silver bullet. There are other anatomical issues that have to resolve themselves before we get to modern humans (narrowed, flattened zygomatic arches, change in placement of key muscle anchors, smaller cheeks, reduced protusion of the lower face) as well as the onset of "culture"--i.e. at what point did habilis/rudofensis start using handaxes and other tools to the point where teeth were far less important to our survival than they might otherwise have been. Was there a "perfect storm"? Did we suddenly have weaker jaws at the right point where we could replace teeth with tools?

We also have to figure out which of the Australopithicines was our ancestor--the more gracile species or the robust (which have the very large muscles you pointed out, extending up to the crest of the skull). That would impact on the severity of the change between this ancestor and the genus Homo.

It's really fascinating stuff, and this discovery will probably play an important role in how we imagine the crossover from the Australopithicines. But, like I said, there's a lot more work to do and a lot more that needs explaining. There is no silver bullet, and I think the researchers would agree with that. The media, on the other hand, will probably stick to the hype--they seem to like simple, gene-based explanations these days. Ultimately, though, the question will be resolved by a confluence of ideas from geneticists and the stones-and-bones folks.

"Theory" is a hard concept to teach

[cagle_.25]

My students always have a mental block when it comes to teaching the distinction between theory and law. Although I state clearly several times that a "law describes, and a theory explains", many students have it firmly fixed in their minds that the progression of the scientific method is

Observation --> hypothesis --> theory --> law

instead of the correct

Observation --> law --> hypothesis --> theory

By the middle of the year, when we start talking about Boyle's Law and other gas laws being explained by the Kinetic Theory, some students start to get straightened out.

I'm not sure why this is so difficult for them, but my hypothesis is that it has something to do with their underlying thought structure. The reason (I believe) that students believe that theory comes before law is that they themselves have trouble distinguishing what they SEE with their BELIEFS about what they see. Therefore, they see no need to distinguish laws from theories. My $.02