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."

Comments

Actually, no.

[Romothecus]

RTFA. Virtually every scientist who read their work was of the opinion that the explanation "mutation to smaller jaw means bigger brain" is incredibly simplistic and that the real explanation is probably far more complex. The change in jaw morphology is probably only one of many contributing factors.

"Discovered"?

[afabbro]

"Discussed a new theory" is more accurate...

Re:"Discovered"?

[bugnuts]

Stop calling everything "theories" as if that word weakens the probability of it being true. That is the same lame tactic used by Creationists.

Theories are developed hypotheses, that have withstood scientific examination. Some theories are stronger than others, such as evolution, which has extremely compelling and a wealth of strong evidence. Examining genes and doing the statistics on them is also extremely compelling. Everything you do with technology including the computer you're using now is supported completely by theories. Does that mean it's not true? Kind of like disproving Zeno's Paradox -- please stand in front of the arrow.

So, sure, it's a theory as opposed to a proof. But you can probably bet your life on it.

Re:"Discovered"?

[Ayaress]

I'm pretty sure I've posted this exact reply to the exact parent a dozen times over.

In science, Theory is the highest level of understanding. Law and observation are, in fact, much lower on the heirarchy.

Observation is dumb: It's just what you see. "Oh, the sky is blue." "It hurts when I hit myself with this rock." "Look, there's another rat." That's observation.

Law is also pretty dumb. It's just a set of rules derived from observation. e=mc^2 fits observation, and it has some interesting connotations, but it doesn't say anything about WHY the equation works, and it says even less about HOW matter and energy are interchangeable.

Theory is an overarching collections of observations, laws derived from observation, and principles deduced from laws.

Theory explains why laws work, and why observations are as they are.

To be granted the distinction of 'theory' an idea must:
1. Explain any and all laws and observations already explained by a previous theory (example: to be valid, Relativity had to encompas Newtonian mechanics);
2. Explain such in an imperical manner (any forces involved must be identified and observed. This is why there is no theory regarding dark energy and dark matter - they have yet to be identified, observed, or quantified);
3.a. Explain something not covered in the existing theory (Darwin's theory explained why rats with their tails cut off did not have tailless offspring, as Lamarck's theory said they shoudl);
OR
3.b. Explain the existing theory in simpler terms (such as the replacement of phlogiston theory and epicycle theory by their more advanced, but much simpler, successors).

Now, by calling an observation a Theory, you are, in your misguided attempt to discredit it, in fact exaulting it to a much higher status than it claims to hold already.

Re:"Discovered"?

[RevAaron]

Indeed. I hate it when people say this or that (usually "evolutionism"; -ism, heh) is "just" a theory. A theory is a big deal. Perhaps creationism is "just" an educated guess, but the definition of the word theory is not. In common usage, people use the word "theory" to mean just that- an educated guess, rather than a hypothesis backed up by a lot of evidence.

And no, like you say, it's not a proof- but proofs really only exist in the world of mathematics. No scientist can say that a particular theory is 100% proven, ever. That is science, and the fact that science can adapt and grow with new information is one of its great strengths.

*sigh*

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.

Uncertain cause and effect

[Chilltowner]

While it's true that Australopithecus species had much smaller brains than anatomically modern humans and other of the Homo genus, this isn't the gene that separates us from the apes--earlier species made that division.

It also seems to me that they may be putting the cart before the horse here. Depending on the feeding habits of our Homo genus ancestors, a smaller jaw could be a decidedly large disadvantage, limiting the kinds of foods that could be eaten by a scavenger species such as our ancestors. 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. This might mean that we were well on our way toward advanced thinking before the loss of muscle mass in the jaw.

Anatomical structures always pretty tricky, especially when it comes to judging cognitive development and other tangential related adaptations. The kinds of mutations that make us human (smaller jaws, larger heads, versatile voiceboxes) also tend to cause of a lot of potential problems (restricted diet, difficult birth, tendency to choke). Weighing the value of one change over another become enormously difficult.

Not to knock their work, though--this is pretty amazing stuff and will definitely be another piece of the puzzle for anthropologists to consider. My only concerns are that we not look at this as a) the great divide between us and the other apes or b) the silver bullet that made us the brainy folks we are today.

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.

Re:Don't buy into the hype

[Ayaress]

Yes, but this comes down to the constant issue with any scientific literiture. There are several versions of every story:

1. What the scientists actually think (what I was addressing in my post).
2. What they tell the people they get their grant money from (to make it sound more profitable)
3. What the damned journalists say when they get ahold of it.

For example, take last week's discovery of sediments on Mars precipitated from salt water:

1. What NASA thinks: "Well, there's the proof of the sea we were looking for. Pity it's not there anymore"
2. What NASA says: "Hey look at this, there used to be water on Mars! And water doesn't just disappear, you know. Imagine what could be done with that much water!"
3. What the journalists say: "OMG OMG OMG TEHER WERE LIFE ON MARZ OMFG!!!111oneone"