Our Diminishing Jaw

[gmacrider]

I understand that the human jaw and teeth are becoming smaller and weaker over recent generations because modern food is soft and mushy and easy to eat (please correct me if I'm wrong in this premise).

 

What is the process that is causing our jaws to diminish? It can't be evolution can it? Evolution would be where a single individual was born with a mutation resulting in a smaller jaw. If the smaller jaw gave the individual an advantage over others, then this trait would be passed on to subsequent generations. But I can't imagine that is what is happening with our jaws.

 

Please enlighten me. I don't understand how reducing the use of a characteristic would cause that characteristic to "devolve" over generations.

 

Thank you!

[herpguy]

From what I've seen on National Geographic Channel, it is evolution. A smaller jaw provides more brain space, which is the advantage.

[Sisyphus]

Yeah, big jaws mean big jaw muscles. Big jaw muscles means a compact, sturdy skull with lots of leverage to attach them. And that means less room for a brain.

 

Also, as a general principle, something which is not actively useful tends to be a drawback, because everything takes resources which could be used elsewhere. Everything is a balance between benefit and cost, and if there is no benefit, even the tiniest cost will eventually win out and change the trait.

[bascule]

I've also read that it helps with speech

[CPL.Luke]

wouldn't it be wild if human teeth eventually evolved to allow for better speech

[Bluenoise]

wouldn't it be wild if human teeth eventually evolved to allow for better speech

 

Who says that they haven't already?

 

Like lets say you had large canines that stuck out of your lips. Wouldn't that have a sluring effect on your speach?

[CPL.Luke]

but I mean teeth that were specifically designed for speech, and served virtually no purpose otherwise.

[bascule]

As is our wisdom teeth (and how poorly they fit) serve as evidence of the compactification of our jaw. Clearly there were selection pressures at work which trumped the need for straight teeth that all fit in the available space.

[gmacrider]

Thanks for the replies. Some good ideas. But isn't it odd that the same mutations (resulting in smaller, weaker jaws) are occurring many times in the population simultaneously?

[Sayonara]

Thanks for the replies. Some good ideas. But isn't it odd that the same mutations (resulting in smaller, weaker jaws) are occurring many times in the population simultaneously?

No, the mutations are already "out there". What we see in the population is the phenotypic expression (or not) of the resulting genotypes.

[Mokele]

The mutation occurred once, back when there were very few humans (or pre-human hominins) and spread through the population via natural selection. So there's no need to postulate multiple simultaneous mutations.

 

Mokele

[gmacrider]

Sorry, I'm confused.

 

If the mutation (genotype) occurred way back in our evolutionary history, then why is the smaller jaw (phenotype) only becoming prevalent in recent generations?

 

I may be making a faulty assumption here about the smaller jaw only becoming prevalent in recent generations. I'll do some research on that. I thought the jaw changes were because recent generations were eating softer, processed foods.

[Atellus]

I would suggest that the phenotype is only present today because people with smaller, weaker jaws are better able to survive on the softer, more palatable modern diet that is now available, with relatively little variation, all year round. There are also numerous dietary supplements available as well.

 

In times past, even in relatively recent history, strong teeth and jaw muscles were needed to masticate a variety of less palatable substances. Even today, in many parts of the world you will find people eating types of plant matter and animal parts which western humans would find very difficult to chew, and if it's not well chewed it doesn't digest as easily or quickly, and the immedicate nutritional benefit is reduced as a result.

 

So, in the past people with weak jaws tended to suffer more from malnourishment related ailments, particularly in winter, and died off in greater numbers which prevented them breeding in the same numbers or as frequently as their square jawed compatriots.

[gmacrider]

Interesting - I think I'm learning something here.

 

It appears that a mutation can occur without being expressed in the phenotype (why wouldn't a mutated gene be expressed?). Until it's expressed I assume Natural Selection would "treat it" as a neutral mutation, so it would continue to migrate through more of the population. Eventually something would cause the gene to be expressed in some members of the population and Natural Selection would come into play. What would cause a gene to suddenly be expressed? An external factor like changes in the texture of foods?

 

Thanks for all the info.

[Mokele]

why wouldn't a mutated gene be expressed?

 

You have two copies of every gene, so a non-mutated copy may "override" the mutated copy.

 

Eventually something would cause the gene to be expressed in some members of the population and Natural Selection would come into play. What would cause a gene to suddenly be expressed?

 

On the simplest level, say you have two individuals who each have 2 copies of a gene, a normal and mutant copy each. When they mate, there are 4 outcomes: Normal/normal, normal/mutant, normal/mutant again, and mutant/mutant. In the 1/4th of the kids who are mutant/mutant, the mutant gene is expressed, because there is no longer a normal gene to cover it up. Those are exposed to natural selection. Either the number of mutants are increased (if it's beneficial) or decreased (if harmful).

 

It should also be noted that a mutant gene can be good or bad depending on environment. For instance, a pure white coat of fur is beneficial in the arctic, but makes you an easy target in the rainforest.

 

Mokele

[gmacrider]

Ah yes! Now it's starting to make sense. I hadn't made the connections between mutations, sister chromosomes, cell division, gene expression, and Mendel's laws of genetics. Amazing stuff. Thanks for the help!

[Mokele]

Sure, that's what we're here for!