Expert: Dana Krempels, Ph.D. - 1/3/2009

Question
What makes each of these;
the appndix, coccyx, muscles that move ears, muscles that make hair, little toe, and wisdom teeth vestigial? Please help me to understand?

Answer
Dear Heather,

None of these are entirely vestigial, though they are not as well-developed in humans as they are in other animals.  The coccyx still serves as a muscle attachment point, the appendix still has some immune system function, and muscles that make ears and scalp still work (and in some people quite well!  I can wiggle my ears.).  But the fact that they are less functional in humans suggests that they are not in great use, and hence, not under a great deal of pressure from natural selection.

Any trait can be:

adaptive - increases its bearer's likelihood of surviving to leave offspring

maladaptive - decreases its bearer's likelihood of surviving to leave offspring

neutral - does not affect its bearer's likelihood of surviving to leave offspring

Let's take the example of wisdom teeth.  In our ancient past, our ancestors had somewhat longer jaws than the average human has today.  As our jaws shortened (for various evolutionary reasons), the molars became more crowded, and this could promote tooth infections, which were often fatal before the days of dental surgery and antibiotics.  That meant that someone with a jaw too short to accomodate the wisdom teeth might well be out of the gene pool, along with his/her short jaw genes.

There must have been some balancing selection, however, that made it advantageous to have a shorter jaw, because most people today still have jaws that are just a little too short to comfortably accomodate
the full arcade of molars.  So why don't those of us with short jaws and wisdom teeth die out?

Today, in most countries, we have access to both dental surgery (to just remove those problematic teeth) and antibiotics (to fight infection).  So now the wisdom tooth trait is simply not really under much selective pressure in Westernized countries with good dental care.  That means that in such populations there are humans with all their molars and no problems fitting them (their jaws are a little longer), those with shorter jaws that eventually have their wisdom teeth removed, and, finally, those who never develop wisdom teeth at all.

The latter category isn't around because our vestigial wisdom teeth are "evolving away."  They're around because some members of our population inherited mutations that kept wisdom teeth from developing at all.  This means that the trait is polymorphic:  there are several different ways it can be expressed.  (Basically:  1 - have wisdom teeth that fit, 2- have wisdom teeth that don't fit, but can be removed and 3 - no wisdom teeth).

The wisdom tooth trait is just not under selective pressure in Western countries any more because the environment has changed (to include dental care).  It has become selectively pretty much neutral.

The same kinds of stories can probably be told about the other traits you have listed, though an explanation for each one would take much longer than one email.  

Suffice to say that the moveable ears and scalp, the appendix, etc. no longer serve their original functions in humans.  And that made them more selectively neutral.  In some cases, the energy spend maintaining those structures was "better spent" on other things (such as building a larger brain, just to pull something out of the hat), and those individuals who lacked movable ears and scalp had an advantage over those who had them *if* they shunted that energy used to build and maintain those not-so-important traits to something that was truly adaptive (even if that was just using that energy for reproductive efforts; but that's not the only possibility).

We can't go back in time to see what happened, exactly, with each of these traits.  But Darwin's ideas go a long way towards explaining how vestigial organs became that way, and why they may or may not still be under selective pressure.

Hope that helps.

Dana