I
like my jaws. They are, perhaps, my favorite thing to use when I am
chewing stuff. They are also incredibly helpful in the whole talking
department, and given that I am a teacher this is a fairly important
adaptation for me to have. Jaws are really swell.
But
how did we come to have these neat little chompers anyway? It turns
out that the story of jaw evolution is fairly simple. You see, once
upon a time about 530 million years ago, there lived a group of critters
named the Agnathans. These were a strange sort of organism what with
their long bodies and sucker faces and no paired fins, but they were
some of the first fish to grace our fair ocean and for this we have to
give them credit.
They
were a happy sort of fish (I assume) and lived their lives happily
filtering their breakfasts, lunches, and dinners through their happy
little sucker mouths. The water would flow in through their open face
hole and then out again through the gills. This provided them with both
food and oxygen which are good things to have if you want to not die.
And not die they did. Or didn't. Or didn't not do. Or whichever
combination of words means that they survived for quite a long time and
can, in fact, be found alive today. The relatively gross Lampreys and
the downright vile Hagfish are both examples of Agnathan (jawless) fish.
Those
little gills that they used to pull oxygen from the water are all
feathery and frilly and, if taken out of the water, will collapse in a
little less than frilly pile. To combat this, these jawless fish used
special cartilage bars called Gill Arches to give the gills a bit of
backing. These gill arches served the purpose remarkably and kept those
little frilly gills open and extracting oxygen, but the relative
stiffness of these gill arches could also be viewed as a bad thing.
Since they couldn't flex too much the fish had to rely on continuous
movement or the occasional currents to keep water flowing over their
gills.
This
constant motion was a major energy drain and could have been a bit of a
problem for these early critters. No matter how streamlined you are,
constant movement is going to require more energy than simply hanging
out. If you will allow me a bit of gross oversimplification, after a
bit of time a mutation or series of mutations in the genes that regulate the formation of these gill arches came about
and, quite literally, changed the world. The gill arches became hinged
so that they could be a bit more flexy. This added flexibility gave
those critters with these mutations an advantage over their less flexy
peers. They could now pump their gill arches like a bellows and bring
flowing water over the gills without having to move around much.
So
now we have flexible gill arches. You might be asking, What does this
have to do with my ability to eat a cow? Actually, it has everything to
do with your ability to eat a cow, or celery, or anything else for that
matter. You see, it was these flexible gill arches that eventually
became the jaws.
In
another gross oversimplification, over time the front gill arches
became more flexible and pushed forward a bit. The flexible gill arches
in the front linked up with the mouth and gave the fish a tool for
trapping food. Now the fish could flex the rear arches to bring water
into the mouth and over the gills, and it could flex the front gill
arches to snap completely shut to catch other critters for food. The
jaw was now basically complete - an apparatus for taking oxygen out of
the water was turned into a weapon.
For a more complicated version of these events complete with fancy science words and a lot more detail, go here.
