Animals tend to have holes in them. Fact.
But… why do some animals have more holes in them than
others? Where did they come from in the first place, and how have they changed
since then? Well, here’s the plan: I’ll tell you over a series of posts,
focussing on a different orifice each time.
“How rude!” I hear you say. You can thank your mouth for
that ability. Human mouths are pretty specialised: as well as the usual
functions of channelling nutrients and air, they have a whole array of sophistications
that allow us to mess about with that air to make a catalogue of weird sounds
that we call “talking”. When considering where a feature came from, however, we
often have to look past its present functions and think about what the basic
prototype of that feature could have done for its owner. Mouths most certainly
did not evolve for speech, or the world would be a noisy place!
Perhaps the best way to consider why most animals need
mouths is to consider why some organisms DON’T. If you look at the very tiniest
organisms (with a good microscope, of course), there is no sign of a mouth.
That’s because the organism is so small that its nutritional needs can be met
simply by absorbing nutrients through its body wall, or engulfing smaller
bodies into the cytoplasm. But as the organism gets bigger, the laws of physics
dictate that its body’s volume increases much faster than its surface area,
and things get more complicated. It’s like a school cafeteria tripling the
number of kids it has to serve, but only opening one extra serving hatch-
there’s just too much demand from the body’s cells beyond a certain size.
That’s when animals came up with an extra, specialised surface to absorb food: the gut. And of course, the gut can’t function without a mouth. We
can get an idea about how the gut and both its ends first came about by
watching it form in developing embryos: a process I’ll come back to in a later
orifice post.
That’s not where the story ends, though. While many animals
have kept to the basic plan, with little more than a small hole present, the
mouth has shown itself to be very flexible to adaptation, allowing it to be
suited to almost any diet an animal could hope to live on. All sorts of
accessories have been attached to it over the years, like a bizarre set of Lego
expansion packs: from spikes to suckers to sensory equipment. There’s one
innovation in particular, though, that has allowed the mouth to take on even
more specialised roles in a familiar group: the vertebrates and their jaws.
Ancient vertebrates got by for quite a while without jaws,
and two groups still manage it today: the friendly-looking creatures known as
hagfish and lampreys.
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| Hagfish |
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| A lamprey. It likes you. |
Hagfishes love rotting flesh, and get around their lack
of jaws by taking advantage of their lack of a skeleton: they literally tie
their bodies in knots to build up the force to tear off tasty morsels of rancid
whale. Lampreys prefer their flesh fresh, with many species making a living by
clamping onto fishes with a ring of mouth hooks and drinking their host’s
fluids. Ancient jawless vertebrates managed to be pretty horrible to each
other, too, even without the ability to bite. Conodonts were a group of jawless
fishes that vanished at the end of the Triassic period. Fossils are patchy:
most of the remains we have to go on are teeth. Really, really nasty teeth. The
points were much sharper than of any organism we see today: twenty times thinner
than a human hair, and conodonts had plenty of these hooked horrors in their
mouths. Although they could only generate small forces upon their prey without
jaws, by concentrating these upon the narrow tooth tips, they were able to
produce some formidable piercing pressure- though with such fragile weapons, it
seems likely that these animals would have to regularly regrow their tooth tips.
Take me swimming with piranhas any day.
The bones that make up the vertebrate jaw did not arise out
of nowhere: as with so many other structures we see today, they used to have an
entirely different function. Fishes respire by moving water through their
gills, which reside in gill chambers. A chamber is no good if it collapses in
on itself, and so the gills are supported by gill arches. But something odd
started happening to the arch closest to the head in some vertebrates - it
started to become bigger and thicker and elongated, spreading forward into the
region of the mouth. Here is a terrible diagram drawn with Microsoft Paint and a mouse (the digital equivalent of drawing with a slightly chewed wax crayon).
As muscles in the area also grew stronger, these lucky
fishes were able to have more control over the movement of their mouths,
allowing them to be opened and closed (they still haven’t quite got over the
novelty of it- just look at any goldfish!). Still, being able to flap your
mouth around a bit isn’t the world’s greatest revolution in feeding. Instead,
it’s thought that mouth movements first caught on because they could flush
water into the gills. Using gills for respiration rather than filter feeding
was probably quite a new development in the vertebrates, helping them to keep
up with oxygen demand from more active lifestyles, and before jaws, fish had to
pump their entire bodies to provide ventilation. Energy is money for organisms,
and the great saving that primitive jawed vertebrates made by pumping with
their mouths instead would have made them a runaway success. Once the suction from
mouth ventilation started bringing in the odd bit of food by chance… well, the
advantages became even greater, with animals sporting more sophisticated jaws
that increased food capture enjoying the largest rewards, thus gradually shaping
jaws to suit their dual functions. From there, jawed mouths were free to
develop into whatever suited their owners best: whether that’s mutilating other
animals, swallowing plankton or chewing on gobstoppers. Thanks, evolution!
But the other gill bars haven’t let bar number one have all
the fun since then: much later, the second arch also changed, providing a
support for the jaw and a link to the rest of the skull called the
hyomandibula. Some fishes have even gone a bit over the top when it comes to
jaws, and developed a second set from other gill bars that lurks deep in their
throats. Some of these resemble something from a horror movie, but that’s a
subject for another post- don’t ruin my fun by Googling it til then, ok?
But the gills didn't just change on the inside... odd things began to happen to the first gill opening, too...
To be continued!
Image credits:
Lamprey: http://www.flickr.com/photos/usfwspacific/7129322663/
Hagfish: http://www.flickr.com/photos/baggis/2388168549/
Diagram: original drawings by Mallatt (1996) used as reference
