We
(humans) tend to be rather biased against any system or trait that is not as
highly evolved as our own. Consider the
comparison of gills with lungs. Both
organs perform virtually the same function but since humans have lungs and
things like fish and clams have gills, then gills must be primitive, simple and
less efficient right? Let's see . . .
Respiration
is not the act of breathing (unless you are taking a CPR class). Respiration is the chemical breakdown of the
food we eat into energy. We all know
that eating sugar gives us energy but sugar is only one component. It is the
process of respiration that takes that sugar and converts it into a form of
energy that our bodies can use. Once starches and sugars are broken down into
glucose, that glucose is further broken down into smaller and smaller
molecules. Each time a molecule breaks, energy is released. That energy is
captured and stored in the form of another molecule called ATP. ATP is really the energy currency of the
cell. Learning this process is the bane of many a first-year biology student
who have to study it in detail. Apart from this drawback respiration is a
rather useful part of our functioning. The chemical that drives this process is
oxygen. Without oxygen your cells cannot produce any energy, no matter how many
twinkies you eat. Since your cells constantly need energy to perform the
everyday cell functions, without oxygen you won't last long.
I
know, you thought I was going to talk about gills and lungs. Be patient.
Obviously
since oxygen is so vital there must be a way of getting it to the cells. The
smaller and flatter you are, the better. Tiny organisms just let oxygen pass
through their skin or membrane by simple diffusion. Larger critters need
specialized organs to get oxygen into the body and then a transport system to
move it around to the cells.
Evolution
has derived two organs for the first part, gills and lungs. Technically the
only difference between the two is that gills are outside the body and lungs
are internal. Both organs work in much the same way. The surfaces of gills and
lungs are extremely thin. Capillaries lie next to the surface allowing for easy
exchange of oxygen in and carbon dioxide out. Also, both organs try to maximize
the surface area that comes into contact with the environment.
Certainly,
gills evolved before lungs, as sea creatures evolved before land creatures. In
that sense you could say gills are more primitive. But when it comes to
efficiency, gills are vastly superior to lungs. If you stopped to think about
it, it would have to be that way because there is not nearly as much oxygen
dissolved in water as there is in air. In fact, gills work perfectly well in
air as long as they are not allowed to dry out. Lungs, however, are not
efficient enough to keep us alive underwater.
The
added energy needed to pump water (a substance much thicker than air) in and
out would cause us to expend more energy trying to breathe than we would get
back. Yes, gills are simpler than lungs
but in a mechanical system, simplicity is an advantage. Why don’t we all have gills? The problem for land animals is that gills
would dry out and no longer work. The surfaces of our lungs remain moist
because they are inside our bodies.
So
when the ancient lobefins first started to crawl out of the oceans and use
their newly formed lungs, a giant step in evolution was taken, but not
necessarily a giant step in efficiency. Human
bias can lead you astray.
Star Liner
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