Gills vs. Lungs

 

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