The human body abounds with what, in one sense; we could call imperfections but, in another sense, should be seen as inescapable compromises resulting from our long ancestral history of descent from other kinds of animal. Imperfections are inevitable when ‘back to the drawing board’ is not an option — when improvements can be achieved only by making ad hoc modifications to what is already there. Imagine what a mess the jet engine would be if Sir Frank Whittle and Dr Hans von Ohain, its two independent inventors, had been forced to abide by a rule that said: ‘You are not allowed to start with a clean sheet on your drawing board. You have to start with a propeller engine and change it, one piece at a time, screw by screw, rivet by rivet, from the “ancestral” propeller engine into a “descendant” jet engine’ Even worse, all the intermediates have got to fly, and each one in the chain has got to be at least a slight improvement on its predecessor. You can see that the resulting jet engine would be burdened with all kinds of historical relics and anomalies and imperfections. And each imperfection would be attended by a cumbersome accretion of compensatory bodges and fixes and kludges, each one making the best of the unfortunate prohibition against going right back to the drawing board.

The point is made, but a closer look at biological innovation might draw a different analogy from the propeller engine / jet engine case. An important innovation (the jet engine in our analogy) is quite likely to evolve not from the old organ that did the same job (the propeller engine in this case) but from something completely different, which performed a completely different function. As a nice example, when our fish ancestors took to breathing air, they didn’t modify their gills to make a lung (as do some modern air-breathing fish, such as the climbing perch Anabas). Instead, they modified a pouch of the gut. And later, by the way, the teleosts – which means just about any fish you are likely to meet, except sharks and their kind – modified the lung (which had previously evolved in ancestors that occasionally breathed air) to become yet another vital organ, which has nothing to do with breathing: the swim bladder.

The swim bladder is perhaps the major key to the teleosts’ success, and it is well worth a digression to explain it. It is an internal bladder filled with gas, which can be sensitively adjusted to keep the fish in hydrostatic equilibrium at any desired depth. If you ever played with a Cartesian Diver as a child you’ll recognize the principle, but a teleost fish uses an interesting variant of it. A Cartesian Diver is a little toy whose business part is a tiny upended cup, containing a bubble of air, floating at equilibrium in a bottle of water. The number of molecules of air in the bubble is fixed, but you can decrease the volume (and increase the pressure, following Boyle’s Law) by pressing down on the cork in the bottle. Or you can increase the volume of air (and decrease the pressure of the bubble) by raising the cork. The effect is best achieved with one of those stout screw stoppers they put on cider bottles. When you lower or raise the stopper, the diver moves down or up until it reaches its new point of hydrostatic equilibrium. You can coax the diver up and down the bottle by sensitive adjustments to the stopper, and hence to the pressure.

A fish is a Cartesian Diver with a subtle difference. The swim bladder is its ‘bubble’ and it works in the same way, except that the number of molecules of gas in the bladder is not fixed. When the fish wants to rise to a higher level in the water it releases molecules of gas from the blood into the bladder, thereby increasing the volume. When it wants to sink deeper, it absorbs molecules of gas from the bladder into the blood, thereby decreasing the volume of the bladder. The swim bladder means that a fish doesn’t have to do muscular work, as a shark does, in order to stay at a desired depth. It is at hydrostatic equilibrium at whatever depth it chooses. The swim bladder does that job, thereby freeing up the muscles for active propulsion. Sharks, by contrast, have to keep swimming all the time, otherwise they would sink to the bottom, admittedly slowly because they have special low-density substances in their tissues that keep them moderately buoyant. The swim bladder, then, is a coopted lung, which is itself a coopted gut pouch (not, as you might have expected, a coopted gill chamber). And in some fish, the swim bladder itself is yet further coopted into a hearing organ, a kind of eardrum. History is written all over the body, not just once but repeatedly, in exuberant palimpsest.

Excerpted from ‘The Greatest Show on Earth’ by Richard Dawkins