One of the things that has driven my work over the last decade has been an interest in analog systems (which I alluded to in an earlier post) that perform what I call physical computation. What that means is that they are not �programmed� except in the sense that, like all objects, they are forced to obey the laws of physics. When physical objects become sufficiently complicated, then they start to behave in interesting ways while still just doing what comes naturally.

That�s interesting because biological organisms fall into this class of physical or analog machines.

Not everone understands this. Many people mistakenly assume that because information is transmitted through the brain by uniform pulses of electricity, that this means the brain is like a digital computer. Not so. In fact they brain is a much more complex machine, where the time between pulses (which can vary continuously and so is analog) carries the information, not the pulses themselves.

But, from my point of view, what makes analog systems special is that they do not, like most digital systems, have to be designed for a specific purpose in order to be useful. In fact, analog systems can be very bad indeed at particular tasks but get better through feedback, because even the smallest trace of a useful function can be exploited. This is what makes them ideal from an evolutionary point of view.

Think about it: mammals with sophisticated light and sound-sensing organs evolved from single-celled organisms. We know that this happened through evolution. But how does a sense organ evolve when there wasn't one there before? Well, maybe a little hair structure vibrates in the presence of certain acoustic waves (because physics says it has to), and these vibrations accidentally trigger the organism to move in a certain way. If this movement is advantageous for the organism during its life, then the hair structure might become more common�and more sophisticated�in future generations: a feedback loop that could eventually lead to a hearing organ.

Let�s get specific. There has much debate about why the hammerhead shark, which has an oblong-shaped head much wider than its body, looks the way it does. Biologists have considered various possibilities. First, there is an obvious sensory advantage in that binocular vision (depth perception) improves with the separation of the eyes. Another explanation is that this head shape has hydrodynamic advantages, including increased stability while turning. Finally, sharks have electrosensory pores that assist them in catching prey. The wider head means there can be a greater number of these pores for the same density.

If you think about the shark as a physical object, then you don't need to pick one of these theories. Evolution can select for all of them. Any physical changes caused by mutation�no matter how small or how seemingly unrelated to either sensing or locomotion�can be exploited. All that is necessary is that this change somehow supply some kind of advantage, allowing the animal to survive and procreate better than its neighbors. So, a slight widening of the head produced by evolutionary accident may have improved the survival of the hammerhead shark precisely because all three abilities (electro-sensing, depth perception, dynamical stability) were enhanced at once.

Unlike with digital technology, there is no need for the improvement to reach some artificial threshold in order to make a difference. With analog systems, even the most miniscule tweak can be bootstrapped into a feedback loop that ends in major change.