Science of Discworld III

climate accurately, we could predict how the finches would evolve. But we can’t predict the climate well enough, and there are reasons to think that this may never be possible.
    That doesn’t prevent evolution from being ‘predictive’, hence a science, any more than it prevents meteorology from being a science. But the evolutionary predictions are contingent upon the behaviour of the climate. They predict what will happen in what circumstances, not when it will happen.
    Darwin almost certainly read Paley’s masterwork as a young man, and in later life he may well have used it as a touchstone for his own, more radical and far more indirect, views. Paley succinctly expressed many of the most effective objections to Darwin’s ideas, long before Darwin arrived at them. Intellectual honesty demanded that Darwin should find convincing answers to Paley. Such answers are scattered throughout Darwin’s epic treatise The Origin of Species , though Paley’s name does not appear.
    In particular, Darwin found it necessary to tackle the thorny question of the eye. His answer was that although the human eye appears to be a perfected mechanism, with many interdependent parts, there are plenty of different ‘eyes’ in the animal kingdom, and a lot of those are relatively rudimentary. They can even be arranged in a rough progression from simple light-sensing patches to pinhole cameras to complex lenses (though this arrangement should not be interpreted as an actual evolutionary sequence). Instead of half an eye, we find an eye that is half as effective at detecting light. And this is far, far better than no eye at all.
    Darwin’s approach to the eye is complemented by some computer experiments published by Daniel Nilsson and Suzanne Pelger 4 in 1994. They studied a simple model of the evolution of a light-sensing patch of cells, whose geometry could change slightly at every ‘generation’, and which was equipped with the capacity to develop accessories such as a lens. In their simulations, a mere 100,000 generations were enough to transform a light-sensing patch into something approaching the human eye, including a lens whose refractive index varied from place to place, to improve its focus. The human eye possesses just such a lens. Moreover, and crucially, at every one of those 100,000 steps, the eye’s ability to sense light got better.
    This simulation was recently criticised on the grounds that it gets out what it puts in. It doesn’t explain how those light-sensing cells can appear to begin with, or how the eye’s geometry can change. And it uses a rather simplistic measure of the eye’s performance. These would be important criticisms if the model were being used as some kind of proof that eyes must evolve, and as an accurate description of how they did it. However, that was never the purpose of the simulation. It had two main aims. One was to show that in the simplified context of the model, evolution constrained by natural selection could make incremental improvements and get to something resembling a real eye. It wouldn’t get stuck along the way with some dead-end version of the eye that could be improved only by scrapping it and starting afresh. The second aim was to estimate the time required for such a process to take place (look at the title of the paper), on the assumption that the necessary ingredients were available.
    Some of the model’s assumptions are easily justified, as it happens. Light carries energy and energy affects chemical bonds, so it is notsurprising that many chemicals respond to light. Evolution has an immense range of molecules to draw on – proteins specified by DNA sequences in genes. The combinatorial possibilities here are truly vast: the universe is not big enough, and has not lasted long enough, to make one molecule of each possible protein as complex as, say, haemoglobin, the oxygen-carrier in blood. It would be utterly astonishing if evolution could not come up with at least one light-sensing pigment, and incorporate it into a cell.
    There are even some ideas of how this may have happened. In Debating Design , Bruce Weber and David Depew point out that light-sensitive enzyme systems can be found in bacteria, and these systems are probably very ancient. The bacteria don’t use them for vision, but as part of their metabolic (energy-gaining) processes. Proteins in the human lens are very similar to metabolic enzymes found in the liver. So the proteins that make the eye did