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 not start out as components of a system whose purpose was vision. They arose elsewhere and had quite different ‘functions’. Their form and function were then selectively modified when their rudimentary light-sensing powers turned out to offer an evolutionary advantage.
Although we now know quite a lot about the genetics of the human eye, no biologist claims to know exactly how it evolved. The fossil record is poor, and humanoid eyes don’t fossilise (though trilobite eyes do). But biologists can offer simple reasons why and how the eye could have evolved, and these alone are sufficient to demolish claims that its evolution is impossible in principle because the eye’s components are interdependent and removing any one of them causes the eye to malfunction. The eye did not evolve one component at a time. Its structure evolved in parallel.
The instigators of more recent revivals of Paley’s doctrine, albeit in less overtly theist tones, have taken on board the message of the eye as a specific case … but its more generic aspects seem to haveeluded them. Darwin’s discussion of the eye, and the Nilsson-Pelger computer experiment, are not limited to eyes. Here is the deeper message. When confronted with a complex living ‘mechanism’, do not assume that the only way it can evolve is component by component, piece by piece. When you see a watch, do not think of hooking up springs and adding cogwheels from some standard box of spare parts. Think more of a Salvador Dali ‘soft watch’ that can flow and distort, deform, split apart, and rejoin. Think of a watch whose cogwheels can change shape, grow new teeth, and whose axles and supports evolve along with the cogs so that at every stage the whole thing fits together. Think of a watch that may have started out as a paper clip, and along the way became a pogo-stick. Think not of a watch that does and always did have a single purpose, which was to tell the time. Think of a watch that once held sheets of paper together and could also be straightened out to form a toothpick, and which later turned out to be great for bouncing, and started to be used for measuring time only when someone noticed that its rhythmic movements could chart the passing seconds.
Yes, proponents of intelligent design understand the eye … but only as one example, not as the basis of a general principle. ‘Oh, yes, we know all about the eye,’ they say (we paraphrase). ‘We’re not going to ask what use half an eye is.
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 not start out as components of a system whose purpose was vision. They arose elsewhere and had quite different ‘functions’. Their form and function were then selectively modified when their rudimentary light-sensing powers turned out to offer an evolutionary advantage.
Although we now know quite a lot about the genetics of the human eye, no biologist claims to know exactly how it evolved. The fossil record is poor, and humanoid eyes don’t fossilise (though trilobite eyes do). But biologists can offer simple reasons why and how the eye could have evolved, and these alone are sufficient to demolish claims that its evolution is impossible in principle because the eye’s components are interdependent and removing any one of them causes the eye to malfunction. The eye did not evolve one component at a time. Its structure evolved in parallel.
The instigators of more recent revivals of Paley’s doctrine, albeit in less overtly theist tones, have taken on board the message of the eye as a specific case … but its more generic aspects seem to haveeluded them. Darwin’s discussion of the eye, and the Nilsson-Pelger computer experiment, are not limited to eyes. Here is the deeper message. When confronted with a complex living ‘mechanism’, do not assume that the only way it can evolve is component by component, piece by piece. When you see a watch, do not think of hooking up springs and adding cogwheels from some standard box of spare parts. Think more of a Salvador Dali ‘soft watch’ that can flow and distort, deform, split apart, and rejoin. Think of a watch whose cogwheels can change shape, grow new teeth, and whose axles and supports evolve along with the cogs so that at every stage the whole thing fits together. Think of a watch that may have started out as a paper clip, and along the way became a pogo-stick. Think not of a watch that does and always did have a single purpose, which was to tell the time. Think of a watch that once held sheets of paper together and could also be straightened out to form a toothpick, and which later turned out to be great for bouncing, and started to be used for measuring time only when someone noticed that its rhythmic movements could chart the passing seconds.
Yes, proponents of intelligent design understand the eye … but only as one example, not as the basis of a general principle. ‘Oh, yes, we know all about the eye,’ they say (we paraphrase). ‘We’re not going to ask what use half an eye is.
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