Pax optica
This is exactly what happened, we now know. Each grade was indeed useful to its possessor, because each grade still exists and still is useful to its owner. Each type of eye is just a slight improvement on the one before. A light-sensitive patch on the skin enables a limpet to tell which way is up; a light-sensitive cup enables a species called a slit-shelled mollusc to tell which direction the light is coming from; a pinhole chamber of light-sensitive cells enables the nautilus to focus a simple image of the world in good light; a simple lensed eye enables a murex snail to form an image even in low light; and an adjustable lens with an iris to control the aperture enables an octopus to perceive the world in glorious detail (the invention of the lens is easily explained, because any transparent tissue in the eye would have acted as partial refractor). Thus even just within the molluscs, every stage of the eye still exists, useful to each owner. How easy then to imagine each stage having existed in the ancestors of the octopus.
Richard Dawkins compares the progression through these grades to climbing a mountain (Mount Improbable) and at no point encountering a slope too steep to surmount. Mountains must be climbed from the bottom up. He shows that there are numerous such mountains – different kinds of eyes in different kinds of animal, from the compound eyes of insects to the multiple and peculiar eyes of spiders – each with a distinct range of partially developed stages showing how one can go step by step. Computer models confirm that there is nothing to suggest any of the stages would confer a disadvantage.
Moreover, the digitisation of biology since the discovery of DNA provides direct and unambiguous evidence of gradual evolution by the progressive alteration of the sequence of letters in genes. We now know that the very same gene, called Pax6, triggers the development of both the compound eye of insects and the simple eye of human beings. The two kinds of eye were inherited from a common ancestor. A version of a Pax gene also directs the development of simple eyes in jellyfish. The ‘opsin’ protein molecules that react to light in the eye can be traced back to the common ancestor of all animals except sponges. Around 700 million years ago, the gene for opsin was duplicated twice to give the three kinds of light-sensitive molecules we possess today. Thus every stage in the evolution of eyes, from the development of light-sensitive molecules to the emergence of lenses and colour vision, can be read directly from the language of the genes. Never has a hard problem in science been so comprehensively and emphatically solved as Darwin’s eye dilemma. Shudder no more, Charles.
Astronomical improbability?
The evidence for gradual, undirected emergence of the opsin molecule by the stepwise alteration of the digital DNA language is strong. But there remains a mathematical objection. The opsin molecule is composed of hundreds of amino acids in a sequence specified by the appropriate gene. If one were to arrive at the appropriate sequence to give opsin its light-detecting properties by trial and error it would take either a very long time or a very large laboratory. Given that there are twenty types of amino acid, then a protein molecule with a hundred amino acids in its chain can exist in 10 to the power of 130 different sequences. That’s a number far greater than the number of atoms in the universe, and far greater than the number of nanoseconds since the Big Bang. So it’s just not possible for natural selection, however many organisms it has to play with for however long, to arrive at a design for an opsin molecule from scratch. And an opsin is just one of tens of thousands of proteins in the body.
Am I heading for a Lucretian swerve? Will I be forced to concede that the combinatorial vastness of the library of possible proteins makes it impossible for evolution to find ones that work? Far from it. We know that human innovation rarely designs things from scratch, but jumps from one technology to the ‘adjacent possible’ technology, recombining existing features. So it is taking small, incremental steps. And we know that the same is true of natural selection. So the mathematics is misleading. In a commonly used analogy, you are not assembling a Boeing 747 with a whirlwind in a scrapyard, you are adding one last rivet to an existing design. And here there has been a remarkable recent discovery that makes natural selection’s task much easier.
In a laboratory in Zürich a few years ago, Andreas Wagner asked his student João Rodriguez to use a gigantic assembly of computers to work his way through a map of different metabolic networks to see how far he could get by changing just one step at a time. He chose the glucose system in a common gut bacterium, and his task was to change one link in the whole metabolic chain in such a way that it still worked – that the creature could still make sixty or so bodily ingredients from this one sugar. How far could he get? In species other than the gut bacterium there are thousands of different glucose pathways. How many of them are just a single step different from each other? Rodriguez found he got 80 per cent of the way through a library of a thousand different metabolic pathways at his first attempt, never having to change more than one step at a time and never producing a metabolic pathway that did not work. ‘When João showed me the answer, my first reaction was disbelief,’ wrote Wagner. ‘Worried that this might be a fluke, I asked João for many more random walks, a thousand more, each preserving metabolic meaning, each leading as far as possible, each leaving in a different direction.’ Same result.
Wagner and Rodriguez had stumbled upon a massive redundancy built into the biochemistry of bacteria – and people. Using the metaphor of a ‘Library of Mendel’, in which imaginary building are stored the unimaginably vast number of all possible genetic sequences, Wagner identified a surprising pattern. ‘The metabolic library is packed to its rafters with books that tell the same story in different ways,’ he writes. ‘Myriad metabolic texts with the same meaning raise the odds of finding any one of them – myriad-fold. Even better, evolution does not just explore the metabolic library like a single casual browser. It crowdsources, employing huge populations of organisms that scour the library for new texts.’ Organisms are crowds of readers going through the Library of Mendel to find texts that make sense.
Wagner points out that biological innovation must be both conservative and progressive, because as it redesigns the body, it cannot ever produce a non-functional organism. Turning microbes into mammals over millions of years is a bit like flying the Atlantic while rebuilding the plane to a new design. The globin molecule, for example, has roughly the same three-dimensional shape and roughly the same function in plants and insects, but the sequences of amino acids in the two are 90 per cent different.
Doubting Darwin still
Yet, despite this overwhelming evidence of emergence, the yearning for design still lures millions of people back into doubting Darwin. The American ‘intelligent design’ movement evolved directly from a fundamentalist drive to promote religion within schools, coupled with a devious ‘end run’ to circumvent the USA’s constitutional separation between Church and state. It has largely focused upon the argument from design in order to try to establish that the complex functional arrangements of biology cannot be explained except by God. As Judge John Jones of Pennsylvania wrote in his judgement in the pivotal case of Kitzmiller vs Dover Area School District in 2005, although proponents of intelligent design ‘occasionally suggest that the designer could be a space alien or a time-traveling cell biologist, no serious alternative to God as the designer has been proposed’. Tammy Kitzmiller was one of several Dover parents who objected to her child being taught ‘intelligent design’ on a par with Darwinism. The parents went to court, and got the school district’s law overturned.
In the United States, fundamentalist Christians have challenged Darwinism in schools for more than 150 years. They pushed state legislatures into adopting laws that prohibited state schools from teaching evolution, a trend that culminated in the Scopes ‘monkey trial’ of 1925. The defendant, John Scopes, deliberately taught evolution illegally to bring attention to the state of Tennessee’s anti-evolution law. Prosecuted by William Jennings Bryan and defended by Clarence Darrow, Scopes was found guilty and fined a paltry $100, and even that was overturned on a technicality at appeal. There is a widespread legend that Bryan’s victory was pyrrhic, because it made him look ridiculous and Scopes’s punishment was light. But this is a comforting myth told by saltwater liberals