The Science of Discworld II

complicated because their genome is shorter. However, this seductive idea can’t be true. For example, the Shannon information content of the human genome is smaller by several orders of magnitude than the quantity of information needed to describe the wiring of the neurons in the human brain. How can we be more complex than the information that describes us? And some amoebas have much longer genomes than ours, which takes us down several pegs as well as casting even more doubt on DNA as information.
    Underlying the widespread belief that DNA complexity explains organism complexity (even though it clearly doesn’t) are two assumptions, two scientific stories that we tell ourselves. The first story is DNA as Blueprint , in which the genome is represented not just as an important source of control and guidance over biological development, but as the information needed to determine an organism. The second is DNA as Message , the ‘Book of Life’ metaphor.
    Both stories oversimplify a beautifully complex interactive system. DNA as Blueprint says that the genome is a molecular ‘map’ of an organism. DNA as Message says that an organism can pass that map to the next generation by ‘sending’ the appropriate information.
    Both of these are wrong, although they’re quite good science fiction – or, at least, interestingly bad science fiction with good special effects.
    If there is a ‘receiver’ for the DNA ‘message’ it is not the next generation of the organism, which does not even exist at the time the ‘message’ is being ‘sent’, but the ribosome, which is the molecular machine that turns DNA sequences (in a protein-coding gene) into protein. The ribosome is an essential part of the coding system; it functions as an ‘adapter’, changing the sequence information along the DNA into an amino acid sequence in proteins. Every cell contains many ribosomes: we say ‘the’ because they are all identical. The metaphor of DNA as information has become almost universal, yet virtually nobody has suggested that the ribosome must be a vast repository of information. The structure of the ribosome is now known in high detail, and there is no sign of obvious ‘information-bearing’structure like that in DNA. The ribosome seems to be a fixed ‘machine’. So where has the information gone? Nowhere. That’s the wrong question.
    The root of these misunderstandings lies in a lack of attention to context. Science is very strong on content, but it has a habit of ignoring ‘external’ constraints on the systems being studied. Context is an important but neglected feature of information. It is so easy to focus on the combinatorial clarity of the message and to ignore the messy, complicated processes carried out by the receiver when it decodes the message. Context is crucial to the interpretation of messages: to their meaning. In his book The User Illusion Tor Nørretranders introduced the term exformation to capture the role of the context, and Douglas Hofstadter made the same general point in Gödel, Escher, Bach . Observe how, in the next chapter, the otherwise incomprehensible message ‘THEOSTRY’ becomes obvious when context is taken into account.
    Instead of thinking about a DNA ‘blueprint’ encoding an organism, it’s easier to think of a CD encoding music. Biological development is like a CD that contains instructions for building a new CD-player. You can’t ‘read’ those instructions without already having one. If meaning does not depend upon context, then the code on the CD should have an invariant meaning, one that is independent of the player. Does it, though?
    Compare two extremes: a ‘standard’ player that maps the digital code on the CD to music in the manner intended by the design engineers, and a jukebox. With a normal jukebox, the only message that you send is some money and a button-push; yet in the context of the jukebox these are interpreted as a specific several minutes’ worth of music. In principle, any numerical code can ‘mean’ any piece of music you wish; it just depends on how the jukebox is set up, that is, on the exformation associated with the jukebox’s design. Now consider a jukebox that reacts to a CD not by playing the tune that’s encoded on it, as a series of bits, but by interpreting that code as a number, and then playing