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Authors: Terry Pratchett
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an incredibly tiny fraction of all those possibilities. So actual human beings have explored the tiniest portion of DNA-space, just as actual books have explored the tiniest portion of L-space. Of course, the interesting questions are not as straightforward as that. Most sequences of lettersdo not make up a sensible book; most DNA sequences do not correspond to a viable organism, let alone a human being.
And now we come to the crunch for phase spaces. In physics, it is reasonable to assume that the sensible phase space can be âpre-statedâ before tackling questions about the corresponding system. We can imagine rearranging the bodies of the solar system into any configuration in that imaginary phase space. We lack the engineering capacity to do that, but we have no difficulty imagining it done, and we see no physical reason to remove any particular configuration from consideration.
When it comes to DNA-space, however, the important questions are not about the whole of that vast space of all possible sequences. Nearly all of those sequences correspond to no organism whatsoever, not even a dead one. What we really need to consider is âviable-DNA-spaceâ, the space of all DNA sequences that could be realised within some viable organism. This is some immensely complicated but very thin part of DNA-space, and we donât know what it is. We have no idea how to look at a hypothetical DNA sequence and decide whether it can occur in a viable organism.
The same problem arises in connection with L-space, but thereâs a twist. A literate human can look at a sequence of letters and spaces and decide whether it constitutes a story; they know how to âreadâ the code and work out its meaning, if itâs in a language they understand. They can even make a stab at deciding whether itâs a good story or a bad one. However, we do not know how to transfer this ability to a computer. The rules that our minds use, to decide whether what weâre reading is a story, are implicit in the networks of nerve cells in our brains. Nobody has yet been able to make those rules explicit. We donât know how to characterise the âreadable booksâ subset of L-space.
For DNA, the problem is compounded because there isnât some kind of fixed rule that âtranslatesâ a DNA code into an organism. Biologists used to think there would be, and had high hopes of learning the âlanguageâ involved. Then the DNA for a genuine (potential) organism would be a code sequence that told a coherent story of biological development, and all other DNA sequences would be gibberish. Ineffect, the biologists expected to be able to look at the DNA sequence of a tiger and see the bit that specified the stripes, the bit that specified the claws, and so on.
This was a bit optimistic. The current state of the art is that we can see the bit of DNA that specifies the protein from which claws are made, or the bits that make the orange, black and white pigments of the fur that show up as stripes, but thatâs about as far as our understanding of DNA narrative goes. It is now becoming clear that many non-genetic factors go into the growth of an organism, too, so even in principle there may not be a âlanguageâ that translates DNA into living creatures. For example, tiger DNA turns into a baby tiger only in the presence of an egg, supplied by a mother tiger. The same DNA, in the presence of a mongoose egg, would not make a tiger at all.
Now, it could be that this is just a technical problem: that for each DNA code there is a unique kind of mother-organism that turns it into a living creature, so that the form of that creature is still implicit in the code. But theoretically, at least, the same DNA code could make two totally different organisms. We give an example in The Collapse of Chaos , where the developing organism first âlooksâ to see what kind of mother it is in, and then develops in
And now we come to the crunch for phase spaces. In physics, it is reasonable to assume that the sensible phase space can be âpre-statedâ before tackling questions about the corresponding system. We can imagine rearranging the bodies of the solar system into any configuration in that imaginary phase space. We lack the engineering capacity to do that, but we have no difficulty imagining it done, and we see no physical reason to remove any particular configuration from consideration.
When it comes to DNA-space, however, the important questions are not about the whole of that vast space of all possible sequences. Nearly all of those sequences correspond to no organism whatsoever, not even a dead one. What we really need to consider is âviable-DNA-spaceâ, the space of all DNA sequences that could be realised within some viable organism. This is some immensely complicated but very thin part of DNA-space, and we donât know what it is. We have no idea how to look at a hypothetical DNA sequence and decide whether it can occur in a viable organism.
The same problem arises in connection with L-space, but thereâs a twist. A literate human can look at a sequence of letters and spaces and decide whether it constitutes a story; they know how to âreadâ the code and work out its meaning, if itâs in a language they understand. They can even make a stab at deciding whether itâs a good story or a bad one. However, we do not know how to transfer this ability to a computer. The rules that our minds use, to decide whether what weâre reading is a story, are implicit in the networks of nerve cells in our brains. Nobody has yet been able to make those rules explicit. We donât know how to characterise the âreadable booksâ subset of L-space.
For DNA, the problem is compounded because there isnât some kind of fixed rule that âtranslatesâ a DNA code into an organism. Biologists used to think there would be, and had high hopes of learning the âlanguageâ involved. Then the DNA for a genuine (potential) organism would be a code sequence that told a coherent story of biological development, and all other DNA sequences would be gibberish. Ineffect, the biologists expected to be able to look at the DNA sequence of a tiger and see the bit that specified the stripes, the bit that specified the claws, and so on.
This was a bit optimistic. The current state of the art is that we can see the bit of DNA that specifies the protein from which claws are made, or the bits that make the orange, black and white pigments of the fur that show up as stripes, but thatâs about as far as our understanding of DNA narrative goes. It is now becoming clear that many non-genetic factors go into the growth of an organism, too, so even in principle there may not be a âlanguageâ that translates DNA into living creatures. For example, tiger DNA turns into a baby tiger only in the presence of an egg, supplied by a mother tiger. The same DNA, in the presence of a mongoose egg, would not make a tiger at all.
Now, it could be that this is just a technical problem: that for each DNA code there is a unique kind of mother-organism that turns it into a living creature, so that the form of that creature is still implicit in the code. But theoretically, at least, the same DNA code could make two totally different organisms. We give an example in The Collapse of Chaos , where the developing organism first âlooksâ to see what kind of mother it is in, and then develops in
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