Possible creatures

Andreas Wagner in Aeon:

ImagesOLXS559UHow do random DNA changes lead to innovation? Darwin’s concept of natural selection, although crucial to understand evolution, doesn’t help much. The thing is, selection can only spread innovations that already exist. The botanist Hugo de Vries said it best in 1905: ‘Natural selection can explain the survival of the fittest, but it cannot explain the arrival of the fittest.’ (Half a century earlier, Darwin had already admitted that calling variations random is just another way of admitting that we don’t know their origins.) A metaphor might help to clarify the problem. Imagine a giant library of books containing all possible sequences of letters in the alphabet. Such a library would be huge beyond imagination, and most of its texts would of course be pure gibberish. But some would contain islands of intelligibility – a word here, a Haiku there – in a sea of random letters. Still others would tell all stories real and imagined: not only Dickens’s Oliver Twist or Goethe’s Faust, but all possible novels and dramas, the biography of every single human, true and false histories of the world, of other worlds as yet unseen, and so on. Some texts would include descriptions of countless technological innovations, from the wheel to the steam engine to the transistor – including countless innovations yet to be imagined. But the chances of choosing such a valuable tome by chance are minuscule.

A protein is a volume in a library just like this, written in a 20-letter alphabet of amino acids. And while protein texts might not be as long as Tolstoy’s War and Peace, their total number is still astonishing. For example, a library of every possible amino acid string that is 500 letters long would contain more than 10600 texts – a one with 600 trailing zeros. That vastly outnumbers the atoms in the visible universe. The library is a giant space of the possible, encoding all the proteins that could be useful to life. But here’s the thing: evolution can’t simply look up the chemicals it needs in a giant catalogue. No, it has to inch its way painstakingly along the stacks. Imagine a crowd of browsers – each one representing an entire familial line – who must blindly explore the library, step by random step. This sounds like a party game, but there’s a grisly twist. A mutation that compromises an essential protein such as haemoglobin is punishable by death. On that ill-fated volume, the bloodline ends.

The challenge, then, is to land on texts that work.

More here.