Current mainstream explanations of evolution are all very well – the standard idea of randomness filtered by fitness – but do they explain it all? Some people have suggested that the random formation of proteins, let alone RNA or DNA or whole cells, is hyperastronomically unlikely. So is there another explanation?
Imagine strewing 1000 shirt buttons around a room. Choose two at random, and connect them with a thread. Repeat many times. Now and again, pick up individual buttons and see how many are connected. X axis number of threads, Y axis the number of buttons in the largest cluster. As the ratio of threads to buttons grows, the largest cluster grows very slowly. Maybe at 400 threads the largest links less than 100 buttons. Suddenly, the size accelerates – around the 500 thread mark, the rate rises very quickly, up to around 900 buttons, then it levels out again. The rise is very sharp, around that 0.5 ratio, and steeper the more buttons are involved. We have a phase transition, a sudden emergence of tight order.
So what? That’s far from life. What’s a minimal definition of life? One might start with what’s known as an autocatalytic set, one where a bounded set of chemicals sustains and catalyses its own reactions, maybe with exchanges of energy only with the external world. Well, a standard way of modelling chemical reactions treats chemicals as nodes, and their reactions as edges connecting them and their product(s). A model not unlike our buttons and threads. If we assume that a random chemical has some set chance of acting as a catalyst on any other reaction, and we vary the number of chemicals and the probability of any two reacting together, we do indeed see similar sharp phase transitions. Suddenly, parts of a random stew start reacting together, and large autocatalytic sets spontaneously emerge. Further, these can grow. They can even split. Even with small differences from their largely similar ‘parents’.
There is much more to this idea of emergent complexity in biology, all kinds of models with striking correspondences with the way life seems to work. They offer a spectacularly successful model of so much that can mystify about natural selection, about how complex structures come about despite nothing in any conceivable intermediate stages offering any survival edge, about the patterns of mass extinctions and bursts of speciation that the world has seen, random boolean networks behaving strikingly like genomic structures, mathematical explanations of the numbers of cell types in different creatures, about so much more.
And that is barely a hint in one area of what complexity theory might offer. I think it’s a shift in our understanding that might compare with those introduced by Darwin or Freud or Einstein.