My daughter just had to learn a lot of stuff for a biology exam. It gave her a hard time. And then I just read an article in another girl’s blog who is just facing the same difficulty. I asked myself: why is life so terribly complicated. Why all those enzymes, that ATP synthase driven by protons flowing through a membrane, that citric acid cycle, why those thylakoids and carboxysomes and all that stuff that gives our kids a traumatic experience.
The theoretical biologist Stuart Kauffman came up with the view of living organisms as autocatalytic systems. A lot of different molecules are interacting and in this web of chemical reactions, many of them are catalyzers of some of these reactions. Catalysis means that a molecule is taking part in a reaction and making it more likely to happen, but comes out of it unchanged. In a living organism, we have what Kauffman calls “catalytic closure”. The whole system is autocatalytic. For example, DNA-molecules are catalyzing, together with some enzyme, the formation of RNA-molecules, those RNA-molecules are catalyzing (together with the ribosomes, a complex of RNA and proteins) the formation of proteins (including those enzymes and ribosomal proteins), a DNA-strand is catalyzing the formation of its complementary strand, and so on.
As a whole, the organism uses some energy and catalyzes the creation of more of itself from some raw materials. A living organism, then, can be regarded as a complex web of chemical reactions that, as a whole, is autocatalytic.
Now, Kauffman realized that if you have a source of energy and a process that synthesizes many different kinds of molecules, some of the molecules would be catalyzers for others. If the number of different types of molecules is high enough, it would become very likely, even almost inevitable, that inside this mix of different molecules catalytic closure would occur, a network of chemical reactions that, as a whole, is autocatalytic. Kauffman realized one could view such a network as a living organism. As long as there are resources (raw materials and energy), such a network would grow, and it would push aside any other competing chemical reactions. It could then evolve by replacing some molecules and reactions by other ones, increasing the efficiency and thus the growth rate of the whole thing. The rest is evolution.
This idea about the origin of life has a number of interesting features:
- Given the right conditions, the emergence of life is very likely, even nearly inevitable. It is not a very unlikely chance event.
- Life would have started out complex from the beginning, with hundreds or even thousands of different molecules involved in it right from the start! Complexity might be a necessary feature of life (sorry, kids). The simplest organisms we can find still contain several hundred genes and you cannot take away much of them again. Viruses are simpler but they need the molecular machinery of another organism to be able replicate.
- RNA might have been involved right from the start, but maybe it wasn’t and only entered the mix later. So the idea to trace the origin of life to a single self-replicating molecule of RNA might be misleading. RNA might have arisen in an environment that was already living in the sense of being autocatalytic, and that might have contained hundreds or thousands of other types of molecules. Its emergence might not have been a very unlikely chance event but something made more likely by a “friendly” catalyzing environment that already existed earlier.
The disadvantage of Kauffman’s idea is that it is relatively abstract. It does not tell you exactly what the initial conditions were and which kinds of molecules where involved. But it might even be possible to let such a process happen in a laboratory.
Another thought that comes up when looking at the complexity of organisms is that it makes the creationist’s view extremely unconvincing. An omnipotent creator would not have to create elementary particles guided by quantum mechanics in order to produce life as an extremely complex web of reactions. An omnipotent creator could simply create living things directly, in some miraculous, vitalistic way, with some Aristotelian physics instead of molecules and electrons and protons and ATP synthase. We would not be living, chemical beasts but some kind of ghosts (some additional thoughts in that direction can be found here).
To the dismay of kids who have to learn biology, life does have a molecular level and at that level is understandable (hence you have to learn about it in school) but, alas, very complex. Life is not possible in any simpler way. But if you step back and look at it, isn’t it also very fascinating?
(The picture, showing the structure of the enzyme ATP synthase, is from http://commons.wikimedia.org/wiki/File:Atp_synthase.PNG.)