Saturday, July 20, 2013
Metabolism & Replication
In Chapter 5 of What is Life? Addy Pross considers theories of the origin of life on earth. There is a lot in here to chew on, but I will focus on a couple of themes. One is the relationship between historical and ahistorical explanations.
Historical explanations of abiotic genesis concern the question of how life actually did emerge from inorganic matter. Ahistorical explanations concern how organically complex systems could have emerged, given some propitious set of circumstances. If we knew how life did emerge, it would obviously help us understand the physical processes that made such an event possible. Likewise, if we understood how life could have emerged it would help us determine what to look for in the geological record. Unfortunately, we don’t have plausible answers to either question. This reminds me of an old joke. There are two ways for a man to deal with a woman. Nobody knows either one of them.
The second theme is the dichotomy involving metabolism first accounts of the origin of life and replicator first accounts. Metabolism is the regulation of chemical reactions that makes all organic processes possible. Materials have to be exchanged with the environment and transformed within cells. Energy must be acquired and expended for this to happen.
Metabolism first explanations of life’s origin hold that it begins with a holistic, autocatalytic reaction among inorganic chemicals. Suppose that molecule A catalyzes molecule B; B catalyzes C; C, D; and D in turn catalyzes A. You know have a potentially self-sustaining cycle. Perhaps that’s how life got started: digestion precedes reproduction.
Replicator first accounts look to molecules that can replicate themselves. Chain A-B-C can catch an additional A, which catches a B, which catches a C. At that point the C-A connection breaks, and we have two A-B-C molecules.
In existing organisms, metabolism and replication support one another. It is very unclear how either could get going by itself, let alone both of them independently. If Pross’ survey is fair, no one knows how either could climb “uphill” against the second law in order to produce even the simplest organisms.
I keep waiting for some sign of how a reductionist account of life might be possible, as Pross promised. I haven’t got to the end yet, but I am getting rather near it. Meanwhile, I am sticking with Aristotle. Life looks to me as if it were ontologically irreducible to the same laws that seem to govern inorganic matter. I am not arguing for some deus ex machina. I think, rather, that the appearance of life tells us something about inorganic matter that we could not possibly guess were life not in evidence.