Origins of Life by Freeman Dyson

Author:Freeman Dyson [Dyson, Freeman]
Language: fra
Format: epub
ISBN: 0511035594
Published: 0101-01-01T00:00:00+00:00

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CHAPTER THREE

A Toy Model

THE MEANING OF METABOLISM

This chapter describes my own attempt to understand the Oparin

theory of the origin of life. The essential feature of the Oparin theory is that it has life beginning with metabolism rather than with precise

replication. In this chapter I shall use the phrase “Oparin theory” to

include both the original Oparin theory and the later versions pro-

posed by W ächtersh äuser and others. The Oparin theory as Oparin

proposed it made no attempt to be quantitative. I am trying to place

the theory within a framework of strict mathematics so that its con-

sequences can be calculated. The essential difficulty arises because

metabolism is a vague and ill-defined concept. There is no such dif-

ficulty with the concept of replication. Replication means exactly

what it says. To replicate a molecule means to copy it, either exactly

or with a stated margin of error. Starting from this well-defined con-

cept, Manfred Eigen was able to formulate his theory of the origin of

life, which is in fact a theory of the origin of replication, as a system of equations that can be solved with a computer. Eigen’s equations

describe the evolution with time of populations of molecules sub-

ject to nonlinear laws of replication. When we try to formulate in

a similarly exact fashion the theory of Oparin, which is a theory of

the origin of metabolism, we run immediately into the problem of

defining what we mean by metabolism.

Doron Lancet has tackled this problem by studying computer

models of the evolution of molecular populations, which he calls

replicative-homeostatic early assemblies (RHEA). In these models,

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A Toy Model

49

metabolism is defined in a general way as the evolution of a pop-

ulation in which some of the molecules catalyze the synthesis of

others. He finds conditions under which populations can evolve

to a high and self-sustaining level of catalytic organization. Many

other computer studies of the evolution of metabolism have been

published. The results are summarized in a recent review article

(Segr é and Lancet, 1999). My own model of molecular evolution

is a very special case of a RHEA model. My model has an antique

flavor because its behavior is simple enough to be calculated with

pencil and paper rather than with computer simulations.

I reduce the Oparin theory to a mathematically precise form in

two stages. The first stage is a formal description of molecular pop-

ulations treating them like a classical dynamical system and making

the dynamical equations precise but leaving the laws of interaction

completely general. The general theory of molecular systems ob-

tained in this way allows us to define what we mean by the origin of

metabolism but does not allow us to predict under what conditions

metabolism will occur. The second stage consists of the reduction

of the general theory to a toy model by the assumption of a sim-

ple and arbitrary rule for the probability of molecular interactions.

The entire intricate web of biochemical processes is replaced in the

model by a couple of simple equations. The habit of constructing

toy models of this sort is one to which theoretical physicists easily

become addicted. When the real world is recalcitrant, we build our-

selves toy models in which the equations are simple enough for us

to solve.

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