From prelife to life: how chemical kinetics become evolutionary dynamics

Abstract

Life is that which evolves. Living systems are the products of evolutionary processes and can undergo further evolution. A crucial question for the origin of life is the following: when do chemical kinetics become evolutionary dynamics? In this Account, we review properties of "prelife" and discuss the transition from prelife to life. We describe prelife as a chemical system where activated monomers can copolymerize into macromolecules such as RNA. These macromolecules carry information, and their physical and chemical properties depend to a certain extent on their particular sequence of monomers. We consider prelife as a logical precursor of life, where macromolecules are formed by copolymerization, but they cannot replicate. Prelife can undergo "prevolutionary dynamics", including processes such as mutation, selection, and cooperation. Prelife selection, however, is blunt: small differences in rate constants lead to small differences in abundance. Life emerges with the ability of replication. In the resulting evolutionary dynamics, selection is sharp: small differences in rate constants can lead to large differences in abundance. We also study the competition of different "prelives" and find that there can be selection for those systems that ultimately give rise to replication. The transition from prelife to life can occur over an extended period of time. Instead of a single moment that marks the origin of life, prelife may have seeded many attempts for the origin of life. Eventually life takes over and destroys prelife.

Figure 1

Prelife with a random fitness landscape. The equilibrium configuration of equation (2) is shown. There are 2^k different sequences of length k. The color code is: k = 1 (black), 2 (grey), 3 (green), 4(cyan), 5 (blue), 6 (red) and 7 (yellow). Longer sequences are not shown. Parameter values: d = 1; a₀ = a₁ = 1; for all other i we have a_i = 1 + sξ_i where s is the intensity of selection, shown on the x-axis and ξ_i is a random number taken from a uniform distribution on the interval [0, 1]. We observe that longer sequences are exponentially less common. As the intensity of selection, s, increases some sequences become more abundant than others.

Figure 2

Prelife and life. The equilibrium configuration of equation (3) is shown. Again there are 2^k different sequences of length k. The color code is the same as for Figure 1. Parameter values: d = 1; a₀ = a₁ = 1; all other a_i = 0.5. Some cutoff in the numerical simulation is needed, and therefore we assume that sequences of length k = 7 (or greater) do not extend. Sequences of length k = 3, 4, 5 can replicate; their relative replication rates, f_i, are random numbers taken from a uniform distribution on [0, 1]. As the overall replication rate r (shown on the x-axis) increases, life replaces prelife. For high replication rate the population is dominated by a particular sequence of length 5, which has the maximum relative fitness f_i. Prelife allows coexistence. Life leads to competitive exclusion. (Note: Σx_i = 1.)

Figure 3

Competition of two prelives as described by equation (4). Sequences of type A are somewhat less stable than sequences of type B. Type A sequences of length less than 6 (cyan) or greater than 10 (blue) cannot replicate. But A sequences of length 6 to 10 (red) can replicate. As the overall replication rate increases A outcompetes B (green). Parameter values: a = b = 1, d_A = 1, d_B = 0.5, f_i = 1 for i = 6, …, 10, f_i = 0 otherwise; maximum sequence length is 50. Hence; different prelives can compete with each other and those ‘fertile’ prelives can be selected that give rise to life.