Abstract
Understanding the emergence of life from (primitive) abiotic components has arguably been one of the deepest and yet one of the most elusive scientific questions. Notwithstanding the lack of a clear definition for a living system, it is widely argued that heredity (involving self-reproduction) along with compartmentalization and metabolism are key features that contrast living systems from their non-living counterparts. A minimal living system may be viewed as “a self-sustaining chemical system capable of Darwinian evolution”. It has been proposed that autocatalytic sets of chemical reactions (ACSs) could serve as a mechanism to establish chemical compositional identity, heritable self-reproduction, and evolution in a minimal chemical system. Following years of theoretical work, autocatalytic chemical systems have been constructed experimentally using a wide variety of substrates, and most studies, thus far, have focused on the demonstration of chemical self-reproduction under specific conditions. While several recent experimental studies have raised the possibility of carrying out some aspects of experimental evolution using autocatalytic reaction networks, there remain many open challenges. In this review, we start by evaluating theoretical studies of ACSs specifically with a view to establish the conditions required for such chemical systems to exhibit self-reproduction and Darwinian evolution. Then, we follow with an extensive overview of experimental ACS systems and use the theoretically established conditions to critically evaluate these empirical systems for their potential to exhibit Darwinian evolution. We identify various technical and conceptual challenges limiting experimental progress and, finally, conclude with some remarks about open questions.
Highlights
Autocatalysis and the Emergence of Life A cell, the widely accepted “unit of life”, is able to propagate by transferring templated genetic information to daughter cells, aided and buffered by a plethora of complex processes and catalytic machinery
Autocatalytic sets of chemical reactions (ACSs), comprising various chemical species, with cooperative catalytic interactions have been proposed [2,4,5,6,7,8] as an intermediate stage (Figure 1) of chemical evolution (Note: while our focus here is on autocatalytic sets of chemical reactions (ACSs), the demonstration of intermediary chemical and evolutionary dynamics has been explored in chemical systems that lack autocatalysis [9])
There have been a few proposals to remedy to this situation: (i) the so-called “nested ACS”, in which an ACS comprised of longer polymers, is reinforced by an ACS comprising of weak but short catalytic polymers [55]; (ii) stochastic fluctuations could lead to the emergence of ACS containing longer polymers [60]—here, the emergence of an ACS corresponds to a phase transition from the inactive to the active states in Dyson’s model, and the stochastic analysis further suggests an optimal size of the system for the possibility of such a transition; and (iii) template-assisted spontaneous ligation can lead to open-ended polymer elongation and autocatalysis [61,62,63,64]
Summary
Autocatalysis and the Emergence of Life A cell, the widely accepted “unit of life”, is able to propagate by transferring templated genetic information to daughter cells, aided and buffered by a plethora of complex processes and catalytic machinery. In contrast to a templated replication system, a self-reproducing unit might be identified by a unique collective composition of a set of chemical reactions (Note: we use the term replication only in the context of a monomer-by-monomer copy of the sequence information occurring via template-directed polymerization and the term selfreproduction otherwise.) In this context, autocatalytic sets of chemical reactions (ACSs), comprising various chemical species, with cooperative catalytic interactions have been proposed [2,4,5,6,7,8] as an intermediate stage (Figure 1) of chemical evolution (Note: while our focus here is on ACSs, the demonstration of intermediary chemical and evolutionary dynamics has been explored in chemical systems that lack autocatalysis [9]). We review studies on the spontaneous emergence of self-reproducing systems (Section 2.3) and evaluate the requirements for an autocatalytic system to undergo canonical Darwinian evolution, the aspects of variation, heredity, and differential fitness (Section 2.4)
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