Selection for propagule formation in prebiotic chemical ecosystems
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Biological individuals on Earth today are cellular, but the simplest cells are too complex to have arisen spontaneously. We explore the hypothesis that the emergence of biological individuality might have been driven by selection for different molecules to move through space together by forming a propagule. We predicted that propagule formation will be advantageous to an autocatalytic system primarily when 1) sites supporting autocatalysis are distantly spaced and 2) the environment undergoes periodic disturbances. We tested this hypothesis using computational reaction-diffusion models of surface-catalyzed autocatalytic systems in patchy, disturbance-prone environments. We considered two competing autocatalytic cycles, each composed of mutualistic subcycles, one of which diverts flux to the production of propagules. Our analysis shows that propagules tend to be detrimental in well-mixed environments or environments where viable surfaces are close enough together to be reliably seeded through single-species dispersal. However, propagule formation is advantageous when long stretches of empty space must be crossed to reach new resources and/or when environmental disturbances are frequent. These findings suggest spatial structure can selectively favor reliable forms of codispersal. Evolution in response to this mechanism could potentially explain the emergence of vesicles that transported many autocatalytic species and eventually became independent protocells.
Author summary
Cells on Earth today are too complex to have emerged spontaneously, meaning they must have developed through a gradual evolutionary process. Early evolution could have occurred through selection on self-amplifying sequences of chemical reactions called autocatalytic cycles. These cycles resemble processes in modern life, such as metabolic cycles and DNA replication. Understanding how these chemical cycles became bound into cells is vital for understanding the origin of life from prebiotic chemistry. We suggest that selection for codispersing between favorable sites by forming a propagule could have eventually led to the emergence of cells. We use computer models to test the idea that propagule formation is favored when the distance between resource-rich sites is long and the environment is periodically disturbed. By showing that realistic environmental settings drive the emergence of propagules, which could then serve as targets of selection for traits such as growth and division, our research helps explain the emergence of the first biological individuals.
