Abstract
The emergence of cellular organisms occurred sometime between the origin of life and the evolution of the last universal common ancestor and represents one of the major transitions in evolutionary history. Here we describe a series of artificial life simulations that reveal a close relationship between the evolution of cellularity, the evolution of metabolism, and the richness of the environment. When environments are rich in processing energy, a resource that the digital organisms require to both process their genomes and replicate, populations evolve toward a state of non-cellularity. But when processing energy is not readily available in the environment and organisms must produce their own processing energy from food puzzles, populations always evolve both a proficient metabolism and a high level of cellular impermeability. Even between these two environmental extremes, the population-averaged values of cellular impermeability and metabolic proficiency exhibit a very strong correlation with one another. Further investigations show that non-cellularity is selectively advantageous when environmental processing energy is abundant because it allows organisms to access the available energy, while cellularity is selectively advantageous when environmental processing energy is scarce because it affords organisms the genetic fidelity required to incrementally evolve efficient metabolisms. The selection pressures favoring either non-cellularity or cellularity can be reversed when the environment transitions from one of abundant processing energy to one of scarce processing energy. These results have important implications for when and why cellular organisms evolved following the origin of life.
Highlights
The cell is, by definition, the basic structural unit of all organisms
The results presented above contribute to our understanding of this important stage in evolutionary history by demonstrating that cellularity and metabolism coevolve in response to environmental change
We found that environments rich in processing energy selected for low cellular impermeability and low metabolic proficiency while environments poor in processing energy selected for high cellular impermeability and high metabolic proficiency and that between these two extremes, the population-averaged values of cellular impermeability and metabolic proficiency were very strongly correlated
Summary
The cell is, by definition, the basic structural unit of all organisms. Cellular organization defines the boundaries between organisms and their environment, allowing organisms to control their internal chemistry, to generate and store their own chemical potential energy, and to build and replicate complex systems with high fidelity. Cellularity confers individuality upon organisms by storing genetic material that can be passed on to future progeny through cell replication. This vertical inheritance, made possible by cellular organization, may have been a prerequisite for the first speciation events in early evolutionary history (Woese 1998). Supporting this notion, strong evidence suggests that the last universal common ancestor of life (Becerra et al 2007; Goldman et al 2013) represents a cellular organism or a population of cellular organisms. The genome of the last universal common ancestor, for example, likely
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