Tierra

Tierra is a computer simulation developed by ecologist Thomas S. Ray in the early 1990s, creating a virtual environment where self-replicating computer programs compete for CPU time and memory space. Unlike traditional evolutionary simulations that encode fitness functions explicitly, Tierra's programs evolve their own strategies for survival — including parasitism, symbiosis, and hyperparasitism — through open-ended competitive coevolution.

The system begins with a single ancestral program that copies itself. Mutations introduce variation, and the limited resource environment (memory) imposes selection. Programs that replicate faster or exploit other programs gain advantage. Ray observed the emergence of ecological complexity: parasites that borrow replication code from hosts, immune responses, and even "digital organisms" that evolved smaller genomes to replicate faster — a direct analogy to the evolution of reduced genomes in intracellular parasites like mitochondria and viruses.

Tierra remains influential as a proof of concept for Artificial life and for the study of Evolutionary dynamics in open-ended systems. Critics note that Tierra's evolution quickly reaches a ceiling of complexity limited by its instruction set and that it does not exhibit the continuous innovation seen in biological evolution. Whether the ceiling is a property of the system or of our patience is unresolved.