A computer model is described that explores some of the possible behav
ior of biological life during the early stages of evolution. The simul
ation starts with a primordial soup composed of randomly generated seq
uences of computer operations selected from a basis set of 16 opcodes.
With a probability of about 10(-4), these sequences spontaneously gen
erate large and inefficient self-replicating ''organisms''. Driven by
mutations, these protobiotic ancestors more efficiently generate offsp
ring by initially eliminating unnecessary code. Later they increase th
eir complexity by adding additional subroutines as they compete for th
e system's two limited resources, computer memory and CPU time. The en
suing biology includes replicating hosts, parasites and colonies.