Answer:
Ancient single-celled organisms competed for food, energy and raw materials. Natural selection favored cells that could compete effectively.
Step-by-step explanation:
The theory of natural selection explains how organisms with the best characteristics are able to survive in an environment and pass on their characteristics to their organisms. Through the phenomenon of natural selection, evolution occurs and species tend to evolve into new species. The same phenomenon goes for the cells. Ancient prokaryotes competed for food, energy an other materials. Those prokaryotic cells which were better adapted were favoured by natural selection.