There are a number of worlds around the galaxy where the evolution of life has been effectively halted by a single species (and sometimes a single organism) that came to dominate the biosphere.  These are known to planetary scientists as monoculture worlds.  In the whole galaxy, not a single monoculture world harbors intelligent life, except where explorers have come from another planet.

Usually, monocultures are dominated by early-evolving plant-like organisms that grow so aggressively as to block out the sunlight that its competitors would need, while producing potent poisons that kill off nascent herbivores.  These dominant organisms often do have other, simpler symbiotic partners that exist only to the extent that they serve the dominant species. 

On a few monoculture worlds, the dominant species has evolved past the need to eliminate competition, and the dominant species prove edible.  The aggressive growth cycle of these species can provide an abundant food source for nearby inhabited worlds and colonies.  This aggressive growth also presents a hazard, for when these dominant organisms are transplanted, they often become invasive pests.