Hypothesize how the evolutionary history of cells might have differed had endosymbiosis not occurred?

Most eukaryotic cells today would less likely have no mitochondria and other organelles because endosymbiosis did not occur. Endosymbiosis theory posits that organelle such as the mitochondria, which originally was a bacterium, was engulfed by the cell and instead of being digested turned to generate ATP and other important component for cellular processes. This advantage prolifically created the cell to become more efficient and high-powered organism which enabled for other processes such as communication and organization with other similar cells.  

stimulus is a change in the environment that causes an organism to react. the response is the reaction to the stimulus. some examples of stimuli for zebras are drought, to many predators, and bad feeding grounds such as dead grass or plants. zebras respond to stimuli just like all other living organisms