While the endosymbiont theory is the best present answer to the question of eukaryotic cell origins, there is still widespread uncertainty regarding the identity of the original cell that hosted the initial endosymbiosis.
Not everyone is enthusiastic about the endosymbiont theory. Young-earth creationist Don Batten, for example, writes that we should expect that there would be many similarities in many of the genes for photosynthesis or respiration between prokaryotes and eukaryotes--they have to achieve the same chemistry ...
A units-of-evolution framework is particularly useful for assessing the evolutionary implications of the endosymbiont theory
, because such a framework explicitly allows for the occurrence of synergistic or antagonistic effects between host and symbiont (e.g., Lewontin 1970; Buss 1987; Maynard Smith 1991).