Answer
Currently, prokaryotic organisms have been found to live symbiotically inside eukaryotic cells. The existence of this type of relationship serves as evidence to support the endosymbiotic theory. If it is possible for these symbiotic relationships to happen today, then they could have been possible with high probabilities in the past.
Work Step by Step
For example, the amoeba Pelomyxa palustris lacks mitochondria but has at least two bacteria inside it that perform functions similar to those of this organelle, such as the complete oxidation of glucose.
Many unicellular eukaryotes lack mitochondria, but have endosymbiotic bacteria that can cover the functions of this organelle.
Other eukaryotes use spirochetes to mobilize, instead of flagella.
