Phagocytosis, a type of endocytosis, was used by a prokaryote to engulf a second prokaryote.
This engulfing process is significant because it is believed to have played a crucial role in the endosymbiotic theory, which suggests that some organelles in eukaryotic cells, such as mitochondria and chloroplasts, originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells. This interaction led to a symbiotic relationship, allowing for the evolution of more complex eukaryotic cells.