Another shared protein, actin, is used in phagocytosis by eukaryotes. Phagocytosis is the ability to engulf and consume another particle. This ability is required for the endosymbiotic origin of mitochondria. Cells with mitochondria is one of the key differences between prokaryotes and eukaryotes. In a widely accepted theory, mitochondria began as bacteria and were gradually incorporated into eukaryotic cells. Chloroplasts had a similar origin.