Origin of Eukaryotes
About 2 bya prokaryotic cells began evolving internal cell membranes. Then, some smaller prokaryotic organisms entered other prokaryotic organisms! Instead of infecting them, like a parasite, they started living inside the larger cell. Over time, they formed a special relationship whereby different prokaryotes developed different abilities. For example, one group evolved the ability to use oxygen to generate ATP. Other prokaryotes carried out photosynthesis. These eventually became the modern day mitochondria and chloroplasts.