Early protists were most likely formed by endosymbiosis. Endosymbiosis is the term for when one organism lives within another organism and they live symbiotically. Early protists likely formed from this manner because eukaryotic organelles like mitochondria are thought to have formed this way - a bacteria may live internally within another cell, and this bacteria later became the mitochondria. It is not photosynthesis because bacteria can also do photosynthesis. It is not conjugation because that is the way bacteria exchange DNA. It is not binary fission because that is a form of cellular division, which bacteria can undergo.