The idea that 3 billion years ago a simple cell engulfed a free-living prokaryote cell, kept it alive and became a eukaryote cell with a mitochondria is difficult to ‘prove’. There are hypothesis that try to explain the origin of the first eukaryote cells. There are predictions and supporting evidence too. The theory of Endosymbiosis is a great illustration of the nature of science. This story of a biologist’s...