Research about new hypothesis that has replaced an older explanation of something in the natural world

For more than a billion years, organisms were about to be the same as today's single-celled bacteria and microorganisms. But then, a new form of life emerged, the entire diversity of multi-cellular life we know today descended from the unicellular organisms. The evolution of the unicellular organisms into multi-cellular or the eukaryotes we can say was said to be done because one cell got inside another cell. Mitochondria were found in multi-cellular organisms being working as an energy supplying factory for the cell, unlike the unicellular organisms. The multi-cellular organisms were found more complex and modified than the unicellular organisms.