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.

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.