The endosymbiotic theory is the theory that says that a prokaryotic organism is encompassed by another prokaryotic organism. In this way, the origin of plastids and mitochondria is explained. It is said that a prokaryotic organism encompassed another and stayed inside, so that the mitochondria formed. This establishes an endosymbiotic relationship.
Think of incomplete dominance of a mixture. Lets say you have a red and a white rose. They would be combined to create the color pink. Same in this situation. A white and black cat would mix to create a grey cat.
Earth and its atmosphere are continuously altered. Plate tectonics shift the continents, raise mountains and move the ocean floor while processes not fully understood alter the climate. Such constant change has characterized Earth since its beginning some 4 billion years ago.
