Five Unsolved Space Mysteries

5. How did life begin and evolve on Earth?

Under­standing the processes that lead to life, however, is complicated by the actions of biology itself. Earth’s atmosphere today bears little resemblance to the atmosphere of the early Earth, in which life developed. Many ideas have been proposed to explain how life began. Most are based on the assumption that cells are too complex to have formed all at once, so life must have started with just one component that survived and somehow created the others around it.