During 4.37-4.20 billion years ago, abundant icy planetesimals bombarded the primitive dry Earth, which caused it to be enveloped by an atmosphere and an ocean. After the appearance of the ocean, plate tectonics started, but the ocean was too saline, acidic, and enriched in heavy metals. Life could not emerge in such a toxic ocean. 4.37-4.20 billion years ago : The formation of the atmosphere and ocean. 4.37-4.20 billion years ago : The initiation plate tectonics.