ossils found in soil layers of different ages show a record of slow, gradual changes in species, with simple organisms gradually being replaced by more complex organisms, apparently by evolutionary processes driven by natural selection. For example, 1000 million years (abbreviate My) ago, the oceans held only simple organisms like algae, while the land was relatively lifeless. Fish fossils appear in strata after about 500-600 My ago; dinosaurs and giant reptiles were on the land by 200 My ago. Mammals were not common until after 65 My ago, and humanlike creatures appeared only in the last 4 My.