As you move towards the surface, you find fish, then amphibians, then reptiles, mammals, birds, and finally modern mammals including humans. ![]() At the bottom of the timeline there are no fossils of modern animals. ![]() It's as if rock layers are a vertical timeline. Basically, scientists have learned that rocks are stacked in layers containing fossils with the oldest fossils at the deepest layers, and the youngest, or most recent fossils, near the top. One evidence is rock layers-specifically, what is called the geologic column. ![]() While everybody understands that black bears are related to grizzly bears and we can even figure they are related to extinct bears, lots of people wonder how scientists can be so sure that bears are related to salmon as well.
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