THE THREE LAYER CAKE
There are thousands upon thousands of layers in the earth’s crust. However, scientists have grouped the layers into major groups. The most recent three layers are the Paleozoic, Mesozoic, and Cenozoic. These layers represent the last 500 million years of life on earth.
In the Paleozoic, you find fish, amphibian, and reptile fossils (in that order), but never dinosaurs, birds, modern mammals, or even flowering plants.
Think of that: despite the billions of plant fossils in the Paleozoic layer, nobody has ever found one fossil of a flower, including any kind of deciduous tree or even a single blade of grass. Why not? The obvious explaination is flowers had not evolved yet.
The next layer, the Mesozoic, is often called the age of dinosaurs. The Mesozoic has dinosaurs like crazy. Of course, dinosaurs are reptiles and that’s why you won’t find any until after the Paleozoic which contains the first reptiles. The Mesozoic also has the first flowering plants, birds, and mammals, though few if any birds or mammals that we know of today.
On top of the Paleozoic and Mesozoic is the Cenozoic. This is the current layer that is still being deposited in oceans, deserts and swamps all around the earth today. The Cenozoic is the first major layer where we find modern mammal fossils like cats, dogs, monkeys and humans. This layer, or “era” is often referred to as the age of mammals.
These three layers make up a sort of 3-layer cake. Just like a cake, the bottom layer went down first, followed by the middle and the top. Since fossils progress from fish at the bottom to humans at the top, we have clear evidence that life evolved through time.