Why do lower levels of sedimenatary rock on earth show fossils of the simpliests life forms?

Question

Why do lower levels of sedimenatary rock on earth show fossils of the simpliests life forms?

Answer 1

Sedimentary rocks are laid down horizontally, and the oldest ones are at the bottom, the youngest at the top - logical really.  So fossils in the oldest rocks tend to be the older, simpler forms.  However, if the rocks are tectonically deformed, folded and overturned, as is possible, the rule will not hold.  It is really only true for undisturbed rock layers.

Answer 2

Because those are the older layers with the beginnings of life that lived and died on them.

The lower the layer, the older the layer.