For example, based on the primate fossil record, scientists know that living primates evolved from fossil primates and that this evolutionary history took tens of millions of years.
The principle of superposition states that in an undeformed sequence of sedimentary rocks, each layer of rock is older than the one above it and younger than the one below it (Figures 1 and 2).
Accordingly, the oldest rocks in a sequence are at the bottom and the youngest rocks are at the top.
However, the age of each fossil primate needs to be determined so that fossils of the same age found in different parts of the world and fossils of different ages can be compared.
There are three general approaches that allow scientists to date geological materials and answer the question: "How old is this fossil?
Layers that cut across other layers are younger than the layers they cut through (principle of cross-cutting relationships).
The principle of superposition builds on the principle of original horizontality.The layers of rock at the base of the canyon were deposited first, and are thus older than the layers of rock exposed at the top (principle of superposition).In the Grand Canyon, the layers of strata are nearly horizontal.This is the principle of original horizontality: layers of strata are deposited horizontally or nearly horizontally (Figure 2).Thus, any deformations of strata (Figures 2 and 3) must have occurred after the rock was deposited.Most sediment is either laid down horizontally in bodies of water like the oceans, or on land on the margins of streams and rivers.