By comparing fossils of different primate species, scientists can examine how features changed and how primates evolved through time.
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.
Each time a new layer of sediment is deposited it is laid down horizontally on top of an older layer.
This is the principle of original horizontality: layers of strata are deposited horizontally or nearly horizontally (Figure 2).
However, by itself a fossil has little meaning unless it is placed within some context.
The age of the fossil must be determined so it can be compared to other fossil species from the same time period.
These eras are separated by catastrophic extinction boundaries, the P-T boundary between the Paleozoic and the Mesozoic and the K-Pg boundary between the Mesozoic and the Cenozoic.
There is evidence that catastrophic meteorite impacts played a role in demarcating the differences between the eras.More recently, however, those eons have been subdivided into eras of their own.Despite seeming like a relatively stable place, the Earth's surface has changed dramatically over the past 4.6 billion years.For example, in the rocks exposed in the walls of the Grand Canyon (Figure 1) there are many horizontal layers, which are called strata.The study of strata is called stratigraphy, and using a few basic principles, it is possible to work out the relative ages of rocks.There are three general approaches that allow scientists to date geological materials and answer the question: "How old is this fossil?