Scientists use the Neo Darwinian theory of evolution. to determine the sequence of the fossils and the uniform theory of geology to determine the age.
The scientists use the Neo Darwinian theory to determine which fossils are older. The simpler the fossil structure the older the fossils is assumed to be.
Charles Lyell's theory of uniform geology assumes long ages to form the geological layers found in the earth. This theory of slow uniform process is used to determine the age of the fossils.