Organisms take up the two isotopes of carbon in about the same ratio that the isotopes are found in the atmosphere. However, carbon-12 does not decay. Scientists use other radioactive isotopes, such as potassium-40, to date older fossils. Uranium-238 is used to determine the age of cores of silt taken from seabeds. By dating fossils and examining geologic strata, scientists have been able to put together a time scale for the history of life on earth.
Fossil evidence indicates that over time organisms of increasing complexity appeared on the earth.
Bacteria and blue-green bacteria are the first fossils that were preserved from the Precambrian era. During the beginning of the Paleozoic era, complex multicellular invertebrates dominated life in the oceans. By the end of the Paleozoic era, plants and animals has colonized the land surface of the earth. The fossil record contains many examples that could be interpreted to mean that species evolved from more ancient organisms.