There are two kinds of dating used by geologists, paleontologists, and paleoanthropologists: relative dating and absolute dating.
Relative Dating is Used to Determine the Order of Past Events
Relative dating consists simply of knowing which fossils are older or younger. It can be easy to determine this based on which geological deposit they come from and the Law of Superposition. The Law of Superposition is simple and states that the older layer lies underneath the younger layer in undisturbed contexts. Thus, fossils from deeper layers are older than fossils from layers closer to the surface of the earth.
Absolute Dating Gives an Actual Age
Absolute dating (or chronometric dating) is based on solar years and gives an actual age reported as “years before present.” There are many ways of finding out the absolute date. Radiocarbon analysis, or carbon 14 analysis, measures the amount of carbon 14 in charcoal, wood, shell, or bone. Carbon 14 decays at a known rate when an organism dies, so the amount of carbon 14 left in the organic material can tell you how long it has been since it died. Radiocarbon dating is only really accurate for materials up to 50 thousand years old.
Other absolute dating methods are used for materials more than 50 thousand years old:
- Potassium-argon analysis dates rocks that have been heated, either by a volcanic eruption, natural fire, or human hearths. It can be used to date rocks from the beginning of the earth 4.6 billion years ago to rocks that were burned about 100 thousand years ago.
- Amino acid racemisation dating is used on ostrich eggshells and is good at measuring ages between 40 and 180 thousand years ago.
- Electron spin resonance measures the number of trapped electrons in bone and shell, which informs scientists of the last exposure of the material to sunlight before being buried.
Researchers Use All the Methods Available to Determine the Age of a Fossil
Which methods are chosen depends on the purported relative age of the specimen in question, and the types of materials available for dating. When the actual fossils can’t be dated, palaeontologists and palaeoanthropologists use the dates of materials from the same geological layer to determine the age of the fossil.
Whenever possible, multiple dating techniques are used to confirm the age of a specimen. Most dates accepted by scientists are supported by multiple lines of evidence, usually for many specimens of the same species from many sites and using many different types of analysis. When the dating results are redundant, scientists can then be confident about the absolute date of a fossil.
Andrew McRae's Talks Origins page has some more information about relative and absolute dating.
If you are interested in what fossils can tell us about evolution, read Fossils are the Hard Evidence for Evolution.
Sources:
Haviland, W. A. and Crawford, G. W. 2009. Human Evolution and Prehistory. Nelson Education Ltd.
