Many times we will hear references to dated fossils; we know generally when the dinosaurs lived, when they went extinct, around when plants and vertebrates became terrestrial, and even when the Earth formed. But how do we know all these dates?
The most common type of dating is known as radiocarbon dating and it is used to determine the age of fossils that used to be animals, plants, or bacteria. It uses isotopes of carbon; an isotope is an atom of carbon that has a different weight (due to, typically, different numbers of neutrons). Some of these isotopes are radioactive, meaning they decay into electrons, neutrons, and/or another atom. The way scientists refer to them is: the parent isotope decays into the daughter isotope. Additionally, we know exactly how long it takes them to decay. We can determine how long it takes for half of the parents to decay into the daughters.
Radiocarbon dating uses an isotope of carbon called Carbon-14. It is radioactive and will decay, and its half life is about 5,700 years. So, scientists can measure the how much of the daughter isotope exists and then determine how many half-lives the carbon has gone through and therefore we can know how old the fossil is.
But, how do we know how much of the parent isotope we started with? To scientists, this is known as the “closure time,” it is the point at which the isotopes can no longer escape from the sample, for example when molten or semi-molten rock cools. Before that decay happens, the leftover “daughter” isotopes escape from the sample and then we know that when there are equal concentrations of parent and daughter isotopes, one half-life has been completed and therefore that fossil is around 5,700 years old. The concentrations of daughter isotope combined with the concentrations of the parent isotope should be equal to the initial amount of parent isotope.
Of course, radiocarbon dating can only be really used for fossils that are around 30,000 years old; after that, it does not tell us much. Other radioactive isotopes can be used to date older fossils; we can use potassium-argon dating, uranium-lead dating, etc. and these can be used to date fossils (such as rock or sediment) that have certain elements that fossils of organisms do not have. There are a few other strategies to date fossils other than using radioactive isotopes as well; to get relative timing of fossils, we can just look at the strata (or layers) in the rock. Those fossils that are deeper in the rock are older than the fossils that are higher up to the ground. However, unlike radiocarbon dating, that only gives us relative timing instead of absolute timing.