Dating Techniques

The study of human evolution was revolutionized by the development of absolute dating methods in the 1950s and 1960s. Absolute dating methods are methods that return a specific date, in years, for the formation of a fossil or sediment layer. Often this date is given as a range of years, for example, 3.5 mya +/- 35ky (that would be read as 3.5 million years ago, plus or minus 35 thousand years). To understand human history, we need to understand the fossil record. But the fossil record cannot be understood without knowing the temporal context. In other words, if you don't know when a critter died, you don't know much about how it relates to our history as a species.

Reading Dates

Anthropologists use the following terms when reporting dates:

Radiometric Dating

Most (but not all) absolute dates are radiometric dates. Radiometric dating techniques are based on the principle of radioactive decay. Essentially, elements, such as carbon (C) or potassium (K), have more than one isotopic form, or variation based on atomic weight. For example, carbon has three isotopes: 12C (or Carbon-12), 13C (Carbon-13), and 14C (Carbon-14). The number refers to the atomic weight of the isotope, which is the combined number of protons and neutrons. The number of protons an atom has determines which element it is, while the variation in the number of neutrons make it a particular isotope of that element.

Some isotopes are unstable; they decay into stable isotopes by emitting some of their subatomic particles. These unstable isotopes are called radioactive isotopes. Radioactive decay is what we call it when these radioactive isotopes emit their subatomic particles as they change into more stable isotopes. Although the rate of radioactive decay depends on which element or isotope you're talking about, each isotope has a fixed, known rate at which it decays into a more stable form. This is its half-life, the amount of time it takes for half of the atoms of that isotope to decay into a different isotopic form.

To greatly simplify, radiometric dates work by counting the number of atoms that have decayed from a radioactive to a more stable isotope and then figuring out how many half-lives have passed. For example, 14C (carbon-14) has a half-life of 5,730 years. If half of the 14C in a sample has converted to 14N (nitrogen-14, the stable isotope to which 14C decays), then about 5,730 years have passed. If 3/4 of the 14C in a sample has converted to 14N, then about 11,460 years have passed. If that number seems off, remember that the half-life is how long it takes half of the isotopes to decay, regardless of how large a sample you begin with. In other words, it takes 5,730 years for half of the original 14C to decay. It takes another 5,730, on average, for half of the remaining half to decay. Therefore, it takes twice as long for 3/4 of the original sample of 14C to have decayed as it did for 1/2 of it to decay.

In this class, we will not go into more detail about how radiometric dating works, since that's covered in other Anthropology classes. What's important for understanding the outline of human evolution is recognizing that there are multiple radiometric dating techniques. These date different materials and different types of events. They also have different ranges over which they are effective (that is, they can't be used for materials that are older or younger than their effective range). To critically evaluate the meaning of a early human fossil or tool, you must understand how the dates were determined and the limits on those dates. More importantly, you need to know the relationship between the item being dated and the fossil or tool. Most of the time, fossils and tools can't be directly dated. We determine their age based on their context. For example, a fossil may be found just below a layer of volcanic ash that can be dated to 2.3mya based on K/Ar dating. That would mean the fossil was older than 2.3mya. Similarly, a fossil might be found with pieces of burned wood that date to 25kya. That suggests the fossil is about the same age.

Below is a chart that lays out the most important radiometric dating techniques used in the study of human evolution.