You've heard us mention radiometric dating. Have you wondered what it is?

As in history, where the temporal context of an event is just as important as the event itself, the chronological position of an early human fossil is critical to the understanding of the species. Without proper understanding of the time period represented by an early human species, no evolutionary relationship can be interpreted. Since evolution is change over time, and time is a variable that must be controlled.

One of the tools at the disposal of researchers is the radiometric dating technique. There are many such techniques, but each relies on the principle borrowed from physics that radioactive elements will decay at a constant rate. So an object can be dated using the clock created by the continual "click" of the radioactive atoms.

One such technique is the now famous "Carbon-dating." Using the naturally occurring radioactive Carbon-14 isotope, scientists can date the age of organic matter, which is comprised of Carbon. Intake of Carbon-14 is presumed to balance the decay of Carbon-14 throughout the life of an organism. After death intake no longer continues, thus, after a plant or animal dies, Carbon-14 will decay at a steady rate. This is measured in "half lives," where one half life is the time it takes for half of the Carbon-14 to break down (into Nitrogen-14). The half life for Carbon-14 is about 5,700 years. This means it is a good age determiner for relatively young samples containing some organic matter: for example, wood or bone artifacts. However, if a site is older than about 60,000 years, there is not enough Carbon-14 left to detect. For older sites, such as the ones we often deal with in paleoanthropology, another technique is needed.

Another technique of particular use to paleoanthropologists is the ⁴⁰K/⁴⁰AR (Potassium-40/Argon-40) technique. This technique is specific to volcanic materials, dating the breakdown of the radioactive Potassium-40 isotope into the stable and inert gas Argon-40.

Here's how it works. A volcano explodes over a landscape four million years ago. During the explosion, Argon gas trapped within the lava is expelled. This leaves the lava Argon-free. As the lava cools, it hardens. Potassium, naturally occurring in the lava, is made-up of a mixture of radioactive Potassium-40 and non-radioactive Potassium-39. Over time the radioactive Potassium-40 breaks down into Argon-40 which is trapped in the structure of the hardened lava. From the amount of Argon found in the lava and the rate at which the Potassium-40 decays into Argon-40, scientists are able to determine the age of the lava.

The reason that this particular method is of interest to many paleoanthropologists is the long half life of Potassium-40 (about 1.3 billion years). In theory, Potassium-40 could be used to date an object as old as the Earth itself with its long half life.

Without radiometric dating techniques (and before they were developed) all dating can only be relative. One could say that a fossil at a deep layer of an archaeological site was older than a fossil found near the surface. One could even correlate layers between sites, producing regional chronologies that allowed for the comparison of fossils across large distances. But all such comparisons lacked the foundation of a precise, absolute date. Radiometric dating techniques allow us, under certain conditions, to affix a specific date in time to finds made in the field, giving us control over the time variable.

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