(WHTM) — On this day in 1940 two scientists would isolate an atom that would revolutionize our studies of life on Earth.

Martin Kamen and Samuel Ruben were chemists at the University of California Radiation Laboratory in Berkeley, California. They were looking for an isotope of carbon, using a device called a cyclotron accelerator, which accelerates high-energy particles and smashes them into targets of stable materials.

With this device they were able to synthesize Carbon-14, an isotope of Carbon, which would lead to a revolution in-

Time for a digression. What, exactly, is an isotope?

Elements are made of atoms, and atoms are made of protons, electrons, and neutrons. (For simplicity’s sake, we will leave the messy matter of sub-atomic particles out of this.) The number of protons and electons determine what element an atom is. In the case of Carbon, there are six protons in the nucleus, and six electrons moving around the nucleus.

While elements always have an equal number of protons and electrons, the number of neutrons (which reside in the nucleus with the protons) can vary. These variants are isotopes. The standard, most common version of Carbon is Carbon-12, with six protons and six neutrons. Then there’s Carbon-13, with six protons and seven neutrons. Both of these are “stabile”-they do not break down into other elements.

Carbon-14 is different. With six protons and eight neutrons, it tends to come apart. Every once in a while, a Carbon-14 atom will cough up an electron and electron anti-neutrino (a messy sub-atomic particle) and morph into a nitrogen atom. This is known as “beta decay”, and proceeds at a slow and steady rate.

“Slow and steady” is the important part.

Back to the revolution. Kamen and Ruben discovered the half-life of Carbon-14 (the period of time during which half of the atoms would decay into nitrogen) was much longer than scientists thought-on the order of 5,568 years. (Later study refined that to 5,730 ± 40 years).

At the University of Chicago, professor of chemistry Willard Libby realized that Carbon-14 might be used to determine the age of once-living matter. Carbon-14 is produced naturally in the upper atmosphere when Nitrogen-14 atoms are bombarded by cosmic rays. Living organisms absorb both Carbon-12 and Carbon-14.

Libby realized all living things-including us-were being “tagged” by this radioactive isotope. (Not too radioactive, fortunately.) When an organism cashes it in, it stops collecting Carbon, but the Carbon-14 will keep decaying. By measuring the amount of Carbon-14 in an object, you could use the half-life to deduce the object’s age. Libby published a paper on the topic in 1946.

Libby and his associates found an interesting way to test the theory-sewer gas. They collected methane from sewage works in Baltimore, and compared it to methane from petroleum. The brand spanking-new methane contained Carbon-14; the multi-million year old petroleum methane, having had more than enough time for its Carbon-14 to decay, contained none.

Since then radiocarbon dating, or carbon-14 tests, have become a go-to for figuring out the ages of once-living organisms. Libby won the Nobel Prize in Chemistry in 1960.

The effect of Carbon-14 dating on the field of archaeology has been profound, and sometimes spectacular. Not only can archaeologists date items at a site with unparalleled accuracy, they can use the results to connect sites together, forging an increasingly dynamic map of human activity. Coupled with (and cross-checked by) other dating methods such as measuring tree rings, Carbon-14 can be used to date items as far back as 50,000 years.

The use of Carbon-14 has been refined over the years, and will continue to be refined as new discoveries are made. Early on, scientists (including Willard Libby) realized a baseline assumption, namely that the ratio of Carbon-14 to Carbon-12 was essentially the same throughout the atmosphere, was wrong. A number of “calibration curves” for Carbon 14 dating have been introduced over the years. IntCal, an international organization that updates and maintains the radiocarbon calibration curves, published its first consolidated list in 1998, with updates in 2004, 2009, 2013, and 2020. So, as the years have gone by, radiocarbon dating has become more and more accurate, as scientists have figured out where, when, and how something might be throwing off their measurements.