The answer to this can be found in Chapter 5. There, Diamond tells us that scientists typically use a method called radiocarbon dating to determine how old an early agricultural community is. All living things contain carbon. One part of carbon is carbon 14, which is radioactive. From the time that a living thing (plant or animal) dies, its carbon 14 content decays into carbon 12. It does this at a steady and predictable rate. Therefore, scientists can look at the ratio of carbon 14 to carbon 12 in the remains of plants and animals to determine how long ago they died.