When carbon-14 falls to Earth, it is absorbed by plants.
These plants are eaten by animals who, in turn, are eaten by even larger animals.
Before the advent of absolute dating methods in the twentieth century, nearly all dating was relative.
Narrow rings grow in cold or dry years, and wide rings grow in warm or wet years.
The rings form a distinctive pattern, which is the same for all members in a given species and geographical area.
Since certain species of animals existed on Earth at specific times in history, the fossils or remains of such animals embedded within those successive layers of rock also help scientists determine the age of the layers.
Similarly, pollen grains released by seed-bearing plants became fossilized in rock layers.
Dendrochronology: Also known as tree-ring dating, the science concerned with determining the age of trees by examining their growth rings.
Half-life: Measurement of the time it takes for one-half of a radioactive substance to decay.
By measuring the amount of carbon-14 remaining, scientists can pinpoint the exact date of the organism's death.
The range of conventional radiocarbon dating is 30,000 to 40,000 years.
With sensitive instrumentation, this range can be extended to 70,000 years.
In addition to the radiocarbon dating technique, scientists have developed other dating methods based on the transformation of one element into another.
Radioactive decay: The predictable manner in which a population of atoms of a radioactive element spontaneously disintegrate over time.