Archaeologists use the exponential, radioactive decay of carbon 14 to estimate the death dates of organic material.

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When an organism dies it ceases to replenish carbon in its tissues and the decay of carbon 14 to nitrogen 14 changes the ratio of carbon 12 to carbon 14.

Experts can compare the ratio of carbon 12 to carbon 14 in dead material to the ratio when the organism was alive to estimate the date of its death.

If a fossil is found between two layers of rock whose ages are known, the fossil's age is thought to be between those two known ages.

Because rock sequences are not continuous, but may be broken up by faults or periods of erosion, it is difficult to match up rock beds that are not directly adjacent.

However, these "molecular clocks" are sometimes inaccurate and provide only approximate timing.

For example, they are not sufficiently precise and reliable for estimating when the groups that feature in the Cambrian explosion first evolved, and estimates produced by different approaches to this method may vary as well.The principle of radiocarbon dating is simple: the rates at which various radioactive elements decay are known, and the ratio of the radioactive element to its decay products shows how long the radioactive element has existed in the rock.This rate is represented by the half-life, which is the time it takes for half of a sample to decay.After 5,730 years, the amount of carbon 14 left in the body is half of the original amount.If the amount of carbon 14 is halved every 5,730 years, it will not take very long to reach an amount that is too small to analyze.In the case of radiocarbon dating, the half-life of carbon 14 is 5,730 years.