The rate of decay is assumed to be relatively constant over time.

Once the decay rate is known, the time required to get from original ratio to present ratio is easy to calculate.

The idea behind radiocarbon dating is straightforward, but years of work were required to develop the technique to the point where accurate dates could be obtained.

Research has been ongoing since the 1960s to determine what the proportion of in the atmosphere has been over the past fifty thousand years.

When they die no new carbon-14 is taken in by the dead organism.

The carbon-14 it contained at the time of death decays over a long period of time.

By measuring the amount of carbon-14 left in dead organic material the approximate time since it died can be worked out.

For example, in 1991, two hikers discovered a mummified man, preserved for centuries in the ice on an alpine mountain.

Later called Ötzi the Iceman, small samples from his body were carbon dated by scientists.