Carbon is essential to life. DNA is a pair of strings of carbon atoms with other atoms and molecule attached. The food we eat is substantially carbon too. Glucose is made up of molecules of C6H12O6 – we use it to make the energy needed to live and grow. We are constantly consuming carbon therefore and absorbing it into our bodies.Some of the carbon we consume is radioactive. The radioactive carbon is produced as a result of cosmic rays colliding with atoms in the atmosphere.
The carbon 14 gradually decays, with a half life of 5730 years, but while we are alive, the proportion of carbon 14 in our bodies is approximately constant.
When we die, we stop consuming carbon in food, but the carbon in our bodies continues to decay. We can use the proportion of carbon 14 remaining to estimate how long ago a person died, or how old a preserved biological specimen is. For us to be able to do this, the specimen must be not too old, or all the carbon 14 will have decayed, and not too recent, or too little will have decayed to produce a reliable estimate of the age of the specimen.