Radioactive carbon-14 trapped in the lens of the eye permits researchers to accurately date the year of a person's birth, Danish scientists report.
The lens contains proteins, called lens crystallines, that are transparent, allowing light to pass through to the retina. These proteins are produced during the first year of life and are unchanged afterward, providing a unique record of the time of birth.
The only other bodily proteins that remain unchanged throughout life are those in the enamel of teeth, but they are formed over a five- to six-year period and are thus less useful in dating.
The key to the dating process is the incorporation into the crystallines of carbon-14, which is found in all the food a person consumes. The Earth's atmosphere normally has a fairly constant level of carbon-14: it decays into nitrogen-14 with a half-life of 5,730 years, but is constantly replenished by the interaction of cosmic rays with nitrogen-14.
Because it is taken up by plants, all living organisms have a concentration of carbon-14 equal to that in the atmosphere. When the organism dies, the isotope decays slowly until none of it is left -- the phenomenon that is used for radiocarbon dating of archaeological artifacts.
During the period of atmospheric testing of nuclear bombs between 1945 and 1960, the atmosphere received a sharp spike of carbon-14, commonly called the "bomb pulse," that has gradually been declining to normal levels as excess carbon-14 has been absorbed into the ocean.
The yearly amount of carbon-14 has been carefully monitored and is well known.
Forensic medicine specialist Niels Lynnerup of the University of Copenhagen and physicist Henrik Kjaeldsen of Aarhus University reported Tuesday in the online journal PLoS One that measuring the amount of carbon-14 in the lenses of 13 corpses using a large nuclear particle accelerator provided the year of birth.
The primary restrictions on the technology are that the person has to have been born after 1950, the lens must be removed within three days after death before it decays too much, and the individual cannot have subsisted primarily on seafood, which raises the readings.