The eyes have it

Posted 31 January 2008

Researchers have discovered that the lens of the eye can reveal the date of birth of a human being. This can be determined by measuring the level of the carbon isotope C-14 in the lens of the eye. The technique provides a useful tool for forensic scientists with which to date an unidentified body and may also have further implications for health science research.

In the first 2 years of life, cells in the lens of the eye build tiny transparent proteins, which allow light to pass through the eye so that we can see. These special proteins, known as lens crystallines, remain essentially unchanged for the rest of our lives and are the only part of the human body apart from teeth which do so. This immutable quality of the lens crystallines is a fact that scientists can now put to good use. By measuring the amount of C-14 trapped in the eye lens, scientists at the Universities of Copenhagen and Aarhus can now establish, with relatively high precision, when a person was born. Carbon-14 enters into the body, when it is absorbed from the atmosphere into the soil and food chain (the Carbon Cycle) and is thereby ingested in the food stuffs that we eat. As the lens crystallines in the eye remain unchanged for most of our lives, they register the amount of C-14 present in the atmosphere at the time of their formation.

In the 1950s, when the superpowers detonated nuclear bombs during the cold war, there was a dramatic increase in the quantity of C-14 in the atmosphere. Since then the amount of additional atmospheric C-14 has decreased rapidly, mainly due to absorption by the oceans. This change in C-14 levels is known as ‘the bomb pulse’ and has been monitored year by year. It is this, however, that gives validity to the method of dating the lens crystallines, since the rapid changes in the atmosphere have provided scientists with a clear record of annual C-14 atmospheric content. By comparing the yearly record of the content of the C-14 in the atmosphere with the content of C-14 in the lens crystallines of the eye, scientists can accurately date a person’s year of birth – providing they are born after 1950.

The method derives from the traditional archeological method of radiocarbondating ancient living matter. But whereas traditional carbondating is useful for establishing age of death, this method enables scientists to establish the age of birth of a living or recently deceased person. Archeological radiocarbondating relies on the fact that for many thousands of years, the concentration of C-14 in the atmosphere remained fairly constant. This has allowed scientists to compare the amount of C-14 left remaining in dead matter to the (previous) fairly even concentration of C-14 in the air, and, by knowing the radioactive decay rate of C-14, work out the time at which that organic matter ceased to be living matter. This current method of dating the eye lens, by contrast, relies on varying changes in the content of C-14 in the atmosphere and enables scientists to identify the corresponding year in which C-14 atmospheric content is as high as C-14 in the eye.

The technique uses a large nuclear particle accelerator to determine the amount of C-14 in as little as one milligram of lens tissue and will be valid for a minimum of a hundred years, until the increased Carbon-14 in the atmosphere finally returns to normal levels. The method offers scientists a more precise means of dating bodies than checking the C-14 content in teeth, since teeth take approximately 6-8 years to develop.

Associate Professor Niels Lynnerup from the Department of Forensic Sciences developed the forensic method together with the Department of Eye Pathology and the Department of Physics and Astronomy at the University of Aarhus, Denmark. Aside from its use by forensic scientists in e.g. disaster situations where identification is difficult, Professor Lynnerup explains that this method also has several other applications:

“As has been pointed out by researchers working in allied fields, we think that carbon dating of proteins and other molecules in the body could be used to study when certain tissues are generated or regenerated. This could, for example, be applied to cancer tissue and cancer cells. Calculating the amount of C-14 in these tissues could tell us when the cancerous tissue is formed and this could further our understanding of such diseases”.