Sex, bones and isotopes: Determining the sex of human remains with isotopes

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  • Published: Jul 9, 2012
  • Author: Jon Evans
  • Channels: Ion Chromatography
thumbnail image: Sex, bones and isotopes: Determining the sex of human remains with isotopes

Male and female

Sex, bones and isotopes: Determining the sex of human remains with isotopes

Archaeologists can tell quite a lot about a historical person from their bones, including where they lived and what they ate. This is because the ratio of the isotopes of certain elements, including carbon, nitrogen and oxygen, can differ in bones according to diet and geographic origin.

But it turns out that the isotope composition of bones can also reveal more intimate information about a historical person: what sex they were.

Up to now, determining the sex of ancient human remains has only been possible by studying the shape of the pelvic bones or by extracting and analyzing DNA from the remains. If the pelvic bone is missing or damaged or no DNA has been preserved, then the sex of the historical person has stayed a mystery.


Down in the cemetery

Inspired by a study that appeared in Science in 2002, a team of archaeologists and geochemists at the Ecole Normale Supérieure de Lyon in France has now come up with another way to determine sex, based on isotopes. The Science study was conducted by two Swiss scientists, who unexpectedly discovered that the ratio of a couple of stable iron isotopes differs in the blood of men and women. Later studies found that the ratio of a couple of copper isotopes differs as well. It turns out that men have a higher abundance of copper-65, compared with copper-63, but a lower abundance of iron-56, compared with iron-54, than women.

Because bones are suffused with blood throughout a person’s life, the French scientists wondered whether this isotope difference would be reflected in bones and, if so, whether it would persist in ancient bones. To find out, they analyzed skeletons excavated from the cemetery of the Saint-Laurent de Grenoble Church, located on the bank of the Isère river in Grenoble. These skeletons were of 31 people buried in the 17th and 18th centuries; as the skeletons were intact and well preserved, the sex of the 31 people could be accurately determined from the shape of their pelvic bones.

The scientists took samples from each of the skeletons, and then extracted the iron and copper with anion-exchange chromatography. Finally, they used inductively-coupled plasma mass spectrometry to determine the abundance of the two stable isotopes of iron and copper.

This revealed that, in general, the abundance of the isotopes did differ in the bones of men and women. As in the blood from living people, the bones from men contained a higher abundance of copper-65 and a lower abundance of iron-56 than the bones from women. What is more, this abundance didn’t seem to be affected by differences in diet, as revealed by analysis of carbon and nitrogen isotopes, or by any kind of environmental contamination over the years.


Wrong side of the line

However, general variations between people, as well as the effects of disease, meant that there was a bit of overlap between some of the male and female skeletons. When the scientists plotted the 31 skeletons on a graph of copper-65 and iron-56 abundances, they found that the male and female skeletons tended to bunch together in opposite corners, allowing them to be divided by a diagonal line. But a few of the male and female skeletons had unusual abundances of the copper and iron isotopes, causing them to be located on the wrong side of the line.

Because of this, the scientists found they could accurately determine the sex of 24 of the 31 skeletons from their isotope abundances. But with an accuracy of 77%, this is still a perfectly valid and useful method for determining sex when no other means are available. Indeed, it has a similar accuracy to studying the shape of the pelvic bone, and so the two methods could be particularly useful when used in tandem.

The scientists are also looking to improve the accuracy rate, by exploring this isotopic difference in more detail, both in living people and in historical human remains, in order to get a better idea about natural variation in copper and iron isotopes.

Related Links

American Journal of Physical Anthropology, 2012, 148, 334–340: "Fe and Cu stable isotopes in archeological human bones and their relationship to sex"

Article by Jon Evans

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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