The hair closest to the root indicates where a person has been most recently. The longer the hair, the more recorded history the researchers have to work with. Hair grows one millimeter every three days, so if the hair is 20 centimeters long, that represents about 20 months’ worth of history, says Ehleringer. “I only have about six months of history,” he says of his own locks.
Joe Berry, a staff scientist in the department of geology at the Carnegie Institute of Science, in Washington, DC, says that Ehleringer’s lab is the world leader in studying isotopic compositions and that the work represents a real breakthrough because it provides a practical tool for law enforcement.
Researchers are currently using the technology to help detectives in the Salt Lake County Sheriff’s Office identify a murder victim found near Utah’s Great Salt Lake in 2000. For the past eight years, she has been a “Jane Doe,” says Todd Park, a detective working on the case. Now, using the new technique, detectives have analyzed strands of the victim’s hair and have been able to determine where she spent the last two years of her life. Park says that this is a huge help and that it narrows his search dramatically.
Ehleringer says that the map can’t define an exact location–only regions or an expansive narrow band, like the one that stretches from Oklahoma to Illinois. Fortunately, some regions, such as Northern Montana and Wyoming, are very small, with a distinct set of isotopic signatures.
This tool will be a “huge breakthrough for law enforcement” not just in helping identify murder victims, but also in tracking the movements of a serial killer, says Park. In 2003, Ehleringer and Cerling cofounded IsoForensics, a company that uses stable isotope analysis of forensic substances to find slight chemical variations.
Stephen Macko, a professor in the department of environmental sciences at the University of Virginia, who was not involved in the research, says that the work is very exciting. But he notes that the researchers still need to better define how foods from different areas impact the isotopic makeup of hair. “If you drink orange juice fresh from Florida or eat meat from a cow in Texas, you are getting the isotope signals of that state’s water,” says Macko. So how does a person get his local signature? In their study, Ehleringer and Cerling note that since things such as soup, coffee, coke, and cooking pasta use the local water, the isotope signals are incorporated into hair. The researchers are currently conducting subsequent studies to further explore such questions, but they’re unwilling to discuss any details.