Teeth as a forensic clock

Here’s something we’re likely to see that endearing techno whiz kid, Abby Sciuto, whip out of her forensic arsenal next season on NCIS. They’re chemical and nuclear technologies to date teeth. And when paired up, new research indicates, they’ll identify not only when people were born but also the age at which they clocked out — thereby pointing to the general date of death.

It’s a bit gruesome to contemplate why coroners and others need these data. We’d all like to hope that when people die, it’s going to be among family or friends who can vouch for the deceased’s identity. But bad things happen to lots of people — sometimes in groups. And identifying them may hinge on knowing their age and how long ago they succumbed — both of which can prove especially challenging when the tissues are decomposing or when all that remains are partial skeletons.

The older of the technologies is known as aspartic acid racemization. A mouthful. The amino acid aspartic acid is a building block of proteins throughout the body. It comes in mirror-image forms — what are conversationally known as left- and right-handed versions. They tend to start out present in roughly a racemic — or 50:50 — mix. Throughout life, all left-handed aspartic acid in the body tends to slowly convert to the right-handed conformation.

This racemization — slow conversion of lefties — is a slow process, Bruce A. Buchholz of Lawrence Livermore National Laboratory and his coauthors note in the May Molecular & Cellular Proteomics. At 25 °C, it would take about 100,000 years for all left-handed aspartic acid molecules in the body to become righties.

But what has made this molecular clock so useful for forensic anthropologists over the past quarter-century is the fact that it stops dead when someone dies. And by focusing on the enamel of teeth, which is laid down over a short period as each adult tooth forms, chemists know when this outside shell of a tooth developed, which would be during the time that tooth erupted — a fairly predictable age.

By analyzing how much racemization of its aspartic acid occurred, scientists can determine how old the tooth’s owner was at death — generally accurate to about 5.5 years, plus or minus 4.2 years.

What it doesn’t tell you is how long ago that death occurred. But for people born since the mid-1940s, there is a second technique that can deliver a fairly precise age (within about one year) of when a tooth’s enamel was laid down. It looks at the ratio of radioactive carbon-14 in that enamel to stable C-12. This technique’s use on teeth was first described by Buchholz and his colleagues in a 2005 Nature paper.

With six protons and six neutrons, carbon normally has an atomic number of 12. But sometimes a cosmic ray will collide with a nitrogen atom, giving it an extra neutron. It quickly becomes carbon-14 (with six protons and 8 neutrons). This radioactive element has a half-life of some 5,700 years. Over time, that carbon-14 will decay to regular carbon-12.

Through most of Earth’s history, the ratio of C-14 to C-12 was fairly constant — at least until the nuclear-weapons era started. Bomb blasts created a surfeit of C-14 that quickly dispersed around the globe. And the enamel of teeth that erupted since the period of those blasts, basically the mid-1950s — has incorporated an elevated ratio of C-14 to -12 in all of its tissues, including tooth enamel.

But the ratio has varied. Over time, some of the excess C-14 has become buried or incorporated in biota around the globe. And by knowing the rate of its relative disappearance, for want of a better term, physicists can date how long since the mid-50s a tooth formed, based on the ratio of the two carbon isotopes within its enamel.

Again, by knowing the age at which a particular type of tooth erupts —  front teeth earlier, molars later — scientists can calculate back from when the tooth formed to determine the year in which a tooth’s owner was born.

Until his group’s new report, scientists hadn’t compared racemization and C-14 analyses on the same teeth, Buchholz says. So they collected teeth that had been extracted by dentists from 40 individuals, people whose age was known (between 13 to 70), and compared the technologies’ relative accuracy in dating choppers.

Overall, C-14 analyses gave superior age-at-birth dates, but only for people whose teeth erupted after the bomb blasts, meaning individuals about 60 and younger. However, when the researchers applied both techniques to teeth, they realized that the racemization offered an additional useful detail, a good gauge of an individual’s age at death.

And they applied it to teeth from a homicide victim in Sweden (where one of the scientists worked). By pairing information from both techniques, they could determine that the victim was born in 1942 and lived for an apparent 46.8 years. That put the victim’s death late in 1988 (plus or minus 2.1 years). Although police have not identified the man, Buchholz’s team reports that owing to the dates they came up with, police think they know who this person might be: “a foreigner believed to be in his forties who was suspected for having set fire to a restaurant in 1988 but then disappeared.”

For people born in the last 50 years or so, the C-14 test can by itself sometimes identify dates of both birth and death, Buchholz notes. Indeed, the Royal Newfoundland Constabulary recently employed the technique to help home in on the age of another homicide victim.

Hikers happened onto a lone skull in a wooded area in far eastern Canada on May 17, 2001. For several years, the police worked to identify the victim using a range of techniques, including DNA analysis, facial reconstruction, dental analyses and more. But they had a hard time narrowing their search because they didn’t know when the man had died.

Recently, RNC Inspector John House was looking for other forensic techniques that might be employed when he ran across a paper by Buchholz’s team on the C-14 analysis. He recruited the scientists’ assistance in analyzing some of the skull’s teeth — and hair.

Because the police had some of the man’s wavy black locks, with roots intact, the scientists could subject them to C-14 dating as well. Explains Buchholz: Because hair grows at about a centimeter per month, “the hair root and about an inch of growth gives a good idea of carbon intake over the last couple of months.” And that allows a fairly accurate date of death. In this case, June 1995, plus or minus 1.7 years. Based on the dental enamel’s C-14 ratio, they calculated that the victim had been born between 1955 and 1961.

Alas, the physicist notes, many skulls don’t come with hair. And in these instance, racemization can really come in handy.

By the time the numbers came in for the Newfoundland victim, House says, “The case was very cold” — as in frigid. Now, he says, “it’s become an active investigation again.” And explains why, he says, C-14 analysis “is something I’d definitely use again.”

It isn’t a panacea. The victim still remains unknown. But based on all of the information House’s group has assembled, his police department was able to issue a poster last December with a projected likeness of the man and a host of information that they hope will bring out new leads in their investigation.

By the way, if you’re curious about why racemization is so much less accurate a clock than C-14 for dating a tooth’s age, part of the explanation has to do with temperature. Unlike C-14, the clock runs faster for aspartic-acid racemization when it’s hot. So being in a fire will totally distort a tooth’s apparent age via this technique — as might being left in a desert. Even the placement of a tooth — in the front of the mouth versus the back — can provide a degree or two difference in the temperature at which it’s incubated during an individual’s life, Buchholz notes. “And over a period of 40 years or so, that few degrees can be significant” — enough to alter a tooth’s apparent age by a several years.