Enamel in a person’s tooth holds the secret to what that person ate as a child. Bone tissue in the tibia shows someone’s diet over a lifetime. Their hair will show what the person ate in the last few months of their life. Putting this kind of tissue through isotopic analysis is contributing towards advances in human forensics.

Written by: Leonie Joubert
Photographs by: Sam Reinders

Archaeologist Hope Chakanetsa analysed bone and teeth samples from 37 unidentified people whose remains are held in the University of Cape Town Human Skeletal Repository, to see if a study of the isotopes in these tissues would shed light on these cold cases.

Hope Chakanetsa plans to pursue doctoral studies that will allow her to continue using isotopic analysis to investigate new archaeological questions.

The methods for sampling tissue and processing these in the Stable Light Isotope Laboratory in the university’s Department of Archaeology has allowed the UCT graduate to learn valuable skills that she could take into a career beyond academia.

Since all mammals’ teeth are laid down early in life, the enamel in them will always carry a record of what that person or animal ate in that specific time.

If Hope Chakanetsa were the lead in a crime drama, she’d be the science geek tinkering away in the police forensics lab when the detective rushes in with a cold case that’s just got very hot. Some new evidence has come in that means they might be able to track down the identity of a woman whose badly decomposed body was found years earlier, abandoned on a plot outside Cape Town, and whose remains have been tucked away in some or other institutional vault ever since.

The coroner was able to say something about this person: a woman, clearly, because the pelvic width says as much. The fact that the roots of her teeth were becoming transparent, and her skull sutures were tightly closed suggest that she was an older adult when she died. Her hair texture suggests she’s a person of colour — this tissue outlasts other soft tissue, such as a person’s nose or lip shape, which might otherwise hint at a person’s ethnicity. The deep gashes in her rib bones suggest a likely cause of death was with a sharp blade. But with no identity documents on her person, there’s no way to know who she was.

In this drama, the next few minutes will follow Chakanetsa as she pores over the skeleton, drilling bone tissue from the woman’s tibia and from a rib, before prising a tooth from her jaw to get a scraping of enamel. In a jiffy, Chakanetsa would put these powdered samples through a mass spectrometer for a stable light isotope analysis and within minutes will show the detective conclusive evidence that the woman was from a small community on the outskirts of Qonce, a small town in the Eastern Cape, but that she’d moved to Cape Town a year before she died. This, by a generous use of artistic license, would allow the detective to rush off in a blur of sirens and flashing lights to arrest the suspect in the woman’s murder.

This is not a police procedural, though, and Hope Chakanetsa isn’t an actor. The isotope science she applies to the field of human forensics also doesn’t script itself this easily — someone doesn’t quickly run an isotope analysis overnight and pinpoint a precise location where someone was born several decades earlier, for instance.

But the unidentified cold-case human remains that Chakanetsa worked with during her Master’s research through the Department of Archaeology at the University of Cape Town would have undergone similar diagnostic scrutiny to the fictional scenarios that often play out on the small screen.

Chakanetsa worked with the remains of 37 people who had been found dead in the greater Cape Town area, and whose identities still remain a mystery. All these individuals have been in safe keeping at UCT’s Human Skeletal Repository in the Division of Clinical Anatomy & Biological Anthropology in the Faculty of Health Sciences.

By looking at the specific isotopic make-up of the carbon, nitrogen, oxygen and strontium in these individuals — in samples taken from their teeth and ribs — Chakanetsa hoped to learn the methods of processing human tissue for this kind of analysis, while also documenting the range of isotopic variation in the people living in modern-day Cape Town.

Each individual would have received similar treatment by forensics analysts as those in the fictional account, as authorities tried to glean as much about the person as possible. Their hair, teeth, skeleton and facial features would all have said something about the individual. If the body wasn’t too badly decomposed, distinguishing features such as tattoos or scars would still have been visible and added to the portrait.

Once that level of analysis is done, though, an isotope study can add more layers of information to the mystery of an unidentified body. Chakanetsa’s research allowed her to process tiny samples of bone and teeth because in this tissue lies the secret to where the person may have lived as a child and if they relocated as an adult.

Elements such as carbon, nitrogen, oxygen, hydrogen and strontium occur in the soil and water in any location, but will often have different isotopic ratios that are unique to a place owing to regional geography. When a person eats plants or animals that are raised in an area, they will ingest these elements and absorb them into body tissue, along with their isotopic ratios.

Carbon isotopes hold the clue to the kind of plants a person ate. If a person eats a diet rich in plants such as wheat, potato or rice, their tissue will be enriched in carbon-12 versus carbon-13, which is the signature of plants that have evolved the C3 photosynthetic pathway for metabolising carbon. If the person has eaten a diet high in maize, sorghum, or sugar, their tissue will have a higher ratio of carbon-13 to carbon-12, due to the C4 photosynthetic pathway.

Nitrogen can say a lot about the kind of protein someone has eaten. Strontium can be linked back to a specific place.

Teeth hold the secret to that person’s diet as a child — their teeth develop in childhood and the enamel remains in place throughout their life, locking away the elements that the body used to build the enamel.

The tissue in a person’s bones shows what they have eaten through the course of their lives, as new bone tissue is added to and replaces older bone as the person grows and ages.

By studying the nitrogen, carbon, strontium and oxygen in these bone and teeth samples, Chakanetsa was able to learn the technicalities of how to run human tissue through such a delicate and precise laboratory process, while also trying to learn more secrets about the people behind the cold case files.

The most distinctive finding was that most of the individuals had eaten a diet with a fair mix of C3 and C4 which might be expected of people eating in today’s food system, although there was clear evidence that some had eaten a more C4-based diet in their childhood, and then graduated to a C3-based diet in their adulthood. This is a trend that is widely seen in South Africa, as people migrate from the countryside towards cities and adopt a more urban diet.

Meanwhile the remains of the 37 people who Hope worked with for this study, in the hope of bringing more information about them to light, are back in safe keeping in the university's Human Skeletal Repository. Perhaps, one day, another leap forward in forensic science will turn these cases from cold to hot and allow the individuals’ identities to be known. Maybe, even, some families would be able to bury their dead.