President’s Spotlight—Stable Isotope Forensics: Applications to Unidentified Human Remains Cases


During the year, we will present presidential spotlights for each section to highlight the latest significant research or improvements within its discipline. Examples include cutting-edge technology, standards, or methods that improve the practice. Alternatively, they may show a historical account/timeline of the discipline’s use of innovative technology or research. These are presented to inform all members of how each discipline is responding to the challenges of a modern forensic science world—our theme for the year. This featured submission is from the Anthropology Section.


Sources: Eric Bartelink, PhD, Melanie Beasley, PhD, Gregory E. Berg, and Lesley A. Chesson, MS

Forensic anthropologists typically contribute to unidentified human remains cases through traditional analyses focused on determining the biological profile (e.g., sex, age, population affinity, stature, and antemortem characteristics). Perimortem trauma analysis, Postmortem Interval (PMI) estimates, and radiographic comparisons are also in the forensic anthropologist’s wheelhouse. When these customary methods fail to result in an identification, what other investigative avenues should analysts explore? Within the past three decades, forensic anthropologists increasingly have used Stable Isotope Analysis (SIA) to predict possible regions-of-origin and life history information of unknown individuals. SIA of various body tissues (e.g., teeth, bones, nails, hair) can provide additional investigative leads that may help resolve new, as well as cold, cases.

SIA capitalizes on the fact an individual’s movement across landscapes can create distinct isotopic “fingerprints” of chemical elements incorporated into body tissues from food, water, and even ingested dust. Isotopes of carbon, nitrogen, and sulfur reflect an individual’s diet since you are what you eat. Other isotope systems, such as oxygen, strontium, and lead, reflect the environment in which an individual resided when a tissue was formed (i.e., your geolocation). Tissues incorporate stable isotopes in a mostly regular pattern during growth and development, and investigators can therefore approach SIA through the lens of “a life’s history.” For example, teeth, bones, nails, and hair can be measured to understand snapshots of time representing childhood, years before death, months before death, and days before death, respectively. Additionally, isotopic data can be used to test hypotheses: Was the individual local or non-local (e.g., a foreign national)? Did this person travel in the weeks and months prior to death? How long has the individual been deceased (i.e., PMI estimate)? Did this person have distinctive dietary habits?

To answer these questions for medicolegal cases, forensic anthropologists have recently focused on the “forensic” aspect of SIA. Since SIA techniques for sample preparation, analysis, and data interpretation primarily originated in bioarchaeology, forensic applications of SIA must be prepared to meet the requirements set forth in the medicolegal community, such as the Daubert ruling and the Federal Rules of Evidence. Consequently, a major aim of new research has been baseline data: How variable is an individual isotopically? What is the isotopic variation between different individuals and populations? Other, necessary questions are being asked regarding data comparability and measurement uncertainty: What is inter-laboratory isotopic variability due to sample preparation? And sample analysis? How should “meaningful (isotopic) differences” be defined and applied?

Recent cases in forensic anthropology have demonstrated the utility of a multi-isotope approach for predicting possible regions-of-origin and life history of unknown individuals. Examples include the investigation of deceased Latin American migrants who perished along the United States-Mexico border, identification of United States soldiers from past conflicts, and casework focused on starvation and maltreatment. Promising and fruitful research also includes measuring nitrogen isotope ratios of fly larvae to estimate the PMI of decedents (on whom the larvae were feeding) and creating tissue-specific isoscapes for human teeth to predict possible regions-of-origin for an individual, based on oxygen isotope ratios. Recently, a freely available comparison tool, IsoLocate, was released. The program computes custom discriminant functions using single or multiple isotopes (available for a variety of tissue types) to compare an unknown individual to various world populations (United States Americans, Latin Americans, Southeast Asians, Japanese, etc.). As shown in Figure 1, it generates easy-to-understand comparison statistics, such as posterior and typicality probabilities, plus likelihood ratios, with a summary of the correct classification rate for the resulting cross-validated model and a predicted group for the unknown individual. IsoLocate can be found at www.anthropologyapps.com.

 

FIGURE 1: A representation of the output generated by the comparison tool IsoLocate. Latin American migrant (UBC) and United States American (US) comparison populations were used for the classification of an unknown individual (starred data point). Classification was based on carbon and nitrogen isotope ratios of bone collagen.

Measuring the isotopic fingerprints of different body tissues in unidentified human remains cases is informative in narrowing missing persons lists and can aid in an identification. As demonstrated within the past 30 years, many forensic anthropologists investigating unidentified individuals have benefited from the application of SIA. The field’s current research focus will ensure SIA techniques—and the interpretations made about an individual “life’s history” based on the measured isotope ratios—are defensible, repeatable, and reliable. As we look into the future, we are excited to see a wider adoption of SIA within the anthropological and medicolegal community.