Stable carbon isotopes and human evolution

Abstract

Paleoanthropologists have long relied on skull and tooth morphology to infer fossil hominin diets, but from the early 1980s, they have also looked to microscopic wear traces in dental enamel, and since the early 1990s, they have looked increasingly to the stable isotope composition of skeletal tissues. The most commonly used stable isotopes are varieties of the light elements nitrogen and carbon. Nitrogen isotope ratios can provide information about the degree of carnivory vs. herbivory (1), but nitrogen can be extracted only from fossil bones that retain protein, which means specimens mostly younger than 100,000 y in temperate latitudes and usually much younger than 25,000 y closer to the Equator. In contrast, antemortem carbon isotope ratios persist indefinitely in dental enamel (2), and the main limitation is that they reflect ancient diets mostly in tropical or subtropical settings where the plants divide subequally between ones that follow a C4 photosynthetic pathway and others that follow a C3 pathway. This is not a problem for paleoanthropology, because early hominins (humans broadly understood) evolved in tropical and subtropical Africa, and their teeth are preserved in numerous African sites. Four articles in PNAS show how stable carbon isotopes have illuminated the diets of hominins that lived in Africa between roughly 4.1 and 1.3 Ma.