Laura Martín-Francés, a researcher at the Centro Nacional de Investigación sobre la Evolución Humana (CENIEH) and Monash University, leads a study published in the American Journal of Biological Anthropology, focuses on the premolars of two fossil populations found at the Atapuerca mountain range sites in Burgos: Homo antecessor from the TD6 level of Gran Dolina, and the hominids from the Sima de los Huesos (SH), reinforcing their relationship with Homo sapiens and Homo neanderthalensis, as they are the only two fossil populations where a mixed pattern in enamel thickness in dentition has been documented.

The main objective of the study was to determine whether the teeth of both populations exhibit thick enamel, considered the "primitive" state present in most of the fossil record and in modern humans, or if, on the contrary, they have thin enamel, a derived characteristic shared by Neanderthals. Furthermore, the authors tested the hypotheses related to these traits: whether thin enamel is associated with more complex dentin morphology, and whether thicker enamel is related to a relatively smaller dental crown.

The results reveal a remarkable variability within the fossil sample of Atapuerca. In the case of the Sima de los Huesos individuals, the upper premolars show thin enamel (like the Neanderthals), while the lower ones have thick enamel, more similar to that of modern humans. In the case of Homo antecessor, the opposite occurs: the upper premolars show variation between individuals, but the lower ones tend towards the condition derived from thin enamel.

174 premolars analysed
The international team involved in this study contributed fossil and modern materials to build a comparative sample that covers as much variability as possible. In total, 174 premolars were analysed, including current human teeth from three continents: Europe, Africa and Asia.

The specimens were scanned using micro-computed tomography (micro CT), an X-ray-based technique that produces high-resolution images (18 microns). This enabled the segmentation of the different dental tissues and their subsequent three-dimensional reconstruction.

"The wide variability we observed in the teeth of Atapuerca is an important warning: we cannot draw definitive conclusions about the thickness of the enamel (and whether it is a primitive or derived condition) from a single tooth," explains Laura Martín-Francés.

The study also confirms that the thickness of the enamel appears to be related to the size of the dental crown: the smaller the crown, the thicker the enamel tends to be, a clear trend in modern humans. In contrast, no evidence was found that thinner enamel is linked to more complex tooth morphology, as has been proposed for Neanderthals.

"It is important to continue investigating this possible relationship in other dental classes, such as the central incisors, which in TD6 and SH also show a complex morphology similar to the Neanderthal. Future research on other teeth and new analyses on the possible causes of this variation will help us understand this aspect of our evolutionary history in the Middle Pleistocene," adds Martín-Francés.