Homo antecessor skull layers resemble modern humans and Neanderthals, fragments suggest

A groundbreaking study has revealed that the cranial structure of Homo antecessor exhibits characteristics similar to both modern humans and Neanderthals, according to research published in the Journal of Human Evolution. Three newly analyzed parietal fragments from the Gran Dolina site in the Sierra de Atapuerca, Burgos, Spain, provide detailed insights into the internal architecture of the skull. These fragments, identified as ATD6-115, ATD6-128, and ATD6-401, originate from adult individuals and mark the first comprehensive quantitative analysis of Homo antecessor cranial thickness on a per-layer basis. The study, led by CENIEH predoctoral researcher Rafael Gallareto-Sande under the guidance of María Martinón-Torres, Emiliano Bruner, and Palmira Saladié, utilized micro-computed tomography to examine the three distinct layers of the cranial bone: the outer table, the diploë, and the inner table. This advanced imaging technique allowed researchers to assess not only the overall thickness of the skull but also the distribution of these layers, revealing key anatomical differences compared to other hominin species. The results showed that the diploë, the spongy middle layer of the cranium, constitutes between 41% and 50% of the total cranial vault thickness in the Homo antecessor samples. This proportion aligns closely with that observed in both modern humans and Neanderthals. However, the outer and inner tables displayed notable variations. The outer table was relatively thinner, while the inner table was relatively thicker than in Neanderthals and modern humans. Such structural differences suggest a unique blend of traits, reinforcing the idea that Homo antecessor occupied a transitional position in human evolution. Originally described in 2009 based on remains from level TD6 of Gran Dolina, Homo antecessor was characterized as having a "mosaic anatomy", a mix of derived facial and neurocranial features alongside more primitive postcranial and dental traits. This classification positioned the species near the common evolutionary root shared by Neanderthals and modern humans. The current findings add another dimension to this understanding by highlighting the specific cranial architecture that contributes to this mosaic nature. Despite these revelations, the researchers emphasize that the conclusions remain tentative due to the limited and fragmented nature of the Homo antecessor fossil record. "The available sample is still limited and fragmentary," Gallareto-Sande noted, "so any new fragment could refine these results." The team hopes that ongoing excavations at the Gran Dolina site will uncover additional cranial remains, enabling further exploration of the species' evolutionary significance. The study underscores the importance of continued paleontological work in the Sierra de Atapuerca, a region renowned for its rich hominin fossil deposits. As more data becomes available, the picture of Homo antecessor's place in the human family tree will likely become even clearer. For now, the findings offer a compelling glimpse into the complex interplay of morphology and evolution that shaped our ancient relatives.