Recent reanalysis of the c.480,000-year-old hominin remains that were excavated at Boxgrove, West Sussex, in the 1990s (see CA 138 and 153) has called into question whether they all come from Homo heidelbergensis as originally thought, or whether they represent two separate populations.
Hominin fossils dating to the Middle Pleistocene, such as those at Boxgrove, are few and far between, so the discovery of at least 29 individuals from this period at Sima de los Huesos (SH) in Atapuerca, Spain, was groundbreaking. Analysis of these Spanish remains has allowed researchers better to appreciate the natural variation in morphology that can occur within a single hominin population, and this, in addition to aDNA analysis of the remains, has revealed that the SH individuals were probably an early population of the Neanderthal clade (although they also share DNA with the Denisovans). These findings have allowed many other remains from this period across Europe to be reconsidered, including the Boxgrove samples.
The remains found at Boxgrove – consisting of two teeth and part of a tibia – were re-examined by an international team using advanced 3D imaging and virtual reconstruction techniques. When comparing the teeth (medial and lateral mandibular incisors) with 22 incisors found at SH, they found that the Boxgrove examples fit comfortably within the range of morphology seen at SH, indicating that this individual may have belonged to an early branch of the Neanderthal clade as well. In contrast, the Boxgrove tibia was markedly different from the lower leg bones found at SH (ABOVE). In particular, while both have thick cortices and bone walls, the Boxgrove example has other features that more closely resemble pre-Neanderthal and modern humans. Based on this mixed morphology, it could be that two separate hominin populations are represented at Boxgrove.
This is an intriguing development. While both the incisors and the tibia date back around 480,000 years, the teeth were found in a lower stratigraphic layer than the tibia, meaning that they probably came from non-contemporary individuals. These dates are at the very far reaches of dating technologies, however, so it is hard to discern just how far apart in time these two hominins would have lived.
These results, recently published in the Journal of Human Evolution (https://doi.org/10.1016/j.jhevol.2022.103253), highlight the complexity of human evolution during the Middle Pleistocene, with the co-existence of multiple human lineages. Our understanding of this period is really only just beginning.