This news release is available in Spanish.
The study focuses on a collection of seventeen fossil human skulls excavated from an archaeological site known as the Sima de los Huesos (Pit of the Bones) in the Atapuerca hill in northern Spain and comes 21 years after the announcement of the first three skulls from the site was published in Nature in 1993.
Over the last four decades, the Atapuerca team has carried out painstaking excavations at the site, which is particularly difficult to access, and patiently reconstructed the skulls in the laboratory, often from large numbers of tiny cranial fragments recovered from the site. These efforts are still ongoing on both fronts, and to date it is estimated that around 30 individuals, comprised of complete skeletons, were accumulated at the site, with the bones being found broken and jumbled together in the sediments. In addition, considerable advances have been made in understanding the geology and dating of the site, which are important to approach the question of how so many cadavers ended up there in the first place. The results of multiple independent dating methods indicate an age of around 430,000 years ago for the fossil assemblage, placing them within the Middle Pleistocene time period.
No other site in the world has yielded so many skulls of an extinct human species. Since the 1980s, the Atapuerca team has maintained that the Sima population is evolutionarily closely related to the Neandertals, and the Sima fossils are the oldest to show clear Neandertal features in the skull. One of the central debates among anthropologists is how the Neandertal skull evolved through time. The study of the Sima fossils confirms the idea, proposed by other researchers, that the evolutionary pattern is "mosaic" in nature. Rather than gradual, steady changes occurring in the entire skull, the earliest changes appeared in the teeth, jaw and face, suggesting that these features together represent a single functional complex related with a specialization in the chewing apparatus. Other regions of the skull, such as the cranial vault, or neurocranium, and the brain housed inside it, underwent changes later in time.
One of the important findings of the study was the homogeneity of the sample of fossils from the Sima. All of the individuals recovered at the site represent the same biological population which makes it possible for anthropologists to study individual variation as well as sexual differences in the skeleton and patterns of growth and development, among other aspects. While considerable differences in size are apparent within the collection, with some larger skulls and some smaller ones, the anatomical features that anthropologists study to examine evolutionary relationships do not vary much within the Sima population. This combination of mosaic evolution and anatomical homogeneity led the authors to favor a branching pattern of evolution, known as cladogenesis in evolutionary studies, in the European Middle Pleistocene.
What species do the Sima fossils represent? This question too is addressed in the recent study, although the authors do not assign the fossils to a particular species. However, they argue that genetic differences from the Neandertals, as seen in the mitochondrial DNA recently recovered from one of the Sima fossils, suggests they are not simply "early Neandertals". Nor should they be placed in the species known as Homo heidelbergensis since the Sima jawbones (mandibles) are distinct from the type specimen of this species, a mandible from the site of Mauer in Germany. This is one of the outstanding questions and one that is sure to inspire considerable debate and controversy within the field. With excavations continuing and new fossils being discovered each field season, there is certainly reason to believe that the Sima de los Huesos will yield more surprising findings in the future.