Atapuerca, Spain, is a unique site for human evolution. In particular, its cave called Sima de los Huesos is the place with the largest number of hominid fossils found. In total 29 individuals have been recovered there so far, with several almost complete skeletons. Their age is estimated c. 430.000 years, the oldest fossils with neandertal features. All the individuals found there belong to the same biological population, which is terribly valuable to analyse their variations, sexual differences and patterns of development.
In 2014 Science Magazine issued  an analysis of the 17 skulls collection of Sima, 7 of them not released before. This study came 21 years after Nature issued the description of the first 3 skulls found in Sima.
What are the key conclusions of this study?
1) About the evolution of the neandertal skulls in Sima. Not all the skeleton parts developed simultaneously. The first neandertal features developed were the teeth, the jaw and the face, which suggest some masticatory specialization.
2) Some neandertal traits are found in the Sima individuals such as: pronounced brow ridge, not sunk over the nose; cheeks projected towards the nose, giving a triangular look to the face; engagement of the jaw to the cranium.
3) The braincases are not neandertal. Their average brain size is less than modern humans and less than neandertals. They developed the brain separately and later.
4) The uniformity of the Sima population contrasts with the variation when comparing to other European fossils in Mid Pleistocene (780-130 K years ago). This can be explained by age and regional variations, also by interbreeding with archaic populations.
This paper followed another study in 2012 by Chris Stringer  , which helped position the Sima fossils outside the taxon Homo heidelbergensis. The Atapuerca team had initially classified the few fossils found in 1993 as Homo heidelbergensis, a species described in 1907 with more primitive features than in Sima. Now this is official: they are not Homo heidelbergensis. But what are they exactly? Are they neandertals, are they an unknown species ancestor of neandertal…?
In any case, the amazing Atapuerca collection and the research effort will keep growing in the following years and will bring new discoveries. For example, we expect many news to come from the Gran Dolina to explain Homo antecessor (a new species of at least 800 Ka.) and the many questions unresolved on its morphology and its position in the evolutionary tree. But this deserves a full new post…
All photos credit: Javier Trueba, Madrid Scientific Films