Fossilised skulls, Sima de los Huesos

Fossilised skulls, Sima de los Huesos. These human skulls are amongst many hundreds of pieces of human fossils found at Sima de los Huesos (Pit of Bones). It is thought that the site was used as a cemetary by early humans, 400,000 years ago. The skulls were classified as Homo heidelbergensis, an extinct species that forms a relatively recent part of the human evolutionary tree. It may have been an ancestor of both Neanderthals and modern humans. There is debate over whether the fossils should be attributed to H. heidelbergensis or H. neanderthalensis, as some anatomical features are transitional between the two species. Sima de los Huesos is one of several sites in Sierra de Atapuerca, Spain, that have been studied on and off over the past century, leading to a massive advancement in the understanding of human evolution. In 2000, Sierra de Atapuerca was made a world heritage site by UNESCO.