Comparison of the jaws of skull 15 (adult) from the Sima de los Huesos (Pit of Bones) site, Sierra de Atapuerca, Spain (right), and a modern human (Homo sapien, left). The modern human jaw has a more prominent chin, with the teeth set further back in the jaw. Thousand of bones and bone fragments, from at least 28 individuals, have been excavated from the Sima de los Huesos site since its discovery in 1984. Skulls from the site, which date to the middle Pleistocene around 430,000 years ago, exhibit a mixture of Neanderthal and more primitive features. The teeth and jaw and facial bones are Neanderthal-like, while the cranium is primitive. This suggests that characteristic Neanderthal features evolved separately from each other, an evolutionary model known as the accretion model.