Declared a World Heritage Site by UNESCO in 2000, this is the most important archaeological site in the world to know human evolution. In Gran Dolina the remains of Homo Antecesor were found, over 800,000 years old: these are the most ancient ones Western Europe. In Sima de los Huesos there is the largest site of human fossils of Homo heidelbergensis or preneanderthal, around 400,000 years old. Traces of occupation were also found here, specially by the Homo sapiens (about 4000 years old) and Neolithic settlements.
Here you have the link to the official website for information and bookings.