After years of intense work, the tenacity of a team of scientists is beginning to uncover the secrets of a very little known period of our history. Due to the scarcity of fossil remains, human evolution during the last million years is still a great mystery. But, 300,000 years ago, a group of 33 human beings met their death in still unknown circumstances, their remains were buried in the deepest part of a cave in the Sierra de Atapuerca, in northern Spain. There, in the place called the Sima de los Huesos, is the largest accumulation of human fossils ever found. In addition, in Atapuerca, the oldest human fossils of the European continent have been found, with more than 780,000 years old.