The Sima Humans Illustration by Mauricio Antón

The thousands of bones recovered from the cave floor at Sima de los Huesos have enabled scientists to create a remarkably complete portrait of the hominids who lived in the hills of Atapuerca around 400,000 years ago. They were reasonably tall—males averaged about five feet, seven inches. And like their close relatives the Neanderthals, they were robustly built. Several of the individuals suffered from disease or injury.

Despite all of this information, one question remains: how did the bones of the Sima hominids end up in the cave? There are no indications that the hominids ever lived in the cave—for example, there is no evidence that they used fire or made tools there. Remains from around 30 individuals have been found, many of them teenagers and young adults. If the Sima hominids had fallen in accidentally, we would expect to see a wider range of ages represented. These bodies may have been intentionally thrown into the pit by other hominids—but we do not know why.