The ongoing excavation at Sima de los Huesos has already yielded fossil remains from approximately 30 individuals. This wealth of information about an entire group of closely related hominids is unprecedented. Paleoanthropologists often have to draw conclusions about an entire species based on one or two fragments from a single individual. But because of the abundance of bones found at Sima de los Huesos, we know more about this group of hominids than we do about most other species of early human.

After collecting the fossil fragments from the Pit of the Bones, investigators took them to a laboratory to be reassembled like dozens of giant puzzles, each with hundreds of pieces. Investigators then analyzed the reconstructed bones for additional clues about the early humans of Sima de los Huesos.

By studying the teeth of the Sima hominids, the Atapuerca team has estimated that roughly half of the Sima humans died between the ages of 10 and 18 and none lived to more than 40. The teeth tend to show extensive wear, suggesting these humans probably held meat, hides or other materials in their mouths to scrape or process them. Judging by the orientation of scratch marks on the teeth, researchers have determined that the Sima humans always held objects so that they could cut with their right hands. In other words, all of the Sima humans appear to have been right handed.

Disposing of the Dead? Illustration by Mauricio Antón

Why are there so many hominid bones embedded in the cave floor of Sima de los Huesos? Researchers have puzzled over this question since the fossils were first discovered.

There’s no evidence that humans lived in the cave—no signs of fire or extensive tool use. The scientists who discovered the Sima fossils have speculated instead that the hominids disposed of these bodies by carrying them over to the cave—perhaps as shown in the imaginative illustration above—and then throwing them in. There is no evidence, however, that this practice was part of a tradition of planned burial. The Neanderthals were the first hominids known to engage in such behavior, beginning under 100,000 years ago.