The Sima de los Huesos hominins lived approximately 400,000 years ago during the Middle Pleistocene. © Kennis & Kennis, Madrid Scientific FilmsThe “Pit of Bones” is located at the bottom of a 43-foot deep vertical shaft, in northern Spain, only accessible by spelunking a third of a mile into the Cueva Mayor cave system.

Archaeologists have recovered more than 5,500 bone fragments, including hundreds of teeth, and 28 skeletons of the inhabitants of the cave believed to be from 400,000 years ago.

Who exactly were the men, women and children discovered in the cave, and what can that tell researchers about early human evolution?

Previous mitochondrial DNA testing suggested a distant relationship to the Denisovans—a group related to the Neanderthals, in the genus Homo, discovered in the Denisvoa Cave in Siberia around 2008.

However, the new bones showed little similarities to other Denisovans remains, especially morphologically, according to a statement released yesterday by the Max Planck Society.

Now, advances in the sensitivity of ancient DNA testing have determined the remains are more closely related to Neanderthals, which provides a fixed marker in early human evolution of a time when the two groups likely split some 430,000 years ago.

When the bones were originally unearthed, scientists were hopeful that future advancement in testing might provide more clues. They carefully removed the specimens, and left them in the original soil and clay to help minimize any decomposition of the DNA after excavation, then stored them for future testing.

Two of these samples recently provided usable nuclear DNA in bones 400,000 years old.   

Watch a video of the excavation (narrated in Spanish):

Svante Pääbo of the Max Planck Institute said, “these results provide important anchor points in the timeline of human evolution,” in a statement. “They are consistent with a rather early divergence of 550,000 to 750,000 years ago of the modern human lineage from archaic humans”.

Researchers believe the remains are some of the earliest examples of ancient burial practices performed by early man living in the cave system at the time. However, other researchers say the bones were likely carried into the pit by streams or flooding, because there a very few small bones in the cave, which would have been likely washed away.

The study, “Nuclear DNA Sequences from the Middle Pleistocene Sima de los Huesos Hominins” was published in the journal Nature.