Researchers at the Museum of Archaeology and Paleontology in Madrid, Spain have uncovered a chamber of a cave filled with 35 skulls of long-dead herbivores. The skulls are assumed to have been placed there by groups of Neanderthals, a close relative to the modern human.

Read More: Who Were the Neanderthals?

The cave, known as the Cueva Des-Cubierta, is in the north Madrid region of Spain. Researchers initially discovered it in 2009, but this collection of skulls was not found until recently – reigniting the debate surrounding just how advanced our genetic relatives were.

Previous Findings in Cueva Des-Cubierta

Neanderthal-related findings are common in the Cueva Des-Cubierta. Researchers believe Neanderthals used the cave periodically as a place of ritual, and maybe even as a burial ground. The cave itself is about 80 meters long and varies in width from two to four meters.

Archaeological findings in the cave have been located on two main levels, according to the main study published in Nature Human Behavior. On Level 2, “an immature human mandible and six deciduous teeth have been recovered,” researchers write in the study. These findings appear to come from a Neanderthal child who was between 3 to 5 years old at the time of death.

In addition to these remains, a variety of tools and other Neanderthal remains have been found at the site since its discovery in 2009.

What the Skulls Suggest

While most archaeological findings related to Neanderthals are usually associated with activities such as hunting, the preparation of tools or the use of fire, these new findings are unique, researchers claim in their study.

Level 3 of the Cueva Des-Cubierta covers about 27 square meters (about 300 square feet) and has a depth of about two meters. In Level 3 of the cave, researchers have uncovered over 2,000 different remains over two centimeters of various animals. Some of the bones showed signs of “thermoalteration,” likely burn marks from being cooked.

In addition to these remains, researchers recovered over 1,400 Neanderthal tools from Level 3 of the cave. These tools include anvils, hammerstones, cores, flakes and shaped tools.

Most importantly, however, is the discovery of a variety of large skulls in Level 3 of the cave. According to an interview with Newsweek, Enrique Baquedano, the director of the Museum of Archaeology and Paleontology in Madrid, claimed that over 35 “crania” had been found in that portion of the cave.

All of the remains belonged to large herbivores that roamed the Earth around that time, and researchers claim that they’re around 42,000 years old. Additionally, evidence suggests that the skulls were initially processed outside of the cave, and not inside.

“The identification of anthropic marks on the crania and the under-representation of zygomatic bones, maxillae, mandibles and teeth suggest that the heads of these animals were first processed outside the cave,” researchers wrote in their study.

The skulls were then brought into the cave for a second round of processing, which was possibly related to the removal of the brain.

These results are significant because they are unlike previous Neanderthal-related findings. According to researchers, other studies related to hunter-gatherer groups have shown that the heads of large animals were usually discarded, implying that the placement of skulls in this instance was deliberate and not related to survival.

These findings are what led researchers to believe that Neanderthals used these crania in a symbolic sense. In this specific instance, the fact that every skull found has some sort of cranial appendage (such as antlers on a deer skull), indicates that Level 3 of the Cueva Des-Cubierta could have been some sort of trophy room, according to the study.

However, given the fact that no Neanderthal-related archaeological site purely dedicated to symbolic use has been found to date, researchers acknowledge that more research is needed.