El Sidrón is a karst cave located in the Asturias region of northern Spain, where the skeletal remains of a family group of 13 Neanderthals were discovered. Physical evidence found in the cave suggests that 49,000 years ago, this family was murdered and cannibalized by another group, the motive thought to have been the marauding group's survival.

The Cave

El Sidrón's cave system stretches into the adjacent hillside at a length of approximately 2.5 mi (3.7 km), with a large central hall approximately 650 ft (200 m) long. The part of the cave containing Neanderthal fossils is called the Ossuary Gallery, and it is ~90 ft (28 m) long and 40 ft (12 m) wide. All of the human remains found at the site were recovered within a single deposit, called Stratum III.

The Ossuary Gallery (Galería del Osario in Spanish) is a small lateral gallery, discovered in 1994 by cave explorers, who stumbled across human remains and named it assuming it was a deliberate burial. The bones all lie within an area of about 64.5 sq ft (6 sqm).

Preservation of the bones is excellent: the bones show very limited trampling or erosion and no there are large carnivore toothmarks. However, the bones and stone tools in the Ossuary Gallery are not in their original location. Geological analysis of the soils in that area suggests that the bones fell into the cave through a vertical shaft, in a massive water-driven deposit, probably resulting from a flood event after a thunderstorm.

Artifacts at El Sidrón

Over 400 lithic artifacts have been recovered from the Neanderthal site at El Sidrón, all were made from local sources, mostly chert, silex, and quartzite. Side scrapers, denticulates, a hand axe, and several Levallois points are among the stone tools. These artifacts represent a Mousterian assemblage, and the makers of the lithics were Neanderthals.

At least 18 percent of the stone tools can be refitted to two or three silex cores: that suggests that the tools were made at the occupation site where the Neanderthals were killed. There were only 51 fragments of non-human animal remains among the collections.

El Sidrón Family

The bone assemblage at El Sidrón is almost exclusively Neanderthal human remains, which account for a total of 13 individuals. Individuals identified at El Sidrón include seven adults (three males, four females), three adolescents between 12 and 15 years of age (two males, one female), two juveniles between 5 and 9 years of age (one male, one undetermined sex), and one infant (undetermined). All skeletal elements are present. Dental investigations suggest that the adults were all fairly young at the time of their deaths.

Analysis of mitochondrial DNA supports the hypothesis that the 13 individuals represent a family group. Seven of the 13 individuals share the same mtDNA haplotype and three of the four adult females have different mtDNA lineages. The younger juvenile and the infant share mtDNA with one of the adult females, and thus they were likely her children. Thus, the men were all closely related, but the women were from outside the group. That suggests this Neanderthal family practiced a patrilocal residence pattern.

Other evidence of close relatedness includes dental anomalies and other physical features which are shared by some of the individuals.

Evidence for Cannibalism

Although there are no carnivore tooth marks on the bone, the bones are heavily fragmented and show cut marks made by stone tools, indicating that the Neanderthals were almost certainly killed and cannibalized by another Neanderthal group, not by animal scavengers.

Cut marks, flaking, percussion pitting, conchoidal scars, and adhering flakes on the bones all provide strong evidence for cannibalism at El Sidrón. The long bones of the people show deep scars; several bones have been cracked open to obtain marrow or brains.

The bones of the Neanderthals also indicate that during their entire lives they suffered from nutritional stress, with a diet made up mostly of plants (seeds, nuts, and tubers) and some lesser quantity of meat. These data together lead researchers to believe this family was a victim of survival cannibalism by another group, who may also have been suffering from nutritional stress.

Dating El Sidrón

The original calibrated AMS dates on three human specimens ranged between 42,000 and 44,000 years ago, with an average calibrated age of 43,179 +/-129 cal BP. Amino acid racemization dating of gastropods and human fossils supported that dating.

Direct radiocarbon dates on the bones themselves were inconsistent at first, but sources of contamination were identified at the site, and new protocols were established for El Sidrón to avoid re-contamination at the site. Bone fragments recovered using the new protocol were radiocarbon-dated, obtaining a secure date of 48,400 +/-3200 RCYBP, or the early part of the geological stage called Marine Isotope 3 (MIS 3), a period which is known to have experienced rapid climate fluctuations.

Excavation History at El Sidrón

The cave of El Sidrón has been known since the beginning of the 20th century. It was used as a hiding place during the Spanish Civil War (1936-1939) by republicans hiding from Nationalist troops. The main entrance to the cave was blown up by the Nationalists, but the republicans managed to escape through minor entrances.

The archaeological components of El Sidrón were accidentally discovered in 1994, and the cave was intensively excavated between 2000 and 2014 by a team first led by Javier Fortea at the Universidad de Oviedo; after his death in 2009, his colleague Marco de la Rasilla continued the work.

Over 2,500 Neanderthal fossil remains were recovered during the excavations, making El Sidrón one of the largest collections of Neanderthal fossils in Europe to date. Although the excavations have ended, additional study of various skeletal elements has and will continue, providing new insight into Neanderthal behaviors and skeletal attributes.