They were initially suspected to be from the Spanish Civil War because Republican fighters used to hide there; however, later analysis shows that the remains actually belong to Neanderthals.

[3] A number of scenarios of how these "members of an extended family" might have ended up in a 6 m2 (65 sq ft) room-sized space, dubbed the Tunnel of Bones included flooding, cave collapse, and disposal by cannibals.

[6] Researchers suppose that they were dropped into the cave in a single event via a collapse of nearby fissures above the site or by influx of stormwater.

The cave is in the northern portion, southern Neanderthals show broader faces with increased lower facial heights.

Additionally, the juvenile specimen exhibits an unusual developmental feature; the neurocentral synchondrosis between his thoracic and C1 vertebrae had not yet fused; typically, this fusion occurs around 2 years earlier in modern humans.

[4] Other paleoanthropologists like Marcia Ponce de León and Christoph Zollikofer have cautioned against interpreting the brain size as a sign of delayed maturation, because of the small case number and lack of statistical evidence, and Tanya Smith noted that the assumption depends on the accuracy of the tooth's age[11] Ancient Neanderthal mtDNA was partially sequenced in the HVR region for three distinct Neanderthals from El Sidrón cave (441, 1253, and 1351c).

[17] The Sidrón Cave Y chromosome coded for several minor histocompatibility antigen genes that differ from that of modern humans.

[17] Recent research investigating the Neanderthal remains recovered from El Sidrón has provided evidence that their diet consisted primarily of pine nuts, moss and mushrooms.