The name of the cave may sound bleak - Sima de los Huesos, Spanish for "Pit of the Bones" - but the site in northern Spain's Atapuerca mountain range is providing a wondrous peek into a remote period in the history of early humans.
Scientists on Thursday described an astonishing collection of 17 fossil skulls unearthed in the cave dating from about 430,000 years ago of an extinct human species closely related to the Neanderthals who later prospered across Europe and Asia from roughly 250,000 to 40,000 years ago.
The skulls, reassembled from jumbled fragments from a small chamber deep within the cave, are the oldest known fossils to show clear Neanderthal features in the skull, although the scientists stopped short of calling them actual Neanderthals.
"Never before had such a tremendous collection of hominin (extinct human) skulls been discovered at a single site. For the first time in history we can study a fossil population, not isolated fossils," said paleontologist Juan Luis Arsuaga of the Universidad Complutense de Madrid, who led the study published in the journal Science.
The site did not yield just skulls. The scientists have pieced together skeletons of at least 28 individuals, Arsuaga said, mostly young adults and teenagers but with a few older adults and children.
Researchers have been conducting excavations at the location - designated a UNESCO world heritage site - over the past four decades and previously described some of the skulls and other remains. There has been a spirited debate about the age of the fossils and the precise species they represent.
The researchers did not assign them to any specific species, noting genetic differences from Neanderthals - formal name Homo neanderthalensis - as evidenced by DNA recovered from one of the Sima fossils. They also said the skulls were not representative of another species that lived at the time, Homo heidelbergensis, because of jawbone differences.
The scientists found Neanderthal-like characteristics in the skulls as well as features associated with more primitive humans. This backs the idea that Neanderthals developed their various defining characteristics separately and at different times - a "mosaic pattern" of evolution, they said.
The Sima individuals lived during the Middle Pleistocene, a span of about half a million years for which scientists are seeking a better understanding of human evolution.
"Phylogenetically, they are early members of the Neanderthal lineage. The specific (species) name is still an open question. I am not in favor of calling them just 'Neanderthals'," Arsuaga told Reuters.
'GAME OF THRONES'
The skulls showed that the earliest changes in the Neanderthal lineage occurred in the teeth, jaw and face, with those characteristics related to a specialization in chewing, perhaps related to meat eating. The skulls retained some primitive traits like a smaller brain case. The Neanderthal trait of an elongated and rounded brain case appeared later.
Arsuaga said the fossils suggest that human evolution in Europe at the time was not a slow, orderly process encompassing uniform changes across the continent's various peoples, but rather something more chaotic akin to the struggles between clans in the fantasy TV series "Game of Thrones."
Neanderthals are the closest extinct relative to our species, Homo sapiens, and disappeared after early modern humans first trekked into Europe from Africa. Genetic evidence shows there was inter-breeding between Neanderthals and Homo sapiens.
The researchers used six sophisticated techniques to establish the age of the Sima fossils, which previously had been estimated as being roughly 530,000 to 600,000 years old, dates that had complicated the question as to the species involved.
"As a result of this study we are able to answer two of the most important questions that surround the Sima de los Huesos fossil assemblage: Who were these people? And when were they living on the landscape?" said another of the researchers, geochronologist Lee Arnold of the University of Adelaide in Australia.
"Both these findings are critical to better understanding the complex patterns of human evolution across Europe during the Middle Pleistocene, not least because the site contains more than 80 percent of the world's known Middle Pleistocene fossil record for the genus Homo," Arnold added.