Sima de los Huesos: The Pit of Bones at Atapuerca, Spain

The Sima de los Huesos, or “pit of bones”, is a small chamber at the bottom of a 13-metre shaft deep inside a cave in the Sierra de Atapuerca, near Burgos in northern Spain. It has yielded more than 7,000 fossil fragments from at least 28 people who lived around 430,000 years ago, the largest collection of human fossils of that age anywhere in the world. These remains, and the ancient DNA recovered from them, have revealed the deep roots of the Neanderthals.

Background: the hills of Atapuerca

The Sierra de Atapuerca is a low limestone range riddled with caves. In the late 19th century a railway cutting for a mining line sliced through the hills, exposing cave fillings packed with fossils and later giving access to sites such as Gran Dolina, where the much older remains of Homo antecessor were later found.

The Sima de los Huesos lies in a different part of the system, in the Cueva Mayor. Reaching it means walking and crawling about half a kilometre from the modern entrance and then descending a vertical shaft. For decades, cavers had visited the pit to dig out bones of extinct bears, damaging the deposits in the process.

Discovery and excavation

In 1976 Trinidad Torres, a mining engineering student studying fossil bears, recovered a human jaw and other human fragments from the disturbed sediment. The palaeontologist Emiliano Aguirre recognised their importance and launched a research project at Atapuerca in 1978. Systematic excavation of the pit began in 1984.

From 1991 the project was led by three Spanish scientists, Juan Luis Arsuaga, José María Bermúdez de Castro and Eudald Carbonell. Working in cramped, humid conditions, excavators haul sediment up the shaft in bags and sieve it meticulously. In 1992 they found three skulls, including Cranium 5, nicknamed “Miguelón”, one of the most complete fossil human skulls in the world. A nearly complete pelvis, nicknamed “Elvis”, followed.

Who were the people of the pit?

The fossils represent at least 28 individuals of both sexes, most of them adolescents and young adults. Almost every bone of the body is represented, which allows researchers to study growth, body size and variation within one population, something that is rarely possible with older fossils.

The people were strongly built, with men estimated at around 1.70 metres tall and body weights often heavier than modern averages. Their brains measured roughly 1,100 to 1,400 cubic centimetres. Their faces, jaws and teeth show many features typical of later Neanderthals, such as a projecting midface, while their braincases remain more primitive. This “mosaic” pattern suggests that Neanderthal traits did not appear all at once but accumulated piece by piece.

What the bones reveal about daily life

Because the collection is so large, the Sima fossils have been used to investigate questions that single finds cannot answer. Studies of the tiny bones of the middle ear, together with reconstructions of the outer and middle ear canals, suggested that these people heard best in a range of frequencies similar to that of modern humans, a result some researchers link to complex vocal communication, although whether they had language is unknowable.

The teeth carry fine scratches and wear that show they were used as a “third hand” to grip material, a habit also typical of later Neanderthals. Most individuals were right-handed. The age profile, dominated by teenagers and young adults, has been compared with patterns of mortality in hunter-gatherer groups, though the small sample makes firm conclusions difficult. Signs of dental disease, healed injuries and growth disturbances add further detail about the hardships of life in Ice Age Iberia.

Dating the fossils

Early estimates placed the remains anywhere from 300,000 to more than 500,000 years ago. In 2014 Arsuaga and colleagues published a detailed study combining uranium-series dating of flowstone above the fossils, palaeomagnetism, and luminescence dating of the sediments. They concluded that the human fossils are about 430,000 years old.

That date fits with the presence of an extinct bear species, Ursus deningeri, an ancestor of the cave bear, whose bones fill the upper layers of the pit. The human bones lie in a distinct layer of reddish clay beneath much of the bear material.

Ancient DNA

The Sima de los Huesos made headlines again in 2013, when a team led by Matthias Meyer at the Max Planck Institute for Evolutionary Anthropology in Leipzig, working in Svante Pääbo’s department, recovered mitochondrial DNA from a thigh bone. Surprisingly, it was more similar to that of the Denisovans, a population known mainly from Siberia, than to that of Neanderthals.

In 2016 the same group sequenced fragments of nuclear DNA from two individuals. These showed clearly that the Sima people belonged to the Neanderthal lineage, after it had split from the Denisovans. The mitochondrial puzzle is usually explained by later replacement of Neanderthal mitochondrial DNA through interbreeding with an early modern human population. The Sima samples remain the oldest human DNA sequenced so far, and they helped to push back the estimated split between the Neanderthal and modern human lines to more than about 550,000 years ago.

A place for the dead?

How so many bodies came to lie in one small chamber is the site’s great mystery. The excavators note that there are almost no remains of herbivores, the usual prey of carnivores, and no sign of people living in the chamber. They argue that the dead were carried through the cave and dropped down the shaft by their companions, which would make it one of the earliest known examples of special treatment of the dead.

In 1998 a single red quartzite hand axe, nicknamed “Excalibur”, was found among the bones. Some team members suggest it was deliberately placed as an offering, but others caution that one tool cannot prove symbolic behaviour. Alternative explanations for the accumulation include accidental falls, carnivores dragging in bodies, or bones washed in by mudflows.

There is evidence of violence and of care. Cranium 17 bears two similar fractures on the forehead that appear to have been made by the same object in a face-to-face encounter, and has been described as a possible case of lethal violence. Cranium 14, from a child with a malformed skull caused by craniosynostosis, suggests that an individual with a serious condition survived for several years.

Timeline

• c. 430,000 years ago: The human remains are deposited in the pit.
• 1976: Trinidad Torres recovers the first human fossils from the Sima.
• 1984: Systematic excavation of the Sima de los Huesos begins.
• 1992: Cranium 5, “Miguelón”, and other skulls are discovered.
• 1998: The quartzite hand axe “Excalibur” is found among the bones.
• 2000: The Archaeological Site of Atapuerca becomes a UNESCO World Heritage Site.
• 2013: Mitochondrial DNA is recovered from a Sima thigh bone.
• 2014: Combined dating places the fossils at about 430,000 years ago.
• 2016: Nuclear DNA confirms the Sima people were on the Neanderthal lineage.

Why it matters

The Sima de los Huesos offers an unrivalled view of a single human population from deep in the Ice Age. It shows when and how Neanderthal features began to appear, provides the oldest human genetic data yet recovered, and raises intriguing questions about how early humans treated their dead. For understanding the origins of the Neanderthals, no other site comes close.