The Sima de los Huesos hominins are a group of pre-Neanderthals that lived about 430,000 years ago. They were discovered at the archaeological site of Atapuerca, Spain. These individuals belong to the "Neanderthal clade," but they are not part of the species Homo neanderthalensis. When scientists first studied these 29 individuals in 1993, they made up about 80% of all human fossils from the Middle Pleistocene era. Every bone from these individuals has been preserved, and the complete remains help scientists understand Neanderthal evolution, how to classify other fossils, and how a single population of early humans varied. Excavation of these remains began in the 1980s, led by Emiliano Aguirre and later by Juan Luis Arsuaga, Eudald Carbonell, and José María Bermúdez de Castro.
These hominins have a mix of traits seen in Neanderthals and traits more similar to older human ancestors. Like Neanderthals, they had large brow ridges. However, their skulls were less robust at the back and had a pointed, "house-like" shape instead of a rounded, "bomb-like" shape. Their brain size averaged 1,241 cubic centimeters, which is on the lower end of the Neanderthal range. Their teeth were similar to Neanderthals, with shovel-shaped incisors and a feature called taurodontism, but their teeth had different shapes in the cusps. Their chests and waists were broad and strong like Neanderthals, but their limbs were longer. They may have been large-bodied, with both males and females about 170 cm (5 ft 7 in) tall and weighing around 90 kg (200 lb).
The Sima de los Huesos, meaning "bone pit," is a chamber in the Cueva Mayor–Cueva Silo cave complex at Atapuerca. It may have been a natural trap for cave bears. Scientists believe these hominins were placed in the pit by other humans, based on how well the bones were preserved and the fact that many of the remains were from adolescents and young adults rather than children or the elderly. All individuals were buried around the same time, and one person may have been killed with a blunt object. Some individuals with serious health problems survived for a while, suggesting the group may have cared for them. Many, especially adolescents, showed signs of diseases linked to poor fat reserves during hibernation, which may have lasted about four months.
This population used Acheulean stone tools and tools that seem to be transitioning to the Mousterian culture, which is typical of Neanderthals. They used these tools for cutting meat, working with hides, and wood, using their mouths as a third hand. The hominins were buried with a large Acheulean handaxe, which may have been a symbolic item. This suggests they may have used an early form of language. They may have been skilled hunters who competed with local hyenas, hunting deer, rhinoceroses, horses, bison, and occasionally cave lions in an open woodland area. They likely ate roots regularly, often squatted, and did not use fire.
Research history
In 1868, Spanish engineers P. Sampayo and M. Zuaznávar wrote a report about the caves in the Atapuerca Mountains, noting that one cave, Cueva Ciega, had human fossils. In 1910, Spanish archaeologists Jesús Carballo García and Saturio González found a cave painting of a horse in Cueva Mayor, which made the area famous worldwide. A railway built nearby, using materials from a place called Trinchera del Ferrocarril, showed that Cueva Mayor was part of a larger cave system: the Cueva Mayor–Cueva Silo complex. In 1962, the Edelweiss Speleological Group explored the area and found artifacts from the Neolithic, Chalcolithic, and Bronze Age. Trinidad Torres searched for bear fossils in Trinchera del Ferrocarril while working on his doctoral thesis but was directed to Sima de los Huesos, a cave marked by Sampayo and Zuaznávar as El Silo, a chamber with a 13-meter vertical shaft in Cueva Mayor–Cueva Silo.
Torres sent four speleologists who found many cave-bear fossils and a nearly-complete human jaw. He told his PhD advisor, Emiliano Aguirre, who organized more digs. Sima de los Huesos was hard to reach, had little oxygen, and had been disturbed by visitors searching for fossils. Sediment removal began in 1984 after lights and infrastructure were added. In 1987, Aguirre’s team installed scaffolding over Sima de los Huesos. Sediment blocks were carried up the vertical shaft, taken to the Arlanzón River, dried, and sorted. Under Aguirre, the Atapuerca team reconstructed three human skulls, one called “Lazarus.”
Aguirre retired in 1990, and his team leaders—Juan Luis Arsuaga, Eudald Carbonell, and José María Bermúdez de Castro—continued the work. In 1992, a nearly-complete skull called “Agamemnon” was found, the most preserved European skull before the Late Pleistocene. Another nearly-complete skull, named “Miguelón” after cyclist Miguel Indurain, was also discovered. In 1994, a nearly-complete pelvis called “Elvis” was found, the only similarly-preserved hominin pelvis before the Late Pleistocene.
Over 7,000 human fossils have been found in Sima de los Huesos, representing every bone in the skeleton. Scientists first thought they came from 32 individuals but now believe 29. In 1993, these fossils were the most complete Middle Pleistocene human record from a single site, making up 80% of the global Middle Pleistocene human fossil record and 85% of Middle Pleistocene postcranial fossils. A similar find, Homo naledi, was discovered in 2013 in South Africa. The fossils showed the range of anatomical variation in Middle Pleistocene Europe, helping scientists understand Neanderthal evolution. UNESCO declared Atapuerca a World Heritage Site in 2000, and the Museum of Human Evolution in Burgos opened to honor the site’s history.
The Cueva Mayor–Cueva Silo complex is the main cave system in the Atapuerca Mountains, stretching over 3 km with three levels. Sima de los Huesos is on the lowest level. The complex has four entrances: Portalón (to Cueva Mayor), Galería de las Estatuas (first level), Sima del Elefante (second level), and Cueva del Silo (third level). Cueva del Silo is closest to Sima de los Huesos, connected via the Sala de los Cíclopes.
Sima de los Huesos is 30 meters underground, 500 meters from Cueva Mayor’s entrance. It is 18 meters long, with a horizontal segment (Sima Top), a 30-degree ramp (Sima Ramp), and another horizontal segment (Sima de los Huesos). The cave’s fill is divided into 12 lithostratigraphic units (LU). Three pits—Alta (SRA), Media (SRM), and Baja (SRB)—were used to collect fossils. Three chimneys exist, but only Chimney 2 is open, leading to Sala de los Cíclopes. Most human and carnivore fossils come from a 4 m × 8 m area on the north side of the ramp. Over 100 human fossils were also found along the ramp.
LU-6 is a red clay layer, up to 50 cm thick, mostly phyllosilicates, including illite. Paleomagnetic dating shows it is younger than the Brunhes–Matuyama reversal (about 780,000 years ago). Fossils are mixed with limestone, speleothems, marl clay, and manganese dioxide, forming a bone breccia. In situ speleothems made of calcite rafts suggest a minimum age of 410,000–460,000 years. LU-7 is dated to 428,000 ± 27,000 to 441,000 ± 25,000 years ago. Red clay from LU-6 has a maximum age of 448,000 ± 15,000 years.
The human fossils likely represent one population. Limited carnivore damage suggests the fossils were buried and inaccessible. Only Chimneys 1 and 2 may have been open at the time of deposit, acting as traps for cave bears. Humans may have been intentionally placed in the cave, with bones sliding down the ramp and collecting at the bottom.
Similar fossils were found in Trinchera Galería’s GII and GIII layers, about 2 km from Sima de los Huesos. This sequence spans 408,000–598,000 to 221,000–269,000 years ago. The 300,000-year-old Trinchera Dolina 10.1 layer in Gran Dolina has a rich stone tool collection and was likely occupied by a population similar to Sima de los Huesos hominins.
Skeleton
The Sima de los Huesos material includes 17 skulls. Compared to Neanderthals, the occipital bone (back of the skull) is less curved and lacks the characteristic occipital bun. Like modern humans and Neanderthals (and unlike H. erectus), the opisthocranion (the farthest-back point of the skull) is above the superior nuchal line. The opisthocranion is at the peak of a flat or slightly convex, semicircular area that extends down to the inion; the area is dotted with circular pits (cratered) that became smaller and more dense with age. In Neanderthals, this area is characteristically sunken and smaller; it is a variable trait in Middle Pleistocene specimens. Below this area is an occipital torus (a horizontal bulge of bone projecting from the occipital bone), much weaker than in Neanderthals. The torus is most noticeable near the center of the occipital bone and terminates before reaching the asterion (where the occipital bone connects with the parietal and temporal bones). The Swanscombe Skull has a similar occipital morphology, but its torus extends farther (like Neanderthals).
When the skull is viewed from the back, its sidewalls are parallel and form a shallow sagittal keel running along the midline of the skull (a house-like contour) like other European Middle Pleistocene specimens. The contour of H. erectus converges more strongly at the top (tent-like), and in Neanderthals it is rounded (bomb-like). Like Neanderthals but unlike many Middle Pleistocene specimens, the brow ridge (supraorbital torus) is double-arched instead of a single, straight bar. The arches are not divided, but some specimens have a depression on the glabella (between the brows). When viewed from the top, the skull projects farthest around the glabella.
The average brain volume for the 15 Sima de los Huesos skulls for which a metric is calculable is 1,241 cc (75.7 cu in), ranging from 1,057 to 1,436 cc (64.5 to 87.6 cu in). The average is comparable to other Middle Pleistocene non-erectus specimens, which are known to range from 1,165 to 1,325 cc (71.1 to 80.9 cu in), and on the lower end of the range for Neanderthals, which are known to range from 1,172 to 1,740 cc (71.5 to 106.2 cu in).
Like other archaic humans, the parietal lobe was proportionally smaller than in modern humans. Compared to Neanderthals, the Sima de los Huesos brain was generally broader (especially towards the back, though this probably does not relate to function). The encephalic rostrum (the front of the corpus callosum) was more expanded than in H. erectus, but thinner and narrower than in Neanderthals. The left hemisphere is typically bigger than the right in Neanderthals and modern humans, related to an expanded Broca’s area associated with language acquisition; in the Sima de los Huesos hominins, brain asymmetry is variable. Like Neanderthals, the temporal lobe (associated with visual and olfactory memory) is narrow. The occipital lobe (the visual cortex) is larger than in modern humans but much smaller than Neanderthals.
The mid-face (the nose and above) exhibits prognathism (juts out) to a similar degree as other Middle Pleistocene specimens, but not as much as Neanderthals. The face and nose are much wider than Neanderthals, and the cheeks are higher. Like Neanderthals and most modern humans but unlike H. erectus, the bottom rim of the piriform aperture (nose hole) is raised. As in Neanderthals (though to a lesser degree), the anterior nasal spine and inferior nasal concha at the base of the piriform aperture are fused; the lateral crests are well-defined, extending to (and connecting with) the nasal spine. Unlike Neanderthals, the floor of the nasal cavity is flat instead of sloping.
Dental and jaw anatomy is generally Neander
Pathology
Cranium 5 shows severe tooth decay, moderate gum disease, and inflammation of the bone around the teeth on the left side. All of its teeth are worn down heavily. There is significant loss of bone in the jaw, which caused the teeth to shift upward, exposing at least one-third of each tooth’s root. This person may have used a toothpick to treat their gum disease, as similar toothpick marks are found on other teeth in the Sima de los Huesos. The lower left third molar is broken. The upper left first premolar was severely broken during life, exposing the inner part of the tooth. This injury likely led to a long-term infection, forming a cyst and a small opening in the bone. The lower left incisors also formed cysts, possibly due to heavy wear and pressure on the teeth. The upper incisors were similarly worn down. The infection may have spread to the eye area, causing swelling, and could have led to a life-threatening infection in the body.
About 40% of the teeth have underdeveloped enamel, which may have occurred during periods of poor nutrition around age 3½. This could indicate the age when this individual stopped breastfeeding, which was likely around four years old for Neanderthals. Neanderthals had more frequent and severe enamel issues because their way of finding food was less efficient or because they lived in harsher environments.
Based on signs of joint wear, "Elvis" may have lived past 45 years old. He had an exaggerated curve in his lower back, a misalignment between the last lower spine bone and the first sacral bone, and enlarged spine bones in the lower back. These conditions would have caused pain and limited movement, suggesting others may have cared for him.
Cranium 14, an adolescent, had a rare condition where the left lambdoid suture in the skull closed too early, causing skull deformities. This condition is extremely rare in modern humans, affecting fewer than six out of every 200,000 individuals. The individual died around age 10, suggesting they received the same care as other children, not being abandoned due to their appearance.
In 2020, scientists Antonis Bartsiokas and Arsuaga proposed that the Sima de los Huesos hominins may have hibernated during harsh winters. They noted that adolescents had signs of chronic diseases linked to poor nutrition, which could occur if they lacked enough fat reserves during hibernation. Adults showed fewer signs of these issues. In hibernating animals, low oxygen levels during hibernation reduce body temperature. This can prevent the body’s natural stress response, allowing temperature to drop without waking.
The Sima de los Huesos fossils show signs of chronic kidney disease and related bone issues, including frequent bone inflammation and a condition called renal osteodystrophy. These are linked to vitamin D deficiency, which can cause low calcium levels and overactivity of the parathyroid glands. These conditions are common in hibernating animals with low fat stores. Fat deficiency stops the release of stored vitamin D, raises stress hormone levels, and lowers calcium, leading to kidney disease. Fat deficiency is a major cause of death during hibernation.
The relevant health issues include:
Like cave bears at the site, the Sima de los Huesos hominins may have hibernated to survive cold, food-scarce winters. One adolescent cave bear had signs of rickets, a condition linked to vitamin D deficiency. The Sima de los Huesos fossils include many adolescents, a pattern also seen in hibernation-related deaths. Neanderthals likely did not hibernate because they were better at surviving cold and could hunt year-round.
Several skulls, including Crania 3, 5, 7, 9, 11, 13, 14, and 17, show fractures that occurred near the time of death. These may have happened while falling down a shaft, not necessarily while alive. About 4% of long bones show similar fractures without major injuries. American archaeologist Erik Trinkaus observed a similar pattern in Neanderthals and suggested that individuals unable to move between caves may have been left behind. If true, only those without serious leg injuries would be found at cave sites. It is unclear if this applies to Sima de los Huesos. The only foot specimen found has a minor injury consistent with a common foot condition in barefoot runners.
Cranium 17 has two similar, rectangular fractures on the left side of the skull, likely caused by a strong object hitting the head. The fractures have different angles, suggesting two separate impacts. The same shape of the fractures suggests the same object was used, making it unlikely the injuries came from falling or falling rocks. The injuries may have been caused intentionally by another human using a tool in their right hand, leading to death.
This suggests the individual represented by Cranium 17 was already dead before being placed in Sima de los Huesos, and may have been buried by others. Dental patterns suggest the Sima de los Huesos hominins were mostly right-handed. In forensic studies, multiple blows to the face above the hat brim are often interpreted as an attack. Fractures on Cranium 5 and Cranium 11 may also have resulted from interpersonal violence. No defensive injuries on the arms have been found.
Interpersonal violence is hard to identify in fossils. By 2015, only a few other human fossils showed clear evidence of this, including the Neanderthal Shanidar 3 and the modern human Sunghir 1.
Culture
The Sima de los Huesos hominins lived alongside a group of many different meat-eating animals, including the cave bear Ursus deningeri (which mainly ate plants), wolf (possibly Canis mosbachensis), red fox, cave lion Panthera fossilis, European jaguar, cave lynx, pine marten, least weasel, European polecat, and European badger. This suggests the area had a very productive environment. Small mammals and large plant-eating animals are rare, which matches the idea that the site was a natural pitfall trap. Cave hyenas, which were common in other areas at the same time, are not found here, which may mean the hominins competed with them for resources. At TD10.1, there is evidence of cave lions being hunted and butchered. This was unusual for the Middle Pleistocene, as hunting carnivores was rare. Ursus deningeri is the most common animal at the site but is not found elsewhere in the Sierra de Atapuerca. These bears may have been more likely to fall into the pit because they needed caves to hibernate in. Three human ribs were found with signs of being bitten, possibly by a bear that fell into the pit.
Well-preserved bat fossils (mostly greater mouse-eared bats and a few Mehely’s horseshoe bats) and large amounts of bat droppings were found, likely from bats that roosted in the area during winter. Early Pleistocene layers suggest bats lived there year-round, but in the Middle Pleistocene, they may have only stayed seasonally as human activity increased. The bats may have been disturbed by humans, as they did not return to the area during warmer periods like other small mammals. Rodent fossils were well-preserved and not digested, indicating they entered the site through a small crevice.
The Gran Dolina TD10.1 and Galería GII and GIII sites had remains of fallow deer Dama clactoniana, red deer, roe deer Capreolus priscus, giant deer Praemegaceros solilhacus, narrow-nosed rhinoceros, wild horses, European wild asses, wood bison (Bison schoetensacki), and Bonal tahr. These herbivores may have been prey for the hominins. Like Neanderthals, the Sima de los Huesos hominins may have eaten a diet high in meat, similar to lions. Galería may have also been a natural trap, and humans may have visited it occasionally to collect animals that fell in. TD10.1 was likely occupied for a long time, with people taking the most nutritious parts of prey back to the cave.
The Sima de los Huesos hominins lived during one of the coldest periods in the last million years, which made Iberia very dry. Winter temperatures in the Atapuerca Mountains today can reach −10 degrees Celsius (14 degrees Fahrenheit), and it may have been colder during glacial times. The animal remains suggest an open woodland environment similar to a savannah. Humans and lions likely followed Europe’s expanding open-woodland areas, and pollen evidence shows grasses were widespread. Pine trees, grasses, and plants like oak, birch, and beech were common. Deep scratches on teeth indicate the hominins often chewed abrasive foods like unwashed roots and stems. Female hominins had longer scratches on their lower teeth, suggesting their diet may have differed from males.
No stone tools were found in Sima de los Huesos, and it was probably never inhabited. The Galería and Gran Dolina sites, however, had many stone tools. Knapping techniques (methods of shaping stone) evolved over time, with Galería tools fitting into the Acheulean industry (a Lower Paleolithic technology). TD10.1 may mark a shift to the Mousterian industry (a Middle Paleolithic technology linked to Neanderthals).
Galería had few tool scraps, suggesting tools were mostly made offsite with only quick adjustments done there, indicating it was a temporary base camp. In GIIa (the older part), tools were simple flakes and unmodified rocks, with retouched tools made of chert and quartz. Large cutting tools like handaxes and cleavers were mostly made of quartzite, and cobble (a type of rock) was used for larger tools. In GIIb, more planning was involved, with tools made from pre-prepared flakes instead of cobble, and quartzite was replaced by sandstone, chert, and limestone. This suggests a different group may have lived in GIIb. These trends continued into GIll, where tools were shorter and wider, and fewer, more efficient strikes were used.
TD10.1 has over 20,000 stone tools, making it one of the richest sites in the Atapuerca Mountains. This likely reflects both long-term and short-term occupation. Flakes and tool scraps are more common than large cutting tools, and all stages of tool-making are present. Tools were mostly made of chert, but sandstone and quartzite became more popular. Some chert came from a source 10 km (6.2 miles) away, which is surprising for early humans. These tools were likely used for butchering, hide-working, and woodworking. Dental wear shows people used their mouths to bite materials like animal hides or plant fibers while cutting with tools.
Knapping techniques were not standardized, but changes in raw materials led to less use of the longitudinal method (striking a core parallel to its length) and more use of the orthogonal method (striking perpendicular to the core). In Galería, the centripetal method (striking from the edge inward) was used with the longitudinal method. In TD10.1, the centripetal and discoidal methods (associated with the Middle Paleolithic Levallois technique) became more common, showing more control over tool shapes.
The Sima de los Huesos hominins likely did not use fire, relying on their large body size and high activity levels to stay warm. Evidence of fire use in Europe appears later, after an interglacial period (Marine Isotope Stage 9, about 340,000 years ago). A site in Barnham, Suffolk, has burned tools and iron pyrite (used historically to start fires), suggesting some European groups could create fire, though this remains debated for non-modern humans.
The way the Sima de los Huesos hominins’