The Sima de los Huesos hominins are a 430,000-year-old population of pre-Neanderthals from the archeological site of Atapuerca, Spain. They are in the "Neanderthal clade", but fall outside Homo neanderthalensis. When first reported in 1993, these 29 individuals were about 80 percent of the human fossil record of the Middle Pleistocene. Every bone is preserved, and the unprecedented completeness of the remains sheds light on Neanderthal evolution, the classification of contemporary fossils, and variations in a single Middle Pleistocene population. Exhumation of the Sima de los Huesos hominins began in the 1980s under the direction of Emiliano Aguirre and, later, Juan Luis Arsuaga, Eudald Carbonell, and José María Bermúdez de Castro.
They have a mosaic of "classic" Neanderthal traits (apomorphies) and more "archaic" traits (plesiomorphies). Like Neanderthals, the brow ridges are inflated; the skull is not as robust in the rear, however, and has a pointed "house-like" profile instead of a rounded "bomb-like" one. Brain volume averages , on the lower end of the Neanderthal range. The teeth are Neanderthal-like, with shovel-shaped incisors and taurodontism, but differ in cusp morphology. The chest and waist are broad and robust like Neanderthals, but the limbs are longer. They may have been large-bodied overall, like other archaic humans, with a height of about and a weight of in both sexes.
The Sima de los Huesos (bone pit) is a chamber in the Cueva Mayor–Cueva Silo cave complex at Atapuerca, and may have been a natural trap for the cave bear Ursus deningeri in particular. The Sima de los Huesos hominins may have been deposited in the pit by other humans, based on the quality of preservation and the predominance of adolescents and young adults over children and the elderly. All were buried at about the same time, and one individual may have been killed with a blunt tool. Some with severe health issues survived for a while, suggesting group care. Many individuals (especially adolescents) exhibited metabolic and nutritional diseases consistent with insufficient fat reserves during hibernation; any hibernation presumably lasted for four months.
This population produced Acheulean stone tools, as well as an industry apparently transitioning to the typically-Neanderthal Mousterian culture. They used these tools in butchering and hide- and woodworking, with the mouth as a third hand. The Sima de los Huesos hominins were buried with a single, large Acheulean handaxe, possibly a grave good with symbolic significance. Symbolic thought could indicate use of an early form of language. They may have been efficient hunters — possibly outcompeting local cave hyenas — who pursued deer, rhinoceros, horses, bison, and (sporadically) cave lions in an open woodland environment. They probably ate roots regularly, habitually squatted, and did not use fire.
Research history Discovery
Spanish engineers P. Sampayo and M. Zuaznávar published an 1868 monograph documenting the caves in the Atapuerca Mountains, noting reports of human fossils in one (Cueva Ciega). Spanish archeologists and Saturio González discovered a cave painting of a horse in Cueva Mayor in 1910, and the area quickly attracted international interest. The eventually-abandoned construction of a railway supplied by material mined from what became known as the Trinchera del Ferrocarril (railway trench) revealed that the Cueva Mayor was part of a larger cave complex: the Cueva Mayor–Cueva Silo complex. Its archeological significance was first understood in 1962 after exploration by the Edelweiss Speleological Group, which identified several sites with Neolithic, Chalcolithic, and Bronze Age artifacts. Trinidad Torres unsuccessfully searched for bear fossils in the Trinchera del Ferrocarril while working on his doctoral thesis, but was pointed by the Edelweiss Spelaeological Group to the unmapped Sima de los Huesos (bone pit) in the Cueva Mayor–Cueva Silo complex. Sampayo and Zuaznávar had marked this as El Silo (storage pit), a chamber down a vertical shaft in the corner of the spacious Sala de los Cíclopes.
Torres sent four speleologists who identified a wealth of cave-bear fossils and a nearly-complete human-jaw fossil. He notified his PhD advisor, Emiliano Aguirre, who organized further digs. The Sima de los Huesos was difficult to access, low in oxygen and had been disturbed by many visitors over the decades looking for bear-fossil trophies, requiring much more excavation to remove the disturbed sediment (revuelto). In 1983, Aguirre found three fossil human teeth (and more bear fossils) in his brief visit to the Sima de los Huesos. Removal of sediment began the following year after lights and other necessary infrastructure were installed. In 1987, Aguirre's team installed suspended scaffolding over the floor of the Sima de los Huesos. Large sediment blocks were transported up the vertical shaft (the only entrance), carried from Cueva Mayor with backpacks, driven to the nearby Arlanzón River, dried, sieved, and sorted. The Atapuerca team under Aguirre had reconstructed three crania, one of which was nicknamed "Lazarus" (the first fossil from the site given a nickname).
Aguirre retired in 1990, and left the excavation to his three team leads: Juan Luis Arsuaga, Eudald Carbonell, and José María Bermúdez de Castro. In 1992, a nearly-complete skull (nicknamed "Agamemnon") was found; at the time, only three other European skulls predating the Late Pleistocene had approached such a level of preservation: Swanscombe, Petralona, and Steinheim. Soon afterwards, another nearly-complete skull was discovered; it was nicknamed "Miguelón", after Spanish cyclist Miguel Indurain. It is one of the best preserved skulls in the human fossil record. In 1994, a nearly-complete pelvis ("Elvis") was discovered; at this time, the only similarly-preserved hominin pelvis predating the Late Pleistocene was the 3.2 million-year-old Lucy.
Over 7,000 human fossils and fragments have been recovered from the Sima de los Huesos since then, representing every bone in the skeleton. The material was first thought to be from 32 individuals, but it is more likely 29. It was the most complete sample of the Middle Pleistocene human fossil record from a single site in 1993, 80 percent of the global Middle Pleistocene human fossil record and 85 percent of Middle Pleistocene postcranial (body) fossils. A similarly-rich Middle Pleistocene human fossil collection was found in 2013 with the discovery of Homo naledi in South Africa. The Sima de los Huesos fossils illustrated the range of anatomical variation that could be present in a given population of Middle Pleistocene Europe across gender and age lines, clarifying the relationships among Middle Pleistocene European fossils and the evolution of Neanderthals. The archeological significance of the Sima de los Huesos and other sites in the cave complex led to the declaration by UNESCO of the archeological site of Atapuerca as a World Heritage Site on 30 November 2000. The city of Burgos, about east, opened the Museum of Human Evolution to commemorate its cultural heritage.
Dental individual Cranium Sex AgeIFYoung adultIIImmature (age 12.9–13.6 years)IIIFImmatureIVFMiddle-agedV15Middle-agedVIFYoung adultVIIMYoung adultVIIIMYoung adultIXImmatureXFImmatureXIFImmatureXIIMYoung adultXIIIMiddle-agedXIVImmatureXV17FYoung adultXVI9FImmature (age 12.6–16.3)XVIIMYoung adultXVIIIMImmature (age 8.7–12.8)XIXFYoung adultXX6MImmature (age 13.3–16.1)XXI5MMiddle-agedXXIIMYoung adultXXIII16FImmature (age 16–17)XXIVFImmature (age 12.6–15.4)XXVFImmature (age 9.6–15.4)XXVI10FYoung adultXXVIIMYoung adultXXVIIIFYoung adultXXIXFImmature
Age and stratigraphy
The Cueva Mayor–Cueva Silo complex is the main cave system of the Atapuerca Mountains, extending over with three levels; Sima de los Huesos is on the lowest one. There are four entrances: Portalón (the entrance to Cueva Mayor) and Galería de las Estatuas on the first level, Sima del Elefante on the second, and Cueva del Silo on the third. Cueva del Silo is the nearest entrance to Sima de los Huesos, via the large Sala de los Cíclopes.
The Sima de los Huesos is underground, and from the entrance to Cueva Mayor. It is long east-to-west, with a mostly horizontal segment (Sima Top), a 30-degree ramp (Sima Ramp) running down , and another horizontal segment (Sima de los Huesos). The cave infill is divided into 12 lithostratigraphic units (LU). The ramp has three pits where fossils were collected: Alta (SRA), Media (SRM), and Baja (SRB). There are three chimneys: Chimney 1 between Alta and Sima Top, Chimney 2 between Media and Alta, and Chimney 3 above Sima de los Huesos. Only Chimney 2 is open, leading into the Sala de los Cíclopes on the west and a low inclined conduit on the east. It is the only entrance. Most of the human and carnivore fossils come from a section on the north side near the base of the ramp, in Sima de los Huesos. Over 100 human fossils were also found along the ramp. A few human fossils were recovered from where an eroded area, which were probably moved.
LU-6 is a red clay layer of variable thickness, up to . It is over 80 percent phyllosilicates, of which more than 60 percent is illite which is weakly magnetic. Paleomagnetic dating of the illite indicates that the layer is younger than the Brunhes–Matuyama reversal (when the Earth's magnetic field last experienced a geomagnetic reversal), about 780,000 years ago. The fossils are mixed with pieces of limestone, speleothems (mineral deposits, probably moved from LU-2 and 4), marl clay chips (possibly from LU-1), and sometimes laminated mud rich in manganese dioxide, forming a bone breccia. There are also in situ (not moved) speleothems made of calcite rafts formed from undisturbed, seasonal pools of water; uranium–thorium dating of the rafts suggests a minimum age of 410,000–460,000 years (probably about 430,000 years ago). The overlying LU-7 is dated to 428,000 ± 27,000 to 441,000 ± 25,000, or 396,000 ± 35,000 to 429,000 ± 32,000 years ago, based on luminescence dating of orthoclase and quartz respectively. Luminescence dating of quartz grains from the underlying red clay suggests a maximum age of 448,000 ± 15,000 years ago.
The human material probably represents a single population. Limited carnivore damage suggests that the human fossils were largely inaccessible after burial. Only Chimneys 1 and 2 (via the inclined ramp) may have been open to the surface at the time of deposit, acting as natural traps for cave bears (Ursus deningeri) in particular. The humans may have been intentionally cast down, and the corpses and bones probably slid slowly down the ramp and collected at the bottom.
Similar fossils have also been recovered from the GII and GIII layers of Trinchera Galería (a partial adult mandible with two molars and an adult braincase fragment, respectively) about from the Sima de los Huesos. This sequence extends continuously from 408,000–598,000 to 221,000–269,000 years ago. The roughly 300,000-year-old Trinchera Dolina 10.1 layer in the Gran Dolina, about north of Galería, has a rich lithic assemblage and were probably occupied by a population similar to the Sima de los Huesos hominins.
Classification