Sima de los Huesos hominins

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 1,241 cc (75.7 cu in), 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 170 cm (5 ft 7 in) and a weight of 90 kg (200 lb) 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 Jesús Carballo García 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 13 m (43 ft) 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).

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