Sima de los Huesos, a Key to Human Evolution

The Sima de los Huesos (Pit of Bones in Spanish and typically abbreviated as SH) is a lower Paleolithic site, one of several important sections of the Cueva Mayor-Cueva del Silo cave system of the Sierra de Atapuerca in north-central Spain. With a total of at least 28 individual hominid fossils now firmly dated to 430,000 years old, SH is the largest and oldest collection of human remains yet discovered. Site Context The bone pit at Sima de los Huesos is at the bottom of the cave, beneath an abrupt vertical shaft measuring between 2-4 meters (6.5-13 feet) in diameter, and located about .5 kilometers (~1/3 of a mile) in from the Cueva Mayor entrance. That shaft extends downward approximately 13 m (42.5 ft), ending just above the Rampa (Ramp), a 9 m (30 ft) long linear chamber inclined about 32 degrees. At the foot of that ramp is deposit called the Sima de los Huesos, a smoothly oblong chamber measuring 8x4 m (26x13 ft) with irregular ceiling heights between 1-2 m (3-6.5 ft). In the roof of the eastern side of the SH chamber is another vertical shaft, which extends upwards some 5 m (16 ft) to where it is blocked by cave collapse. Human and Animal Bones The sites archaeological deposits include a bone-bearing breccia, mixed with many large fallen blocks of limestone and mud deposits. The bones are mainly composed of at least 166 Middle Pleistocene cave bears (Ursus deningeri) and at least 28 individual humans, represented by more than 6,500 bone fragments including over 500 teeth alone. Other identified animals in the pit include extinct forms of Panthera leo (lion), Felis silvestris (wildcat), Canis lupus (grey wolf), Vulpes vulpes (red fox), and Lynx pardina splaea (Pardel lynx). Relatively few of the animal and human bones are articulated; some of the bones have tooth marks from where carnivores have chewed on them. The current interpretation of how the site came to be is that all the animals and humans fell into the pit from a higher chamber and were trapped and unable to get out. The stratigraphy and layout of the bone deposit suggest  the humans were somehow deposited in the cave before the bears and other carnivores. It is also possible—given the large amount of mud in the pit—that all the bones arrived in this low place in the cave through a series of mudflows. A third and quite controversial hypothesis is that the accumulation of human remains might be the result of mortuary practices (see the discussion of Carbonell and Mosquera below). The Humans A central question for the SH site has been and continues to be who were they? Were they Neanderthal, Denisovan, Early Modern Human, some mixture we havent yet recognized? With the fossil remains of 28 individuals who all lived and died about 430,000 years ago, the SH site has the potential to teach us a great deal about human evolution and how these three populations intersected in the past. Comparisons of nine human skulls and numerous cranial fragments representing at least 13 individuals were first reported in 1997 (Arsuaga et a.). A large variety in cranial capacity and other characteristics were detailed in the publications, but in 1997, the site was thought to be about 300,000 years old, and these scholars concluded that the Sima de los Huesos population was evolutionarily related to Neanderthals as a sister group, and could best fit into the then-refined species of Homo heidelbergensis. That theory was supported by results from a somewhat controversial method redating the site to 530,000 years ago (Bischoff and colleagues, see details below). But in 2012, paleontologist Chris Stringer argued that the 530,000-year-old dates were too old, and, based on morphological attributes, the SH fossils represented an archaic form of Neanderthal, rather than H. heidelbergensis. The latest data (Arsuago et al 2014) answers some of Stringers hesitations. Mitochondrial DNA at SH Research on the cave bear bones reported by Dabney and colleagues revealed that, astonishingly, mitochondrial DNA had been preserved at the site, much older than any other found to date anywhere. Additional investigations on the human remains from SH reported by Meyer and colleagues  redated the site to closer to 400,000 years ago. These studies also supply the surprising notion that the SH population shares some DNA with the  Denisovans, rather than the Neanderthals they look like (and, of course, we dont really know what a Denisovan looks like yet). Arsuaga and colleagues reported a study of 17 complete skulls from SH, agreeing with Stringer that, because of numerous Neanderthal-like characteristics of the crania and mandibles, the population does not fit the  H. heidelbergensis  classification. But the population is, according to the authors, significantly different from other groups such as those at Ceprano and  Arago  caves, and from other Neanderthals, and Arsuaga and colleagues now argue that a separate taxon should be considered for the SH fossils. Sima de los Huesos is now dated to 430,000 years ago, and that places it close to the age predicted for when the split in hominid species creating the Neanderthal and Denisovan lineages occurred. The SH fossils are thus central to the investigations concerning how that might have happened, and what our evolutionary history might be. Sima de los Huesos, a Purposeful Burial Mortality profiles (Bermudez de Castro and colleagues) of the SH population show a high representation of adolescents and prime-age adults  and a low percentage of adults between 20 and 40 years of age. Only one individual was under 10 at the time of death, and none were over 40-45 years old. Thats confusing, because, while 50% of the bones were gnaw-marked, they were in fairly good condition: statistically, say the scholars, there should be more children. Carbonell and Mosquera (2006) argued that Sima de los Huesos represents a purposeful burial, based partly on the recovery of a single quartzite  Acheulean handaxe  (Mode 2) and the complete lack of lithic waste or other habitation waste at all. If they are correct, and they are currently in the minority, Sima de los Huesos would be the earliest example of purposeful human burials known to date, by ~200,000 years or so. Evidence suggesting that at least one of the individuals in the pit died as a result of interpersonal violence was reported in 2015 (Sala et al. 2015). Cranium 17 has multiple impact fractures which occurred near the moment of death, and scholars believe this individual was dead at the time s/he was dropped into the shaft. Sala et al. argue that placing cadavers into the pit was indeed a social practice of the community.   Dating Sima de lost Huesos Uranium-series and Electron Spin Resonance dating of the human fossils reported in 1997 indicated a minimum age of about 200,000 and a probable age of greater than 300,000 years ago, which roughly matched the age of the mammals. In 2007, Bischoff and colleagues reported that a high-precision thermal-ionization mass spectrometry (TIMS) analysis defines the minimum of deposits age as 530,000 years ago. This date led researchers to postulate that the SH hominids were at the beginning of the  Neanderthal  evolutionary lineage, rather than a contemporary, related sister group. However, in 2012, paleontologist Chris Stringer argued that, based on morphological attributes, the SH fossils represent an archaic form of Neanderthal, rather than  H. heidelbergensis, and that the 530,000-year-old date is too old. In 2014, excavators Arsuaga et al reported new dates from a suite of different dating techniques, including Uranium series (U-series) dating of speleothems, thermally transferred  optically stimulated luminescence  (TT-OSL) and post-infrared stimulated luminescence (pIR-IR) dating of sedimentary quartz and feldspar grains, electron spin resonance (ESR) dating of sedimentary quartz, combined ESR/U-series dating of fossil teeth, paleomagnetic analysis of sediments, and biostratigraphy. Dates from most of these techniques clustered around 430,000 years ago. Archaeology The first human fossils were discovered in 1976, by T. Torres, and the first excavations within this unit were conducted by the Sierra de Atapuerca Pleistocene site group under the direction of E. Aguirre. In 1990, this program was undertaken by J. L. Arsuaga, J. M. Bermudez de Castro, and E. Carbonell. Sources Arsuaga JL, Martà ­nez I, Gracia A, Carretero JM, Lorenzo C, Garcà ­a N, and Ortega AI. 1997.  Sima de los Huesos (Sierra de Atapuerca, Spain). The site.  Journal of Human Evolution  33(2–3):109-127. Arsuaga JL, Martà ­nez, Gracia A, and Lorenzo C. 1997a.  The Sima de los Huesos crania (Sierra de Atapuerca, Spain). A comparative study.  Journal of Human Evolution  33(2–3):219-281. 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