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WERE THERE RITUAL BURIALS IN THE SIMA DE LOS HUESOS OUTCROP? (Atapuerca Range, Burgos, Spain)

(modified 11/01/2013 to Geobios) WERE THERE RITUAL BURIALS IN THE SIMA DE LOS HUESOS OUTCROP? (Atapuerca Range, Burgos, Spain) David Rabadà i Vives Museu de Geologia del Seminari de Barcelona Diputació, 231 08007 Barcelona, Spain ABSTRACT The Sima de los Huesos site, Atapuerca, contains an important sample of fossilised human bones of Homo heidelbergensis. The nature of the Sima de los Huesos human remains was interpreted as human burials but geological, taphonomical and paleoecological data revision have given rise to a new interpretation. According to this new data revision this outcrop was originated by different taphonomical mechanisms. Competition between Homo and other predators for the cavity, accidental death by falling into the pit and a feeding trough for felines and canines while the cave had other entrances blocked nowadays, could explain this fossil association. The bears could fall by accident or die while hibernating there. Homo could suffer the same fate but they were more probably victim of large felines or others that carried the corpses inside or around the pit. Foxes and other scavengers came later for feeding. The water flows in the karst during heavy rainfalls produced dispersion, mixing and abrasion in all these skeletal remains. During all these processes a low sedimentation rate or a continuous supply of corpses could produce the observed fossil concentration. Key words: Taphonomy, human burials, Homo heidelbergensis, Sima de los Huesos, Atapuerca, Pleistocene. INTRODUCTION The Pleistocene site of Sima de los Huesos at Atapuerca contains the highest concentration of primitive Neanderthals or Homo heidelbergensis around the world. In fact, the most appropriate name for this lineage is Homo neanderthalensis, although it is also possible to admit, only for practical reasons, the succession of two chronospecies, Homo heidelbergensis and Homo neanderthalensis (Bermúdez de Castro et al., 2004). In Sima de los Huesos site more than thirty-two individual remains were found in only four cubic meters of sediments. This condensation of hominid fossils was explained as human burials by this primitive Homo who threw their dead relatives to the cave site as a symbolical act (Arsuaga & Martínez, 1999; Arsuaga, 1999; Carbonell et al., 2003; Carbonell & Mosquera, 2006). If this hypothesis is true we would be at the first human funeral rite. This paper attempts to deconstruct this interpretation reviewing the current references about the site in three aspects, its taphonomy, its paleoecology and its geology. At the end we will join them to discuss a new interpretation. The Sima de los Huesos fossiliferous site was originated between 205,000 and 325,000 years ago (Parés et al., 2000) but other investigations talk about 400,000 and 500,000 years ago (Bischoff et al., 2003; Bermúdez de Castro et al., 2003). This fossil site is located at the end of a dark gallery of 400 meters where it opens a chasm of 13 meters high as cave profiles have been shown by different authors (Arsuaga et al., 1993; Arsuaga et al., 1997; García & Arsuaga, 2011). Inside the cave only predators have been identified including a group of hominids. These were attributed initially to archaic Homo sapiens (Arsuaga et al., 1993; Bermúdez de Castro et al., 1997) but later they were identified as Homo heidelbergensis (Pérez et al., 1999). The high variability found in the Sima de los Huesos site indicates that Neanderthals contain the old heidelbergensis without any taxonomical problem (Arsuaga & Martínez, 1999). At the moment these authors keep the name Homo heidelbergensis instead of Neanderthal in their publications for practical reasons but not taxonomical (Arsuaga & Martínez, 1999; Bermúdez de Castro et al., 2004). The primitive Homo neanderthalensis concentration in the Sima de los Huesos site is associated with bone remains of other carnivores, especially bears and foxes (Arsuaga & Martínez, 1999). The absence of herbivore remains was considered a difficult question to resolve but related with human burials (Arsuaga, 1999). The problem of Sima de los Huesos site is the absence of similar outcrops with equivalent geology and fossil assemblage. Therefore it is very difficult to distinguish natural bone accumulations from human burials. The main findings about treatment of the dead during the Pleistocene age come from repeated studies of cut marks, scraping marks, bone breakage of human bones regarded as purposive mortuary practices (Ullrich, 1999) but all these factors are not observed at the Sima de los Huesos site. The occurrence and situation of this outcrop has very few known parallels for the Palaeolithic record. The only remotely analogous finding might be the Cueva del Angel, near Córdoba in Andalusia (Barroso et al., 2011), where a very deep cleft has been identified with animal food refuse, hand axes and human remains inside. At any rate, Sima de los Huesos site only contains human and carnivores remains while Cueva del Angel represents something very different. The assemblage of large herbivores in the Cueva del Angel corresponds to an accumulation of anthropic origin during a long period from the end of the Middle Pleistocene to the beginning of the Upper Pleistocene where humans brought large quantities of meat into the cave from essentially horses and bovids in the form of dismembered and cut (Barroso et al., 2011). Cueva del Angel fossil assemblage shows an appreciable number of cutmarks and striations (9% of the material) related to defleshing, filleting and disarticulation, and high proportion of burnt elements (88% of the material) while this characteristics doesn’t exits at Sima de los Huesos fossil assemblage. Therefore this outcrop hasn’t direct taphonomical relation with Sima de los Huesos outcrop. Cueva del Angel was a site of intense and continuous occupation for butchering and cooking of animal meat resources predated and transported into the cave by humans and Sima de los Huesos wasn’t it. TAPHONOMIC DATA If we want to understand fossil assemblages inside caves there are two important taphonomical processes, the first involves only presence of vertebrate fossils in their sediments and the second generates skeletal concentrations. The Sima de los Huesos outcrop shows a high quantity of bone remains with excellent bone preservation. Therefore, the discussion about this site has to be focused on concentration mechanisms of bones. There are many causes which accumulate skeletal remains in caves and caverns (Weigelt, 1989; Brain, 1958; Behrensmeyer, 1978; Brain, 1981; Andrews, 1990; Domínguez-Rodrigo, 1994). The presence of vertebrate fossils in caves can be caused by rodents or predators carrying the bones, hibernation, reproduction, natural traps, mud flows, debris flows, sudden death by karst collapse, flooding inside cavity or illness of a group of organisms. All these mechanisms have explained the presence of skeletal remains inside caves, but not necessarily its concentration. There are three contexts which can produce this fact. First context is continuous supply of corpses for a short period of time (Andrews, 1990), second is eventual supplies for a long time (Andrews, 1990) and third a low sedimentation rate (Rabadà, 1990). Combinations of these three contexts accumulate and concentrate bones inside caves. If we want to know what happened in the Sima de los Huesos site we have to consider the characteristics of their human remains. At the moment human remains are mixed with others mammals bones and without anatomical connections (Arsuaga et al., 1993) and complete human skeletons were computed from tooth, jaws and limbs (Arsuaga et al., 1997). For instance, the human remains from Sima de los Huesos, but never entire bodies, have allowed to reconstruct 27 complete long bones to estimate for each bone an average body stature about 163.6 cm for the Sima de los Huesos hominids (Carretero et al., 2012), therefore there aren’t complete human skeletons with anatomical connections in the Sima de los Huesos outcrop until today. The total number of skeletal parts and the inferred number of human individuals show us how partial is the Sima de los Huesos fossil record. The thirty-two individuals were identified from only 1300 skeletal parts but, the total number of skeletal parts of thirty-two heidelbergensis should be much higher, more than 3.000. This fact presupposes two things, the first more than sixty percent of the original remains is missing, especially skulls, phalanges and vertebrae (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001), and the second this association fossil shows a dispersion of large bones. If human remains were carried as a sepulchral rite we have to suppose that these were whole bodies and not parts of them. Therefore, we should find dispersion and dismantling lower than this sixty percent of missing remains. On the other hand, there are many cranial remains with abrasions and fractures. Some authors have suggested that these injuries were caused by impacts of stones when they were fighting or playing to each other (Arsuaga, 1999), but it seems more logical that these scars were produced when falling into the Sima de los Huesos pit or when water currents reworked the human remains because there are no signs of cranial healing. Weigelt (1989) described bone trauma in fallen animals in potholes as Atapuerca fossil association seems to indicate. But there is a problem with this interpretation. When some animals fall in a place inaccessible to predators they tend to generate fossil associations without bite marks. The Sima de los Huesos human remains show many bite marks on the majority of bones (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001). The abundance of bears found with humans might suggest that these predators perpetrated these bite marks (Rabadà, 2001), but bears chew bones producing a characteristic pattern with rounded epiphyses (Weigelt, 1989; Domínguez-Rodrigo, 1994) which is not found on the association of the Sima de los Huesos human fossils but is found in bears’ skeletons and it has been conclusively demonstrated that the bears entered the Sima separately from the humans (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001) because there are bite marks from bears on bears bones but not over human bones (Fernández-Jalvo, 2003) and therefore bears didn’t produce the human bone accumulation in the Sima de los Huesos site. On the other hand, a very close relative of the extinguished bear cave, the current European bears, eats its preys outside caves without moving them to the cavities. In fact, they used to hibernate inside caves where they sometimes died and ate themselves producing the rounded epiphyses. This process could explain the accumulation of bear bones inside cavities (Haynes, 1983; Weigelt, 1989; Domínguez-Rodrigo, 1994), although the work done on these in the Sima (García & Arsuaga, 1997; García, 2003) was mainly taxonomic, and it did not provide convincing evidence as to how bears entered to the Sima. At the moment we can say that bears didn’t produce the human bone accumulation in the Sima de los Huesos site but they ate themselves inside the cave. If we want to understand human bones concentration at Sima de los Huesos site we need to study the Atapuerca paleoecological data for answering who caused this human remains accumulation. PALAEOECOLOGICAL DATA The Sima de los Huesos fossil association contains only predators, but not herbivores. The main Atapuerca predators with 50 percent of individuals estimated are bears, followed by humans with 11 percent and foxes with 8 percent. The rest, felines, wolves and weasels are a minority (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001). Therefore, the Sima de los Huesos fossil association has a general absence of herbivores and a clear predominance of bears, hominids and foxes. The absence of herbivores in the old Atapuerca ecosystem was impossible. Something avoided his presence inside the cave. We will try to explain it later. Another important aspect is the close relation between bears and humans. These animals and Homo heidelbergensis or neanderthalensis used caves as a temporary shelter for millennia, bears as a hibernation place and humans as a campsite. When humans stayed in cavities, they carried preys inside for consumption, producing many cut marks on the bones and collections of stone tools as Cueva del Angel (Barroso et al., 2011). This fact doesn’t happen in the Sima de los Huesos site (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001), although one hand axe was found as a symbolic and ritual object (Carbonell et al., 2003; Carbonell & Mosquera, 2006) but we will see later that it was a reworked element. Therefore, the common situation between bears and hominids was their specialized omnivorous and carnivorous strategies as predators, their competing for similar food resources and caves as a resting place. The Atapuerca range represented an area of ecological competition between these two species. Homo, being nomadic, periodically revised the cave entry for various uses while bears hibernated in some cavity near the Sima every year and perhaps some of them fell inside the pit. It has been observed chasms trap other predators when they come to inspect a cave for eating falling corpses (White et al., 1984). The same could have happened if Homo tried to inspect the Sima de los Huesos. We have to remember that this pit is located at the end of a long and dark gallery (Arsuaga et al., 1993; Arsuaga et al., 1997; García & Arsuaga, 2011). In other words, bears and hominids shared the same accident risk under the same competition for habitat and this situation could caused the accumulation of bears and human corpses in the pit (Rabadà, 2001). The only hand axe found in the Sima de los Huesos could be a tool brought by a Homo near the cave and reworked with the rest of skeletal remains. But, this mechanism did not introduce the majority of corpses inside the site. In fact, the distribution of human ages shows new information and involves other interpretation. Fossils can reflect the mortality rate of the original population. Infant mortality rate in current hunter-gatherer is very high followed by old people (Jones et al., 1992; Howell, 1979) which also occurred in Neanderthal populations (Trinkaus, 1995). If the Sima de los Huesos site was generated as intentional burial, it should contain a high number of children and elders, but according to published data, 52 percent of human remains were adolescents and young adults. On the other hand, 60 percent of them were less than 19 years old and 90 percent less than 27 (Bermúdez de Castro & Nicolas, 1997; Arsuaga, 1999). There are only the testimonial presence of one individual under the age of 10 years in the Sima de los Huesos site (Bermúdez de Castro et al., 2004). The fragile and delicate remains of infants and children are more severely affected by the action of biostratinomic and fossildiagenetic agents than bones from adolescents and adults. Moreover caves minimize weather action (Hill, 1979; Haynes, 1980). Therefore the fossil preservation in Sima de los Huesos site was excellent and the distribution of human ages should be equivalent with the original corpses number. Therefore, humans between 0 to 11 years old and over 27 were underrepresented in the Sima de los Huesos site. This predominance of middle age and scarcity of infants and old individuals doesn’t indicate a mortality rate, but something very different, an accident risk rate (Rabadà, 2001). Teens and young adults tend to be fearless and they go away from households more than babies and old people. This fact involves high risk of contingencies for young humans but this accident was not falling inside the hole because the low skeletal fractures rate observed invalidates this hypothesis (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001). Thus, the main input mechanism of human remains was another. For instance, other authors discarded the hypothesis that the site was a primitive cemetery but they suggested instead that the mortality distribution could be the result of a catastrophe as a sudden death for all the Sima de los Huesos human individuals, although they did not specify the origin of the accumulation and didn’t explain this human remains concentration (Bocquet-Appel & Arsuaga, 1999). The last reasoning involves a paradox and a new question, did humans die suddenly in the pothole while many bears, foxes and others didn’t it? In fact, the percentage of bones shows us a new interpretation. There is a clear predominance of arms and legs remains in the Sima de los Huesos site. The fossil assemblage shows high amount of humeri and tibiae which can indicate accumulation by scavengers. Teeth, jaws and limb bones as femurs, humeri and tibiae are more frequent than ribs, vertebrae and metacarpials, which are very scarce. All these percentages involve a partial transport of bones at Sima de los Huesos site (Andrews & Fernández-Jalvo, 1997, Fernández-Jalvo & Andrews, 2001). The fact that some 60 % of the bodies’ bones are missing, particularly vertebra, cranial elements and phalanges without presence of coprolits (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001) seems to support that the Sima de Los Huesos fossil human accumulation had little to do with ritual burials that involved carrying of the corpses. Human phalanges remains are scarce, which seems to buttress some selective transport. Perhaps lions, leopards and hyenas produced the first bone human accumulation because limbs contain a large quantity of meat for feeding while ribs, vertebrae and metacarpials don’t (Bailey, 1993). Metacarpials contains relatively little nutritional value, carnivores rarely fragment them. Phalanges are nearly absent at many modern hyena dens, and fossil carnivore accumulations often exhibit low numbers of phalanges (Hutson, 2008). Moreover fifty percent of the human remains at Sima de los Huesos are affected by bite marks, especially femora at 96 percent and, in general, on limb bones. The Sima de los Huesos site contains a bone-bearing breccia with clayish matrix mainly composed of Ursus deningeri (MNI: 176), Vulpes vulpes (MNI: 21), Canis sp. (MNI: 1), Panthera leo cf. fossilis (MNI: 3), Panthera sp. (jaguar-size; MNI: 1), Felis silvestris (MNI: 1), Lynx pardinus spelaeus (MNI: 2) Martes sp. (MNI: 1), Mustela nivalis (MNI: 2), Mustela putorius (MNI: 3) and Meles meles (MNI: 1) (García, 2003). According with this last list, large carnivore as felines was present as the first producers of Sima de los Huesos human remains accumulation. Opportunistic predators as lion or leopard used to concentrate bone preys. Since Pleistocene hominid hunter-gatherers are regarded as apex carnivores in a competitive carnivore guild (Eaton, 1994). The total of some 32 individuals at Sima de los Huesos site as preys would not be out of character with a comparatively low rate of predation on ancient humans by medium or large carnivores at the site. Therefore large carnivores could hunt human individuals during Pleistocene and many carnivore species was involved. Spotted hyenas, and probably their Pleistocene ancestors, Crocuta spelaea, are known to dig up buried humans in Ethiopia, and equivalent examples come from some Late Pleistocene caves with Neanderthal remains in Southwest France, and also in Monte Circeo, Italy. Moreover hyenas use to break bones with high intensity (Palmqvist et al., 2011). According to low fracture level in the Sima de los Huesos fossil association perhaps hyena was not the main bone-cracking producer. On the other hand, the carnivore taxonomic diversity found in Sima de los Huesos is particularly high likely corresponding to high ecosystem productivity, although the absence of specialized large scavengers, such as hyaenas, is striking (García, 2003). Therefore we have to suppose that there was high carnivore biodiversity at Atapuerca range. In any case, carnivore who made bite marks over human remains at Sima de los Huesos site is not clear (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001) but some authors think that Panthera leo fossilis and Vulpes vulpes were probably carnivores which produce the main bite marks (Bermúdez et al., 2004). At the moment we can say that incisions over human bones were caused by wolves, foxes and some big feline that had access to the remains before canines (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001). Therefore, a big feline ate the human remains inside or near the cave. The problem is, how are they so scarce in Sima de los Huesos shaft? In addition, the most important thing, how did they come in the pit and go out from it if Homo neanderthalensis threw the corpses inside the chasm as a ritual demonstration? GEOLOGICAL DATA Some felines or others carnivores, could transport the human remains of the Sima de los Huesos site as prey. This fact raises two questions. First, how did the concentration of bones happen in the cavity? Second, how did predators come in and go out from this pit? The concentration of skeletal remains in caves may be due to continuous supply of corpses for a short period of time, eventual contribution for a long time or low sedimentation rate. The first, continuous supply of dead bodies is common in colonial animals. An example of this are bats (Andrews, 1990), but the hunter-gatherers current population density is very low (Jones et al., 1992; Howell, 1979) as well as social predators as wolves and felines (Vicente et al., 1999). Predators need large areas for survival which involves a very low population density. On the other hand, the absence of cut marks in Sima de los Huesos bones indicates that there was not a substantial hominid colony inside or near the pit. Therefore, the concentration of corpses in the cave was given by other processes. The second reason for the concentration of bones, slight remains contribution for long periods, has many examples like feeding regurgitation by owls and eagles (Andrews, 1990), feeding troughs (Brain, 1958), falls into trap chambers (Morris, 1974; White et al., 1984; Andrews, 1990), troughs (Vrba, 1980) and hibernating places (Kurtén, 1958; Kurtén, 1976). Falls inside the pit and feeding troughs were an easy interpretation for human remains in the Sima de los Huesos outcrop. The cave morphology with a chasm at the end of a dark gallery and the bite marks in bones could indicate these interpretations but the low skeletal fractures rate observed are not contingent with falling into the Sima de los Huesos deep pit (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001). Perhaps continuous and eventual supply of corpses for decades could produce the accumulation. The third context, low sedimentation rates could allow too the observed concentration of bones. Cave clays belong to limestone non-soluble fraction. Cavities are due to calcium carbonate dissolution. High percentage of limestone is soluble in water, but a small part of clays are not. Therefore, clay generation during a cave formation is scarce. Therefore karst cavities don’t usually involve rapid burial mechanisms (Smith, 1975; Atkinson & Smith, 1976; Sorriaux, 1982). This context could involve the observed low sedimentation rate inside cavities. The idea is simple, many bones inside a low clay percentage produces a level of bones condensation. This new explanation is contingent with three more aspects from Atapuerca site. The first is the observed abrasion in 24 percent of human remains (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001). The low sedimentation production allowed a high bone transport rate inside karst, which involved the observed bone fragmentation and abrasion. More than 24 % of the bones are eroded by water currents which transported this first fossil assemblage inside the chasm (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001). These abrasions over fractures was post buried which suggests that these remains was inside sediment and mineralized before the abrasion. In addition, there are bite marks from bears on bears bones but not over human bones. On the contrary, there are bite marks from felines or foxes on human remains but not over bear bones while there are not coprolits in the fossil association. All these reasons indicate that bear and human remains were separated at the first time and was mixed later by debris flows which involved the observed abrasion (Fernández-Jalvo, 2003). Therefore coprolits and phalanges could disappear and spread far away during this water transport. The second reason is the mixed fauna found in the Sima de los Huesos site which happens very often in condensation levels. And the third aspect was the different sedimentation ages found in the outcrop. According to a first sample this age was between 325,000 to 205,000 years old (Parés et al., 2000), but another measure proposed an age between 400,000 to 500,000 years old (Bischoff et al., 2003; Bermúdez de Castro et al., 2003). Condensation levels mix fossils from different ages as in the Sima de los Huesos could happen. Perhaps this data dispersion was due to a low sedimentation rate in the Sima de los Huesos pit. Given the actual time scale suggested by palaeomagnetism and uranium thorium, much of this fossil human accumulation would cover a substantial time span, perhaps in excess of 100,000 years. But this slow sedimentation rate of the Sima sediments did not necessarily imply that the human bone accumulation was also slow. The human bones accumulated first of all in another part of the cave system, probably close to a cave entrance where there may have been a carnivore den (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001). The thirty individuals may have been the prey remains of a single big feline or family of them, and they may have remained in the cave entrance for some time before being transported into the Sima. It is possible that the time of transport was long, although it would be strange if it took place over more than a few hundreds of years according to bone preservation. Therefore a continuous supply of corpses for a long period of time could produce too the observed fossil concentration. The next problem to solve is how big felines came in and out of the pit. They probably killed young and inexpert humans while canines came later for feeding on human flesh (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001). Geological data could explain how all these predators and scavengers came inside and escaped from the chasm. Water does not dilute limestone in a karst system entering and exiting through the same place. The cavities formation produced accesses higher than others. Water flows inside by first entrances and escapes by lower. Collapse and debris flows happen during this process burying old entrances and exits. Therefore, there were more accesses in the Sima de los Huesos. Felines and canines came in and went out of the cavity through these old buried caves by slumps. One such passage is apparent to anyone who enters the Sima. At the bottom of the vertical shaft which gives access to the Sima at present, there is an opening at the end of the Sima furthest from the bone pit and topographically one or two meters above it. This passage is completely blocked by large boulders at present, but clearly at some time in the past it was not blocked, for it was formed by water flow, although without being able to see into the passage it cannot be identified as phreatic. This passage has not been investigated for the good reason that the infilling boulders are very large and it would be extremely dangerous to try and shift any of them. What needs to be done is to investigate this and other possible passages from the other end, for example where they might be open to the outside. This and other entries were described in the Sima de los Huesos site (Arsuaga et al., 1997), but these were discarded by those authors. They believed that these entries were sealed 400,000 years ago. Nevertheless some rocks falling are around the Sima area which involves an old upper entrance during the Middle Pleistocene (Carbonell & Mosquera, 2006). Nowadays there is no age data about it and the slump age is not studied. In fact, at the end of the excavation in the Sima de los Huesos we will see new entries very probably. We can suppose that all those cavities were the access at that time, explaining entrances and exits of felines to the pit or a feeding place around the cave which produced the primary accumulation transported and reworked by water currents later. Perhaps this interpretation could explain the absence of herbivores in the Sima site. If those big felines haunted herbivores they would kill them in the plains around the current Atapuerca range. Herbivores don’t stay a long time inside mountains and forests because they prefer open fields to avoid predators and inside caves there is not grass for them. On the other hand, large predators as lions, leopards and hyenas move corpses only hundred meters (Schaller, 1972; Kitchner, 1991; Bailey, 1993) but the Sima de los Huesos was a long way from the plains. All these reasons could explain the absence of herbivores in the Sima de los Huesos fossil association but many caves are known in a great variety of circumstances, and it is unusual to find one like this with carnivores being the only fossil animals. On the contrary, there are some caves with huge numbers of herbivores, usually but not always mixed with carnivores. It’s very probable that there were other passages and chambers connected to the Sima and that the answer to the problem lies with these. Perhaps the passage leading to the Sima was too small for most herbivores and accessible mainly to smaller sized carnivores. The problem of course is to be able to prove this and in their absence, all we can do is make educated guesses. Therefore we will be able to make a much stronger case if we get new evidences on the sedimentology of the Sima deposits, on the structure of the cave system and in particular the discovery of any connecting cave passages, or even on the nature of the bear assemblages to show similarities or differences with the human remains. At the moment the context was different between hominids and bears because humans frequently lived near the range caves and they were potential victims for large predators near these cavities. This interpretation also could explain the greater abundance of bears at the top of the fossil formation. When the pit was closed by slumps, the only mechanism of bear bone contribution was accidentally falling by hibernation. Similarly, the greater abundance of hominids in the bottom of fossil formation could occur while the lower or upper access remained opened with predators bringing corpses inside or around the chasm. All these bones were reworked by water flows. The lack of phalanges and the abrasions over 24 percent of the remains indicate a selective transport and an intense reworking of fossil association after initial burial (Andrews & Fernández-Jalvo, 1997). These authors interpret the Sima de los Huesos remains as a mixture of different fossil associations (Fernández-Jalvo & Andrews, 2001) and probably with different ages (Rabadà, 2007). The water passage along caves during heavy rainfall events would involve the dispersion, mixing and abrasion observed on these skeletal remains. On the other hand, these water currents dispersed and destroyed carnivore coprolites which are an indicator of carnivore den or feeding place. The only one hand axe found in the Sima de los Huesos site could be reworked with the rest of skeletal remains. Traceology studies reveal that the hand axe does not show use-wear traces because there is microscopic erosion on the edges of the hand axe. According to experimental data, this abrasion seems to have been produced by sandy sediments (Carbonell & Mosquera, 2006). Therefore, this hand axe was not a symbolic object, it was a reworked element as the rest of human remains. CONCLUSIONS A monolithic interpretation does not explain natural processes because these are consequences of a network of causes. The Sima de los Huesos fossil association was considered only as a human burial site by other authors, but according to geological, taphonomical and paleoecological data revision this outcrop could be originated by different taphonomical mechanisms. 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