THE SIMA DE LOS HUESOS SITE: A TAPHONOMICAL REVIEW (Atapuerca Range, Burgos, Spain)

THE SIMA DE LOS HUESOS SITE: A TAPHONOMICAL REVIEW (Atapuerca Range, Burgos, Spain) David Rabadà i Vives Museu de Geologia del Seminari de Barcelona Diputació, 231 08007 Barcelona, Spain drabada@hotmail.com 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 taphonomical review 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, explain this fossil association. The bears fell by accident or die while hibernating there. Homo could suffer sometimes the similar fate but they were victim of large felines or others that ate human remains inside or around the pit. Foxes and other scavengers came later for feeding. The water flows in the cave during heavy rainfalls produced dispersion, mixing and abrasion in all these skeletal remains. During all these processes a low sedimentation rate and a continuous supply of corpses produced the observed fossil concentration. Key words: Taphonomy, human burials, Homo heidelbergensis, Homo neanderthalensis, Sima de los Huesos, Atapuerca, Pleistocene. INTRODUCTION In Sima de los Huesos site more than thirty-two Homo remains were found in only four cubic meters of sediments. 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) and the Sima de los Huesos site contains the highest concentration of these primitive Homo around the world (Bermúdez de Castro et al., 2004). Nowadays this condensation of hominid fossils has been 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 reviews the current taphonomical references about it. THE SIMA DE LOS HUESOS SITE The Sima de los Huesos fossil association 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 bottom of a chasm of 13 meters high which falls at the end of a dark gallery of 400 meters as cave profiles have been shown by different authors (Arsuaga et al., 1993; Arsuaga et al., 1997; García & Arsuaga, 2011). 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. This and other entries were described in the Sima de los Huesos site (Arsuaga et al., 1997). For instance, some rocks falling are around the Sima area which involves an old upper entrance during the Middle Pleistocene (Carbonell & Mosquera, 2006). Inside the Sima de los Huesos site only predators have been identified. 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). Therefore this Homo heidelbergensis concentration in the Sima de los Huesos site is associated with bone remains of other carnivores, especially bears and foxes where the main predators with 50 percent of individuals estimated were 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. On the other hand, and when Homo 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 only one hand axe was found (Carbonell et al., 2003; Carbonell & Mosquera, 2006). The Sima de los Huesos human remains are mixed with others mammals bones and without anatomical connections (Arsuaga et al., 1993) and thirty-two individuals 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 infer 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 that some 60 % of the bodies’ bones are missing, particularly vertebra, cranial elements and phalanges (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001). The fossil assemblage shows high amount of jaws and limb bones as femurs, humeri and tibiae which are more frequent than scarce ribs, vertebrae and metacarpials. Therefore there is a clear predominance of arms and legs remains in the Sima de los Huesos site. On the other hand, and according to the tooth studies, the 52 percent of these human remains were adolescents and young adults. In fact, 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). Human bones surface shows a lot of information too. There are many human cranial remains with abrasions and fractures caused by impacts over human remains (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001). In addition, more than 24 % of the bones are eroded showing abrasions over surfaces of fractures. (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001). On the other hand, 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. RITUAL BURIALS AT THE SIMA DE LOS HUESOS Different articles propose that there were a ritual burials in the Sima de los Huesos site. In summary, there are four aspects which involve this recognized interpretation. The first is the absence of herbivores inside this outcrop, the second the highest concentration of Homo heidelbergensis around the world, the third an Acheulean handaxe and the fourth the human mortality distribution. The absence of herbivores would involve that carnivores didn’t used the site as a den and therefore they didn’t introduce the human remains into the cave in order to eat because not one single herbivore was ever carried (Arsuaga et al., 1997). It is more likely that human and bear bodies were already there and that carnivores came into the cave to eat the carrion. However, the large number of foxes, and the presence of other carnivores, does not favour the hypothesis that carnivores visited the site only to scavenge and eventually died there by natural causes, and would favour the natural trap hypothesis to explain the carnivore accumulation. Moreover, the age-at-death profile of the bear sample in Sima de los Huesos fits better with a catastrophic profile than with an attritional pattern (García et al., 1997). In the end, if human bodies didn’t introduced by big predators which involves that they were carried by human (Arsuaga et al., 1997). The second aspect about human burials at Sima de los Huesos site is the bone remains concentration. These fossil remains are mostly concentrated inside a quite discrete sedimentary level, which cannot be explained by any kind of catastrophic event (Carbonell & Mosquera, 2006). In Sima de los Huesos site more than thirty-two Homo heidelbergensis remains were found in only four cubic meters of sediments, the highest concentration of Homo heidelbergensis around the world and the most complete collection of Middle Pleistocene Homo too (Bermúdez de Castro et al., 2004). This condensation of hominid fossils indicates 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). The third aspect about ritual burials at Sima de los Huesos site is an Acheulean handaxe. Carbonell defends that Sima de los Huesos was never a hominid occupation place, since no traces of habitation has been discovered, nor a carnivores net, because there are not herbivores remains (Carbonell & Mosquera, 2006). In fact, Sima de los Huesos contains a large variety of carnivores, such as foxes, large felidae, wolfs, mustelids, and bears. The presence of these specimens may be explained as several events of natural falling, hibernation and catastrophic death, particularly clear for the bears’ case. This may be supported by the fact that all these specimens are present along the whole sedimentary sequence. On the contrary, human remains are mostly concentrated inside a quite discrete sedimentary level. The finding of an Acheulean handaxe at the Sima de los Huesos cave site casts light on the evolution of human behaviour during the Middle Pleistocene. It is a finely flaked quartzite handaxe, which is associated with the hominid assemblage. The particular nature of the deposit involving its taphonomy, palaeontology, and technology points to a symbolic meaning both of the tool and the human accumulation. This would support the hypothesis of human mortuary practices at the Sima around 400 kyr ago. This discovery allows us to extend human complex behaviour and symbolism of mortuary rituals 300 kyr earlier than broadly heretofore accepted (Carbonell & Mosquera, 2006). A use-wear analysis could not demonstrate conclusively as to whether this object was actually used, due to erosion of piece’s edges; however, it would seem quite clear that it was not made to be used in the Sima, since the latter was clearly not employed as an occupation site and it was thrown to the chasm as a symbolic act during a ritual burial (Carbonell & Mosquera, 2006). The last aspect about an anthropic origin for the accumulations at the Sima, and according to the tooth studies, is the lack of infants and children with an abnormally high percentage of adolescents and prime-age adults. Also, the number of adults over the age of 20 years is lower than expected in normal models (Bermúdez de Castro & Nicolas, 1997; Arsuaga, 1999). This mortality distribution could be the result of a catastrophe as a sudden death for all the Sima de los Huesos human individuals before the ritual burial (Arsuaga, 1997). Bocquet-Appel and Arsuaga discarded the hypothesis that the site was a primitive cemetery, suggesting instead that the mortality distribution could be the result of a catastrophe, although they did not specify the origin of the accumulation (Bocquet-Appel & Arsuaga, 1999). Considering the biases of the human bone collection, the presence of one hand axe, the human remains concentration and the absence of herbivores, the recognized interpretation is that in the Sima de los Huesos there was an accumulation of human bodies where carnivores were not responsible for the accumulation of the human remains which would indicate the accumulation of human remains could be either catastrophic or the result of a mortuary practice and this explanation should be considered the null hypothesis for future tests (Arsuaga et al., 1993). TAPHONOMICAL DATA REVISION The problem of Sima de los Huesos site is the absence of similar outcrops with equivalent geology and fossil assemblage. 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, 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. Therefore, and at the moment, we have to focused our attention in taphonomical and geological data of the Sima. Lost human remains At the Sima were identified thirty-two individuals from 1300 skeletal parts (Arsuaga et al., 1997). This fact offers a very important information. The total number of skeletal parts of thirty-two Homo heidelbergensis is much higher than these 1300 skeletal parts. In fact, they would have to be more than 3.000 which indicates 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 within in a pit 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. All these percentages involve a partial and selective transport of human bones at Sima de los Huesos site (Andrews & Fernández-Jalvo, 1997, Fernández-Jalvo & Andrews, 2001). Abrasions on bones There are many cranial remains with abrasions. Some authors have said 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 water currents reworked the human remains in the cave because there are no signs of cranial healing. People would think that these abrasions were caused by impacts when human bodies fell in the chasm. Weigelt (1989) and White (White et al., 1984) described bone trauma in fallen animals in potholes as a trap but there is a problem with the Sima. When some animals fall in a pit inaccessible to predators they tend to generate fossil associations without bite marks while human remains in the Sima are full of them. On the other hand, and according to the hypothesis that other humans threw dead bodies to the 13 m shaft, it was performed an experiment using human bones to determine the degree of breakage by dropping them from a 13 m height on to a hard concrete surface (Andrews & Fernández-Jalvo, 1997). In the event, most of the bones did not break or were damaged in any way. Altogether the proportion of complete bones was 82%, very different from the 3% in the fossil assemblage of the Sima. This fact does not support the hypothesis that other humans threw dead bodies into the pit (Andrews & Fernández-Jalvo, 1997). It should be added that fossils from the undisturbed deposits showing evidence of rounding because they had 24 % of transverse breaks rounded and 20 % of spiral breaks (Andrews & Fernández-Jalvo, 1997). It is assumed that transverse breaks occur when bones are partially mineralized, and spiral breaks occur at or soon after death. The greater degree of rounding on the transverse breaks indicates that the process producing the rounding must have operated at a time considerably after burial and it suggests reworking as a result of movement of the bones in the sediment. It has been demonstrated that abrasion by silty clay matrix may produce rounding of fossilized bones more rapidly than fresh bone, which again suggests modification some time after deposition (Andrews & Fernández-Jalvo, 1997). In summary, the fossil association of the Sima were mixed and eroded by the cave water currents. Bite marks on bones 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 suggested that those 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. Namely, 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. All these reasons indicate that bear and human remains were separated at the first time and was mixed later by water currents which involved the observed abrasion on bones. For instance, occasional excavation of in situ and ex situ deposits near a vertical cave entrance at the formerly mined Plio-Pleistocene Gondolin paleocave system has yielded large and diverse samples of faunas including isolated hominin and non-hominin remains mixed and eroded by cave water currents (Adams et al. 2007). This study highlights the variation in taphonomic processes that can occur within a single cave system, and the complex pre- and postdepositional geological and hydrological processes that can influence the history of karstic fossil assemblages. At the moment it has been conclusively demonstrated that the bears entered the Sima separately from the humans (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001; 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 explains the accumulation of bear bones inside cavities (Haynes, 1983; Weigelt, 1989; Domínguez-Rodrigo, 1994) and bears fossil association in the Sima where they ate themselves inside the cave (García & Arsuaga, 1997; García, 2003). Another important aspect between bears, Homo and other carnivores is the close relation between these species. Bears, Homo heidelbergensis, lions and hyenas used caves as a temporary shelter for millennia, bears as a hibernation place, lions and hyenas as den and humans as a campsite. In summary, the common situation between them was their specialized carnivorous strategies as predators, their competing for similar food resources and caves as a resting place. Therefore Atapuerca range represented an area of ecological competition between these species. In this context, the distribution of human ages shows new information. Human ages distribution 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 or Homo heidelbergensis populations (Trinkaus, 1995). In fact, the most appropriate name for Homo heidelbergensis is Homo neanderthalensis because some authors talk about a gradual succession of two different chronospecies, Homo heidelbergensis first and Homo neanderthalensis later (Bermúdez de Castro et al., 2004). This data involves that Homo heidelbergensis and Homo neanderthalensis are the same species. In addition, the high variability found in the Sima de los Huesos site indicates that Neanderthals contain the old heidelbergensis chronospecies without any taxonomical problem (Arsuaga & Martínez, 1999). Recently Chris Stringer thinks that the Sima de los Huesos material belongs to the Neandertal clade, and perhaps represents a primitive form of Homo neanderthalensis (Stringer, 2012). Even so, the last spanish authors keep the name Homo heidelbergensis instead of Neanderthal in their publications for practical reasons (Arsuaga & Martínez, 1999; Bermúdez de Castro et al., 2004). In any case, and if the Sima de los Huesos site was generated as intentional burial by Neanderthal populations, 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). 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). In fact, two hyoid bones and nearly 30 middle ear bones were found in the Sima de los Huesos site (Martínez et al., 2012). Therefore the fossil preservation in Sima de los Huesos site is excellent and the distribution of human ages has to reflect the original corpses number. Accordingly, 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 because the mortality distribution was the result of a catastrophe 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 percentage of human bones shows us a new perspective about this kind of accident. Percentage of bones We have seen that there is a clear predominance of human arms and legs remains in the Sima de los Huesos site. 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 (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001), seems to support too this selective transport. In addition, limbs contain a large quantity of meat for feeding while ribs, vertebrae and metacarpials don’t because they contain relatively little nutritional value (Bailey, 1993). Lions, leopards, hyenas or others produced this kind of bones accumulation with predominance of arms and legs. In fact, carnivores rarely fragment metacarpials. For instance, phalanges are nearly absent at many modern hyena dens, and fossil carnivore accumulations often exhibit low numbers of them (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. Different patterns between anatomical elements and size of tooth marks have found on human bones (Andrews & Fernández-Jalvo, 1997). Femora, tibiae, humeri and radii were more chewed by small scavengers. On the other hand, pelvis, and lumbar vertebrae were chewed by big predators. In fact chewing-marks by small carnivores are absent on the last elements. Therefore, these marks follow, from high to low abundance, the consumption sequence for flesh ingestion, with highest percentages on pelvis and lowest on ulna and radius (Blumenschine, 1986). Therefore, a big carnivore is considered to have had priority access to the human bodies of SH, because hindquarter (axial parts) are affected by their teeth in higher proportions than is present in other elements. Small carnivores activity, therefore, is consistent with scavenging rather than predation (Andrews & Fernández-Jalvo, 1997). All these carnivores produced the first human bone association with high percentage of limbs. Later cave water currents transported and eroded the bones. Carnivores 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 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. The last of them had access to the remains before canines (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001). 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 hunted human individuals during Pleistocene and many carnivore species was involved. Spotted hyenas, and their proximal 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 hyena was not the main bone-cracking producer. At the moment, big felines ate first human flesh near the Sima de los Huesos site and later scavengers chewed the rest. Absence of herbivores The absence of herbivores in the Sima site is not strange. Some caves are known in a great variety of circumstances where it is unusual to find carnivores being the only fossil animals. For instance, the reconstructed taphonomic and paleoenvironmental contexts of 4 million-year-old partial hominid skeleton (Stw 573) from Sterkfontein Member 2 shows a mammalian faunal assemblage associated stratigraphically with the hominid dominated by cercopithecoids (Parapapio and Papio) and felids (Panthera pardus, P. leo,Felis caracal, and Felidae indet.) but not herbivores. In addition, the assemblage is characterized by a number of partial skeletons of bones across all taxonomic groups. In fact there is scant indication of carnivore chewing in the assemblage (Rayne et al. 2004) as the Sima occurred. Another example is the late Pleistocene asphalt seeps of Rancho La Brea where their assemblage of carnivores with large number of dire wolf and sabertooth cat specimens are more frequent than herbivores remains. The hypothesis that predators engaged in intense competition for trapped prey, explains their predomination and the mechanism of this predator trap (McHorse et al. 2012). Therefore the predominance of carnivore specimens in the Rancho La Brea deposits has long been explained by a scenario in which a prey animal was trapped and attracted large numbers of carnivores who became trapped (Spencer et al. 2003). However, patterns of skeletal part representation for the seven most common species demonstrate that complete skeletons are not present. Water transport, as Sima de los Huesos site, is ruled out as the primary process responsible for removing skeletal elements based on abrasion data. Instead, the feeding activity of carnivores and the ecological context appear to have been an important factor in the formation of the assemblage (Spencer et al. 2003). In summary, very different landscapes and contexts explain the predominance or absence of herbivores in cave deposits. Therefore, the scene context is very important for knowing bones association factors. The Sima was located on small range far away from the plains. If those big felines haunted herbivores they killed and ate 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. In addition, large predators as lions and leopards move corpses only hundred meters (Schaller, 1972; Kitchner, 1991; Bailey, 1993) but the Sima de los Huesos was a long way from the plains. On the other hand, the carnivore taxonomic diversity found in Sima de los Huesos is particularly high likely corresponding to high ecosystem productivity (García, 2003) which involves two things: there were a lot of herbivores living in those plains and the Atapuerca range represented an area of ecological competition between all theirs carnivores, Homo included. The last suffered a high risk of contingencies as depredation by big carnivores which explains the predominance of middle age and scarcity of infants and old individuals. In and out The next problem to solve is how big felines came in and out of the pit if they really ate human flesh at the bottom of the chasm. They ate young and inexpert humans while canines came later for scavenging (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001). Nowadays the Sima de los Huesos site is located at the bottom of a chasm of 13 meters high but caves change a lot during geological processes. In fact, 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 or around the actual cavity through all these old buried caves by slumps. In fact, there are different blocked passages in 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. This and other entries were described in the Sima de los Huesos site but they were discarded and considered as old passages sealed 400,000 years ago (Arsuaga et al., 1997). 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 any case, all those cavities, or one of them, were the access at that time explaining entrances and exits of carnivores to the pit or a feeding place around the cave. They produced the primary accumulation of human remains which was transported and reworked by water currents later. When the cave was closed by slumps, the only entrance was the actual pit where accidental falling introduced bears during the hibernation while humans frequently lived near these cavities being potential victims for large predators. Concentration of bone remains The Sima de los Huesos outcrop shows a high concentration of human bone remains with excellent fossil preservation (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001). 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 and 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. 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 Sima 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). Feeding troughs near the Sima site were an easy interpretation for human remains in the Sima de los Huesos outcrop. This situation involves continuous and eventual supply of corpses producing the bones concentration. The third context, low sedimentation rates allowed 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 and karst cavities don’t usually involve rapid burial mechanisms (Smith, 1975; Atkinson & Smith, 1976; Sorriaux, 1982). This context involved 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 by reworking. 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). The second reason is the mixed fauna found in the Sima de los Huesos site which happens very often in fossil 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 happened. This data dispersion was due to a low sedimentation rate in the Sima de los Huesos pit. Given the actual time scale by palaeomagnetism and uranium thorium, much of this fossil human accumulation covered a substantial time span in excess of 100,000 years. For instance, the Rancho La Brea tar pits reveals a complex history of accumulation, concentration and diagenesis for specimens found there. Radiometric dating of 46 bones from Pit 91 documents at least two episodes of deposition, one from 45,000 to 35,000 yr and another, shorter interval from 26,500 to 23,000 yr. (Friscia et al. 2008). In summary, a continuous supply of corpses for a long period of time and a low sedimentation production produced the observed fossil concentration at the Sima de los Huesos. One handaxe The only one hand axe found in the Sima de los Huesos was considered as a symbolic act during a ritual burial when Homo heidelbergensis threw this tool into the chasm (Carbonell & Mosquera, 2006). 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 was produced by sandy sediments (Carbonell & Mosquera, 2006). Therefore, this hand axe 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 and taphonomical data review this outcrop was originated by different mechanisms. 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