miércoles, 15 de febrero de 2012



David Rabadà i Vives
Museu de Geologia del Seminari de Barcelona

The Sima de los Huesos site, Atapuerca, contains an important sample of fossilised human bones of Homo neanderthalensis. The nature of the Sima de los Huesos human remains was interpreted as human burials but geological, taphonomical and palaeocological evidences have given rise to a new interpretation. Ecological competition inside the cave site between Homo and predators’ accidents by falling into the cavity and old passages used as access to the chamber by human predators as felines and canines, explain this accumulation of human bones.
Key words: Taphonomy, human burials, neanderthal bones remains, Sima de los Huesos, Atapuerca, Pleistocene.


The Pleistocene site of Sima de los Huesos at Atapuerca contains in only four cubic meters of sediments the highest concentration of primitive Neanderthals around the world. More than thirty-two individual remains were found in that rock volume. This condensation of hominid fossils was explained as human burials by primitive Neanderthals who threw their dead relatives to the cave site as a symbolical act (Arsuaga & Martínez, 1998; Arsuaga, 1999; Carbonell et al., 2003). If this hypothesis is true we would be at the first human funeral rite. This paper attempts to deconstruct this interpretation reviewing three aspects of the site, 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 deposited 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 (Arsuaga et al., 1993, Arsuaga et al., 1997). 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) but later they were identified as Homo heidelbergensis (Perez et al., 1999; Arsuaga & Martínez, 1999). In fact, Homo heidelbergensis is an early variety of Homo neanderthalensis. The high variability found in the Sima de los Huesos site (Arsuaga & Martínez, 1999) indicates that Neanderthals contain the old heidelbergensis without any taxonomical problem. 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).

The Homo neanderthalensis concentration in the Sima de los Huesos site is associated with bone remains of other carnivores, especially bears and foxes (Arsuaga & Martínez, 1998). The absence of herbivore remains was considered a difficult question to resolve (Arsuaga & Martínez, 1998; Arsuaga, 1999) but later we will see an easy interpretation.


There are two important taphonomical processes inside caves, the first involves presence of vertebrate fossils in their sediments and the second generates skeletal concentrations. On the other hand karst cavities don’t usually involve rapid burial mechanisms (Smith, 1975, Atkinson & Smith, 1976; Sorriaux, 1982) but caves minimize weather action which breaks and disperses skeletal remains (Hill, 1979; Haynes, 1980). Therefore the discussion about the Sima de los Huesos remains has to be focused on concentration mechanisms of bones. There are many causes which accumulate skeletal remains in caves and caverns (Weigelt, 1927; 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 (Weigelt, 1927; Brain, 1958, Vrba, 1976; Maguire, 1976; Behrensmeyer, 1978; Brain, 1981; Andrews, 1990, Domínguez-Rodrigo, 1994). All these mechanisms can explain the presence of skeletal remains inside caves but not necessarily its accumulation. There are three contexts which can produce the concentration of bones . 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 (Rabada, 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. 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 Neanderthals were identified from 1300 skeletal parts but the total number of skeletal parts of thirty-two individuals should be much higher. This fact presupposes two things, the first more than sixty percent of the original remains is missing, especially skulls and vertebrae (Andrews & Fernández-Jalvo, 1997, Fernández-Jalvo & Andrews, 2001), and the second this association fossil shows a dispersion of large bones. If Neanderthal remains was carried as a human burial rite we have to suppose that these were whole bodies and not parts of them. Therefore we should find a dispersion and dismantling lower than this sixty percent of missing remains. On the other hand there are a lot of cranial remains with abrasions and fractures. Some authors have suggested that these injuries were caused by impacts of stones which Neanderthals threw to each other (Arsuaga, 1999), but it seems more logical that these scars were produced when falling into the Sima de los Huesos pit because there are no signs of cranial healing. Weigelt described bone trauma in fallen animals in potholes (Weigelt, 1927) 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 a lot of bite marks in the majority of bones (Andrews & Fernández-Jalvo, 1997, Fernández-Jalvo & Andrews, 2001). The abundance of bears found with Neanderthals might suggest that these predators perpetrated these bite marks (Rabada, 2001) but bears chew bones producing a characteristic pattern with rounded epiphyses (Weigelt, 1927, Domínguez-Rodrigo, 1994) which is not found on the association of the Sima de los Huesos human fossils but is found in bears’ skeletons (Andrews & Fernández-Jalvo, 1997; Fernández-Jalvo & Andrews, 2001). On the other hand a very close relative of the extinguished bear cave, Ursus arctos, 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 (Weigelt, 1927, Domínguez-Rodrigo, 1994) but not the concentration of human remains. Therefore bears didn’t produce the human bone accumulation in the Sima de los Huesos site and we need to study the Atapuerca paleoecological data for answering who caused it.


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 explain it later.

Another important aspect is the close relation between bears and humans. These animals and Neanderthals used caves as a temporary shelter for millennia, bears as a hibernation place and humans as a campsite. When Neanderthals stayed in cavities they carried preys inside for consumption, producing a lot of cut marks on the bones and collections of stone tools. This fact doesn’t happen in the Sima de los Huesos site (Andrews & Fernández-Jalvo, 1997, Fernández-Jalvo & Andrews, 2001) although one handaxe was found (Carbonell et al., 2003). 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 Sima de los Huesos site represented an area of ecological competition between these two species. Neanderthals, being nomadic, periodically revised the cave for various uses while bears hibernated in the cavity every year with high risk of falling inside the Sima de los Huesos pothole. 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 Neanderthals tried to inspect the Sima de los Huesos. We have to remember that this pit was at the end of a long and dark gallery. 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 (Rabada, 2001). The only handaxe found in the Sima de los Huesos (Carbonell et al., 2003) could be a tool brought or thrown by a Neanderthal. But this mechanism did not introduce the majority of corpses inside the site. The distribution of human ages shows new information. Fossils can reflect the mortality rate of the original population. Infant mortality rate in current hunter-gatherer is very high followed by old people (Blurton 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 Neanderthal remains were adolescents and young adults. On the other hand 60 percent of them were less than 19 years old and 90 percent older than 27 (Bermúdez de Castro & Nicolas, 1997; Arsuaga, 1999). Therefore Neanderthals between 0 to 11 years old and over 27 were underrepresented in the Sima de los Huesos site. This predominance of middle age and partial absence of children and old people doesn’t indicate a mortality rate but something very different, an accident risk rate (Rabada, 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. On the other hand bears died in that dark abyss when hibernating and falling inside the hole. Perhaps the same accident could have happened to young Neanderthals but 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. Other authors explained this human remains accumulation as a sudden death for all the Sima de los Huesos Neanderthals (Bocquet-Appel & Arsuaga, 1999) but this reasoning involves a paradox and a new question, did humans die suddenly in the pothole while many bears, foxes and lions didn’t ? In fact the percentage of bones shows us a new and easier interpretation. There is a clear predominance of cranial, arms and legs remains in the Sima de los Huesos site. Teeth, jaws and limb bones as femurs, humerus and tibias are more frequent than ribs, vertebrae and metacarpials, which are very scarce. All these percentages involve a partial transport of bones (Andrews & Fernández-Jalvo, 1997, Fernández-Jalvo & Andrews, 2001). Lions and leopards produce this kind of bones accumulation because limbs contain a large quantity of meat for feeding (Bailey, 1993). Moreover fifty percent of the human remains are affected by bite marks, especially femora at 96 percent. These incisions were caused by wolves, foxes and some big felines that had had access to the remains before (Andrews & Fernández-Jalvo, 1997, Fernández-Jalvo & Andrews, 2001). Therefore a big feline brought the human remains inside the cave site for feeding. But if felines were the first to eat the human meat, how are they so scarce in Sima de los Huesos shaft? And the most important thing, how did felines and canines come in the pit and go out from it?


Some predator transported 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? And 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 (Blurton 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 the pit. Therefore, the concentration of corpses in the cave was given by another mechanism.

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, 1976) and hibernating places (Kurt, 1958, Kurt, 1976). Falls inside the pit and feeding troughs were the most probable mechanisms 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 indicate these interpretations. On the other hand, the third context, low sedimentation rates, allowed 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. This context involves 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 two 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. The second 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; Bermudez 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.

The next problem to solve is how big felines came in and out of the pit. They probably killed young and inexpert Neanderthals 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 was another lower access in the Sima de los Huesos pit. Felines and canines came in and went out of the cavity through this old buried cave by slumps. Two entries were described in the Sima de los Huesos site (Arsuaga et al., 1997) but they were discarded by those authors. They believed that these entries were sealed 400,000 years ago. Nowadays there is no age data about it and the slump age is not studied. We can suppose that those cavities were the access at that time, explaining entrances and exits of felines to the pit. This interpretation explains the absence of herbivores in the Sima de los Huesos 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 and leopards move corpses only a few hundred meters (Schaller, 1972; Kitchner, 1991; Bailey, 1993). All these reasons explain the absence of herbivores in the Sima de los Huesos fossil association. This context was different for Neanderthals because they frequently lived near the range caves and they were potential victims for large predators near these cavities. This interpretation also explains the greater abundance of bears at the top of the fossil formation (Andrews & Fernández-Jalvo, 1997, Fernández-Jalvo & Andrews, 2001). When the pit was closed at its base, 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 access remained opened with predators bringing corpses inside the cave. All these bones were reworked by water flows. The abrasions over 24 percent of the remains indicate 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.


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 this outcrop was originated by different taphonomical mechanisms. Competition between Neanderthals 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 died while hibernating there. Neanderthals could suffer the same fate but they were more probably victims of predation by large felines that carried the corpses inside. 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. At the end a low sedimentation rate produced the observed fossils concentration.


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