Atapuerca - Patrimonio de la humanidad

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Sima de los Huesos

Por Ignacio Martínez, Ana Gracia y Juan Luis Arsuaga

The largest deposit of human fossils in history

ATAPUERCA. Sierra de Atapuerca nevadaThe Sima de los Huesos is a small pit situated in one of the corners of the Cueva Mayor in the Sierra Atapuerca, a modest rise in the terrain located approximately 12 kilometers east of the city of Burgos. At 3,700 meters, the geological complex denominated Cueva Mayor-Cueva de Silo is one of the longest in the Duero River Basin and has two entrances. One of them, known as the Portalón, contains one of the largest Bronze Age sites in the northern Spanish Meseta. An enigmatic cave painting in red lines representing the shape of a horse’s head can be seen on one of its walls.

At the eastern edge of Portalón is the access to the Galería del Sílex, which contains a spectacular prehistoric sanctuary from the end of the Neolithic and the Bronze Age. This sanctuary was discovered intact at the end of the 1970’s by members of the Grupo de Espeleología Edelweiss (GEE), a cave exploration society from Burgos, after the opening of a passage closed by natural causes thousands of years ago. As we will see, the GEE has always played an outstanding role in the great discoveries made in the Sierra de Atapuerca.

Sala del Coro (Cueva Mayor, Atapuerca)On the western edge of the Portalón there is access to another corridor that, after a narrow passage, opens into a colossal cavern with a high ceiling, named the Sala del Coro. At the other extreme of this cavern, about 100 meters from Portalón, three main galleries cross one another and diverge in different directions: to the north, the Galería de las Estatuas, to the northwest the Galería Baja, and to the south the Galería del Silo. The Galería Baja crosses the Trinchera del Ferrocarril at the site called Sima del Elefante, which was one of the ancient entrances (now sealed) to the Cueva Mayor.

Following the Galería del Silo, and after passing through a set of very narrow, tight passageways, another huge gallery is reached, the Sala de los Cíclopes, located about 600 meters from the cave mouth at the Portalón. The excavations in this cavern have revealed that this was a place chosen by bears to hibernate. The bears belonged to a species that went extinct more than 120,000 years ago and their scientific name is 'Ursus deningeri'. Fossils of this bear species were found in the Sala de los Cíclopes, along with circular impressions in the clay floor (hibernation beds) which were left by the animals when they hibernated. In addition, on the still fresh clay walls there are traces of bear clawmarks and paw prints.

Paso de la Gatera en la Cueva MayorIn the southeast extreme of the Sala de los Cíclopes a steep ramp ascends to reach a small blind recess that houses a deep pit around four meters wide and thirteen meters deep. This is the Sima de los Huesos. At the base of this pit descends a clay ramp with a low ceiling around three meters wide by nearly ten meters long. This sector of the site is denominated SR (Sima-Rampa) and it ends in a small rectangular room, of around seven meters in length by three meters in width, denominated SH (Sima de los Huesos). This area, systematically excavated since 1984, contains the largest accumulation of human fossils ever discovered.

History of a Discovery

Although the Sierra de Atapuerca is internationally known for its Pleistocene sites, the earliest investigations were focused on more recent prehistoric periods. In 1910 Jesús Carballo published the first archaeological and paleontological discoveries of the Cueva Mayor, mentioning the existence of the horse’s head, painted in red, in the Portalón, which Henri Breuil classified as Franco-Cantabrian Paleolithic art. Other important researchers of the time, such as Hugo Obermaier, Emilio Alcalde del Río or Martínez Santa-Olalla, also studied this site.

Primera excavación en la Cueva Mayor (Atapuerca) en 1973, a cargo de J.M. ApellánizLater, in 1964 Francisco Jordá and Geoffrey Clark in 1971 carried out the first systematic excavations in the Portalón, documenting an important stratagraphic sequence. After opening a blocked access which had been closed for millenia, in 1972 the GEE discovered the Galería del Silex which dates from the Neolithic to the Bronze Age and preserves burial chambers, numerous examples of rock art, interesting ceramics, and evidence of flint extraction. All of this was preserved intact up to the present day. Between 1973 and 1983, José María Apellániz carried out new excavations in Portalón, investigating a much more extended surface area than before, and also coordinated the research carried out in the Galería del Sílex.

Coinciding with the excavations of Apellániz, in 1976, an unexpected event changed the orientation of the excavations in the Cueva Mayor forever. That year Trinidad Torres, an expert in bear evolution in the Iberian Peninsula, studied the Pleistocene levels of the Trinchera (in the sites Gran Dolina and Galeria) searching for mammal fossils, particularly bear remains. During his fieldwork, members of the GEE alerted Torres about the large quantity of bear fossils in Sima de los Huesos.

Pintada del S.XVI en la Cueva Mayor de AtapuercaIn fact, as graffiti on the cave walls attested, the area of Cueva Mayor where the Sima de los Huesos is located has been visited for centuries by people from neighboring villages. The Sima de los Huesos was a particularly attractive place because of its richness in bear remains, with the bear canines being the most coveted prize for the people that visited over the decades.

Encouraged by these reports, Torres organized a short excavation campaign with the GEE in the Sima de los Huesos that same year. Unexpectedly, a human jawbone was found among the numerous bear remains that were recovered. The connection between this human fossil and the bear fossils, which belong to a species extinct for over 120,000 years, suggested a considerable antiquity for the human mandible.

Realizing the significance of the finding, Torres took the human mandible to his thesis adviser, an expert in human evolution, Emiliano Aguirre, who recognized the presence of archaic features typical of European human populations of the Middle Pleistocene, highlighting its relevance. Later on, a more detailed inspection of the fossils acquired in the 1976 excavation, resulted in the recognition of more human fossils: some teeth, new fragments of jaw, skull, and long bones.

Atapuerca foto. Mandíbula humana AT-1, el primer fósil humano identificado en la Sima de los Huesos en el año 1976Two years later in 1978, Aguirre started the new task of excavating the Pleistocene sites of the Sierra de Atapuerca, located in the Trinchera del Ferrocarril and in the Sima de los Huesos. To do so, he organized an interdisciplinary team, consisting of specialists in geology, archaeology, and paleontology, which started to work at the sites of the Trinchera del Ferrocarril. Although the Sima de los Huesos, where human fossils had been found, was the most attractive site, its particular conditions did not allow systematic excavation until 1984. However, in 1983 a small sample was collected from the site in which new human fossils were discovered.

El interior del yacimiento de la Sima de los Huesos en plena excavaciónSima de los Huesos has no comparison with any other site in the world. At no other site does one have to walk through a kilometer of winding subterranean passages, descend to the bottom of a deep pit and work in an extremely oxygen-poor atmosphere. In addition, the activities of the amateur spelunkers that have descended to the site over the years searching for coveted bear fossils have altered the superficial levels of sediments, breaking many bones and mixing them up with trash and limestone blocks in a uniform jumble. As a result, before starting to plan a systematic and rigorous excavation, it was necesary to evacuate tons of these altered sediments and limestone blocks from the site in order to access the intact levels.

In a short visit to the site in 1983, a few kilos of sediment were collected to test if there were still human fossils in Sima de los Huesos. The discovery of two human teeth while carefully cleaning and screening the obtained sediments confirmed the great scientific potential of the site. In light of these findings, systematic excavation of the site was begun during the next field season.

Los 14 metros de altura del pozo que conduce al yacimiento de la Sima de los Huesos, se descienden a través de una escala.In the 1984 field season, a new excavation strategy was determined and the basic infrastructure was installed to carry it out. The altered sediments were removed in backpacks, each one carried out of the Sima by members of the excavation team. The team then took the sediments to the edge of the neighboring Arlanzón River were they were washed on screens to recover the bone fragments. These fragments were later sifted to more carefully search for the human remains.

To work at the site, it was necessary to install an electric light in the Sima de los Huesos, since the use of gas lamps was already consuming much of the oxygen present at the site. To solve this, the team ran an electrical cable of almost a kilometer in length, from the Portalón to the Sima de los Huesos, where they installed a simple electrical setup consisting of a few sockets and lightbulbs. They also installed a hanging grid system, attached to the ceiling of the cave, in order to locate and map future fossil discoveries following proper archaeological protocols.

During the final days of the 1984 excavations, a small area of unaltered sediments was discovered at the western edge of the site, where the level of the disturbed sediments was very thin. In this zone (know as Area A), four human fossils were discovered, adding to the 78 specimens that were recovered while washing and screening the overlying disturbed sediments.

During the 1985 field season, Area A was systematically excavated, yielding three new human fossils. The level with human fossils, however, was very thin and was quickly exhausted. It was evident, then, that the main efforts had to be concentrated in the removal of the disturbed sediments which occupied most of the site, with the hope of finding the same level with human fossils discovered in Area A under the layer of altered sediments at other parts of the site.

This work, already begun in 1984, was carried out from 1985-1989, during which time more than 12 tons of limestone blocks and sediments were extracted from the Sima de los Huesos, without the use of any type of machinery, resulting in the discovery of 131 new human fossils.

In 1987, the team designed and installed a suspended platform at the site, anchored to the walls, which allowed work to proceed without stepping on the floor. In addition, a perforation was drilled from the outside through the ceiling of the neighboring Sala de los Cíclopes to facilitate the removal of sediments and improve the ventilation in the Sima de los Huesos.

Muchos de los fósiles que se han encontrado en la Sima de los Huesos han aparecido en el lavado de los sedimentosThis removal of altered sediments was completed during the 1989 field season. However, the most important discovery of that year was that the level of human fossils found in Area A in 1984, and apparently exhausted in 1985, extended across a larger area than was originally suspected. The last phase of the 1989 campaign and part of the 1990 field season was dedicated to the systematic excavation of this level, yielding 47 new human fossils.

The 1990 campaign marked a turning point in the excavations in the Sima de los Huesos. On one hand, the extension of the fertile level of human remains in the A area was delimited, and turned out to be quite small, giving the impression that the site was close to be exhausted. Secondly, the precise location was located within the cave of where the disturbed sediments were secondarily dumped after their removal from the Sima by the original team which excavated in 1976. These sediments were removed to the outside during the 1989 and 1991 campaigns. The cleaning and sifting of this material produced 161 new human remains. Finally, during the last days of the 1990 excavation six human remains in another area of the Sima were found next to the northern wall. This new excavation area was called Area B. The newly recovered human remains in Area B were the most complete and best preserved among all the discoveries to date.

Between 1984 and 1990 in the Sima de los Huesos, 389 human remains had been found. Of these, 335 came from sediments which had been disturbed by the amateur spelunkers, and 54 had been found among undisturbed sediments in Areas A and B. Although the number of fossils was impressive, being larger than all of the other sites of the Middle Pleistocene (780,000-120,000 years ago), most of the fossils were small fragments that apparently could not provide much information (as some of our colleagues liked to remind to us, a bit maliciously). Yet, this was a false impression. There were two fairly abundant skeletal elements in the Sima de los Huesos sample, which provided very relevant information.

Los dientes, gracias a su resistencia y al gran número de piezas por individuo, son un fósil muy abundantes en Sima de los Huesos. Representan al menos a 28 individuosOn the one hand were the teeth, that helped establish the presence of at least 20 individuals (or dentitions) within the sample. This was a very large number at any site containing human remains, regardless of the location or chronology. Human remains are among the rarest of paleontological discoveries, making them very valuable. Only a small fraction of archaeological sites have yielded human remains, and one can count with one hand the number of sites with the same antiquity of the Sima de los Huesos where the minimum number of individuals found exceeds half a dozen. The significance is not simply to break a record (“my site has more individuals than yours”), but rather takes on a singular scientific importance. If the number of individuals in a sample is very small, it is not possible to know the variability of the original population, and therefore it is very difficult to evaluate the research results. On the contrary, when the number of individuals is large, it is possible to determine the causes and degree of biological variability and arrive at firm conclusions about the human remains. In this context, one can understand the significance of the large number of individuals preserved at the Sima.

However, on the other hand, it is not very useful to have identified many individuals by their teeth if there is not much left of their skeletons. At this point, the other abundant skeletal elements in the sample from the Sima de los Huesos gains significance, most notably the phalanges of the hands and feet. These small bones are among the most fragile and delicate in the human skeleton, and they are exceptional in sites as old as the Sima de los Huesos. In fact, among all the archaeological sites of the Middle Pleistocene, just a single human phalanx has been found, from the site of Zhoukoudian in China (the well known “Pekín Man” site). Over the years, more than 60 phalanges were recovered from the Sima de los Huesos, an amazing number. This suggested to the members of the excavation team that if these fragile bones were found at the site, it was logical to believe that other parts of the skeleton would also be preserved.

Todas las regiones del esqueleto están reperesentadas en la Sima de los Huesos. Si las juntasemos podriamos tener un esqueleto compuesto como el de la imagen, si bien muy probablemente no represente un único individuoThe 1991 field season began under a new scientific leadership. Until 1990, the year in which he retired, Emiliano Aguirre was the director of the excavations at the diverse Pleistocene sites of the Sierra de Atapuerca. During those years, the excavation team had been organized around three outstanding collaborators of Aguirre: Juan Luis Arsuaga, José María Bermúdez de Castro and Eudald Carbonell. This situation led the team to aquire a different and relatively original structure, with a collaborative leadership under the direction of the three leaders of the different teams. Following this reorganization, Arsuaga, who had been in charge of the excavation in the Sima de los Huesos under Aguirre’s management, continued this work.

In the 1991 campaign, the team completed the excavation of the dumped deposits from the original 1976 excavation (adding 54 new human remains), and the systematic excavation of Area B was undertaken, where as mentioned previously, six very complete human fossils had been found in 1990. Soon after, it was realized that the level where those fossils were found had not been altered by the amateur spelunkers, which made the team hopeful about finding new complete and well preserved human remains in Area B. The expectations were entirely justified since by the end of the campaign, in a surface area of just an eighth of a square meter (1/8 m2) excavated to a depth of only 20 cm, they had recovered 112 new human fossils. However, the most important fact was not the number of fossils found in an intact geological level, but the extraordinary state of preservation of the fossils. In addition, bones from parts of the skeleton were found that had never before been recovered (or were extremely rare) at any other site of the same time period. The Sima de los Huesos started to become recognized as an exceptional site, for both the number of fossils and their scientific significance.

Excavación del Cráneo número 4 (Agamenón) en el año 1992But this new situation also presented new working conditions and new challenges. The fossils coming from the intact sediments were much more fragile than the ones from the distrubed sediments, which made their excavation a much more delicate task. A hardening solution was needed to extract the skeletal remains from the sediments without them deteriorating. Given the very high relative humidity in the air at the Sima de los Huesos, it was not possible to use the most common hardening solution, which is dissolved in water, and it was necessary to make new ones dissolved instead in acetone. On the other hand, to excavate in the intact levels a different, more systematic methodology is necessary compared to working with disturbed sediments. In essence, the objective is to register as accurately as possible the three-dimensional position of each fossil before it is removed. This method is commonplace in any archaeological excavation.

But there is no single excavation in the world comparable to the Sima de los Huesos. At no other site do they recover hundreds of human fossils each field season. Thus, it was decided to modify the traditional archaeological method in an important detail. While the excavation maps at most other sites are made at a 1:10 scale, in the Sima de los Huesos they are drawn at a 1:1 scale. This change of scale implies a relative increase in the prescision of the maps, given that 1:1 scale is ten times more exact than that of 1:10. But this increase of precision also significantly increased the work necesary to make the maps at this new scale.

Una vez excavados, los fósiles de la Sima de lo Huesos son mapados. De este modo, podemos conocer la posición exacta en la que se encontraban los restos antes de su extracciónIn addition to modifing the field methods during the excavation, it was also necessary to change the work plan in the laboratory. There were now many more fossils to clean, preserve, and catalog, all without losing the exact place of origin of each fossil. To this end, all the excavation and laboratory methods were reconsidered and new protocols were established to optimize the labor within the reduced time during the field season.

The excavation of 1992 was planned with an extremely ambitious objective. On the one hand, the systematic excavation of the dense levels of human fossils in Area B was continued, and at the same time, excavation in more limited and selected (test) areas along the access ramp to the site (designated Sima-Rampa, or SR) were initiated. The excavation in these test points (named SRB, SRM, SRA) contributed very valuble information in understanding the stratigraphy and formation of the site. In addition, 16 human fossils were recovered in SRB and SRM, some of which could be joined to other fragments in sediments from SH, making it clear that the human remains were originally deposited on the ramp.

Although these finds were important, the most incredible discoveries during this year came from Area B – finds that would catapult the Sima de los Huesos Site, and in turn the Sierra de Atapuerca, to the forefront of the scientfic world.

Un momento histórico: despues de trabajar durante toda la campaña de 1992 en su extracción, el Dr. Martínez entrega al Profesor Arsuaga el Cráneo 4 de la Sima de los HuesosIn the first fews days of excavation a complete neurocranium was discovered (Cranium 4), informally baptized “Agamemnon,” in homage to classical archaeology. To demonstrate the importance of this find, it is sufficient to point out that there is only a single skull of comparable antiquity in the whole archaeological record of Europe. The task of excavating this fossil occupied almost the entirety of the field season and occupied an area of around one quarter square meters (1/4 m2), to a depth of some 25cm. In this same area, an additional 200 human fossils were subsequently excavated, including a second human skull (Cranium 5), named Miguelón in honor of the 5-time champeon of the Tour de France, Miguel Indurain. In contrast to Agamemnon, Miguelon also preserved the facial bones and, to this day, is the best preserved and most complete fossil human skull ever discovered. In addition, several skull fragments found this year, together with others recovered during previous field seasons, allowed the reconstruction of a third very complete skull belonging to an individual who died at around 13 years of age and named "Rui", in honor of the legendary figure from Burgos, Rodrigo Diaz de Vivar.

Atapuerca foto. El Cráneo 5 de la Sima de los Huesos, aparecido en el año 1992.
Archivo AtapuercaThe 1992 finds proved to be an important turning point in the international recognition of the site, which now began to be considered one of the most important sites for human fossils in the world, and key to the understanding of human evolution in Europe during the Middle Pleistocene.

Since then, the excavations have continued uninterrupted in the Sima de los Huesos, summer after summer. Over the years, the site has continued to produce incredible finds. In 1994, the same year that the first fossils of 'Homo antecessor' were found in level TD6 of the Gran Dolina, the only practially complete and undeformed human pelvis (Pelvis 1) in the fossil record was recovered from the Sima. Following the tradition of giving informal names to exceptional fossils, it was coined Elvis, in memory of the pelvic gyrations of the King of Rock and Roll, Elvis Presley. In the last 14 years we have encountered thousands of human fosils at the Sima de los Huesos, up to a provisional total of 5,500. This is an extraordinary number, that excedes the total number of human fossils found the rest of the sites on the planet.

La Pelvis 1 ('Elvis') de la Sima de los Huesos (Burgos, Atapuerca). Es la más completa del registro fósil mundial y pertenece a un individuo masculino de la especie 'H. heidelbergensis'Above all, the most intriguing find, and perhaps the most far-reaching of all in the Sima de los Huesos, has not been a human fossil. In 1998, in the same level as the human fossils, the only stone tool ever found at the site was recovered: a stone handaxe (or biface), made of red quartzite, which we named Excalibur.

The type of flaking corresponds to Mode 2, or Acheulean, technology, which coincides with the human type (see below) found in the Sima de los Huesos. This type of technology is widely found in other sites of Atapuerca, in the Trinchera de Ferrocarril, of a similar geological age, such as Galeria and the upper levels of Gran Dolina. It’s interesting to point out that at these sites, similar-sized tools made of the same raw material as the handaxe at the Sima de los Huesos are rare.

All these discoveries, Elvis, Excalibur and most of the thousands of new fossils came from Area B in SH, where excavation is far from being completed. The new field seasons (2002-2005) have been carried out pursuing a new scientific strategy. Instead of continuing work in Area B, whose richness in human remains is well established, major efforts have been focused on the SR and other locations, different from Area B, in SH. The aim is to enhance the data on other relevant aspects of the site: to confirm the proposed stratigraphy, to refine the dating of the site and to determine the geometry of the deposit.

La única pieza de industría lítica encontrada en la Sima de los Huesos: un bifaz realizado en cuarcíta roja y apodado ExcaliburAs the result of these activities, several dozen new human remains have been found in the highest area of SR (in a location called SRA), which undoubtedly belong to the same individuals that were found in SH, confirming the results of prior campaigns in the original place of deposition of the skeletons. In addition, in this spot (SRA) it has been possible to date the fossil deposit with absolute accuracy at more than 350,000 years old. Later we will discussin detail the question of dating the fossils from the Sima de los Huesos. However, we will first devote some lines to a question that inspires curiosity in the general public: How much is still left to excavate in the Sima de los Huesos?

This is a question without a definitive answer given the nature of the site. In fact, many years of work were necessary to begin to understand the geological complexity from the data recovered during many field seasons.

Entre los fósiles humanos de la Sima de los Huesos aparecen mexclados los restos de al menos 167 osos, pertenecientes a la especie 'Ursus deningeri', un antepasado del oso de las cavernasThe human remains appear, together with bear remains (the Ursus deningeri" species), in a red clay stratum. This sedimentary level was deposited on an irregular relief already present, resulting from prior periods of sediment erosion and deposition. This level of red clay rich in human remains was in turn altered by subsequent water circulation at the site, resulting in a new irregular relief. Later on, another level containing fossils of the same bear species and of other carnivores (see below) covered the sediments rich in human remains. In this way, different levels do not have a constant thickness throughout the site, nor are they horizontally located one above the other, but make contact laterally.

To this irregular geometry of the deposit, a consequence its complex geological history, it must be added that the human remains are not homogeneously distributed in their corresponding red clay level. On the contrary, they have accumulated in some parts of the SH, such as Areas A and B, while in other areas of the site there are hardly any fossils. This chaotic distribution is the result of the bones which reached the SH being carried there by small mud slides from their original place of deposition in the ramp (SR).

Una vez que los fósiles son recuperados del yacimiento y lavados, se extienden hasta que se secan por completoAs a result, it is not possible to predict exactly how long it will take to complete the excavation of the site. However, there is one very interesting piece of information. The team working at SH has estimated the percentage of human remains which have been recovered in relation to the total initially accumulated in the site. This calculation is based on the certainty that in the Sima de los Huesos there has been an accumulation of complete skeletons of at least 28 individuals. This information, along with other studies on the pattern of fracturing of human remains at the site, allows a prudent estimate of the number of human remains expected in the site. The team believes that to date just a third of the existing human remains at the site have been recovered.

Dating a Site

One of the most fascinating questions for the general public in the field of paleontology is the age of the fossils. It seems amazing that it is possible to know the date of such remote events that no one witnessed with such precision. Although a detailed explanation of the different techniques that scientists use to date fossils is beyond the scope of these pages, we explain some general considerations that are useful to understand the basics of the methods and evaluate the results.

In paleontology there are two ways of dating an event. One is to establish a relative date, that is, to situate an event within a temporal order of events. For this procedure, the antiquity of the object is established in terms of “before, or after,” but does not imply knowledge of the exact date. This concept can be illustrated with an example from daily life. The terms, grandmother, mother, and daughter establish a relative date between three women - the grandmother the oldest, the mother in the middle, and the daughter being the youngest – but we don’t know the exact age of the women. Establishing the exact (or reasonably precise) age of the events is the objective of absolute dating.

Los materiales y métodos de excavación son el punto de partida para el trabajo de gabinete y la posterior publicación de los resultadosIt is a common mistake to think of the word “absolute”, when refering to a date, as indicating “definitive” or “incontestable.” In paleontology, definitive or incontestable dates do not exist, since they are always subject to contrast with the results of other techniques. Only when the dates obtained by various methods (relative or absolute) agree can they be thought to reasonably estimate (although constantly improved by new techniques) the date of a fossil or site.

Establishing the relative date of sites (and, thus the fossils contained in them) is a task done through the use of biostratigraphy. Essentially, this attempts to construct a chronological framework (or chronobiostratigraphic scale) that temporally organizes the distinct species that have lived throughout the history of biological life. To do this scientists employ concepts and methods from the field of geology (such as the law of stratigraphic superposition) and biology (like the irreversible nature of the evolutionary process). In this way, the different fosilized species that appear in a site indicate to a biostratigrapher the temporal period to which they correspond.

For their part, the techniques commonly used to obtain absolute dates are based on natural radioactive phenomona to establish how much time has passed since the occurence of a certain event. The relative porportions of certain isotopos (which give their name to each technique) are measured, since these proportions change as a direct function of the amount of time passed. If the event dated is the death of the organism that produced the fossil (as in the case of the well-known Carbon 14 technique), the antiquity of the fossil can be directly established. More commonly, the event to be dated by these methods is the formation of certain types of rocks, such as volcanic basalt (Potassium/Argon technique) or flowstones, stalactites and stagmites (Uranium-series technique).

Gracias al mapado de los fósiles en la Sima de los Hueso, podemos dónde y en qué posición se encontraba cada fósil en el yacimientoThere is one very important question when attempting to understand and analyze dates obtained by these methods: the maximum reach, limit or range of the method. This is the maximum age that can be reached, with acceptable precision, with each given technique. Currently, the limit of the method of the Carbon 14 technique is around 40,000 years. Beyond this date, the results of the technique are not reliable and it is not used to date events that are older than this. Essentially, the maximum effectiveness of each technique has is limited by its capacity to precisely measure the exact number of atoms of the corresponding isotopes. Inevitably, all measurement instruments have a margin of error, and the magnitude of this error determines the realiability of the results based on a determined date, which varies with each method. When a technological advance is produced that reduces the error and changes the precision of the measurement, then the maximum age limit is increased. This happened, for instance, when the reach of Carbon 14 increased by employing particle accelerators in association with the measuring instruments.

After this brief foray into the world of dating techniques, let’s return to the case of the Sima de los Huesos. As mentioned previously, when the first human mandible was discovered at the site in 1976, its association with Ursus deningeri bear fossils suggested a great antiquity. Since this species of bear went extinct around 120,000 ago, this appeared to be the minimum age attributable to the human fossil. However, the fact that the levels of the deposits from which both the bear and human fossils derived may have been disturbed by amateur spelunkers called this date into question. Some researchers have suggested that the bear fossils could hve come from a different stratigraphic level than that of the humans and, for this reason, they have different dates.

This doubt was not put to rest until the 1991 excavation when, once the disturbed sediments were cleared away, the intact levels were exposed and confirmed that they contained both 'Ursus deningeri' and human fosils. As such, the association between both species was confirmed and established a minimum age of 120,000 years for the site. While this highly significant date confirmed the great antiquity of the human fossils, it was still much too imprecise. At that time, no datable rock had been found at the site that showed a clear stratigraphic relationship with the human fossils, which led to two independent lines into the question. On the one hand, the rest of the faunal remains from the site were studied. At the same time, new experimental techniques were applied to date the fossils. The results of both methods coincided with an older minimum age for the Sima de los Huesos fossils of around 250,000 years ago.

Atapuerca foto. El Cráneo número 5 apareció fragmentado en distintas piezas, una de ellas fue el hueso frontal de la imagen.
Archivo AtapuercaFinally, after more than 10 years of excavation in different areas at the site, during the 2001 field season a discovery was made that allowed for a more precise date. That year a horizontal stalagmite was found in the upper portion of the ramp (in the place designated SRA) that was deposited directly over the fossil-bearing levels which contained dozens of fossils of the same individuals found in SH. This type of sedimentary rock can be reliably dated by the uranium-series dating technique mentioned above. The results obtained from the stalagmite at SRA showed its age exceeds the maximum reach of that technique, established at 350,000 years, and as a result the human fossils are older than this date.

There is one final consideration to conclude this section on the antiquity of Sima de los Huesos. It is a common mistake to confuse the minimum age of a site with its absolute date. A minimum date of 350,000 years does not mean, as it is often mistaken in the media, that the site is exactly 350,000 years old, but rather that it is older than this date (without knowing how much older). This is an important point to make clear since we are sometimes approached by people who claim that we continue to change the date of the site over the years, making it older each time. This is not correct. What has happened is that the minimum age has been refined from 120,000 years ago in the beginning to 350,000 years ago currently. When technological advances lead to improvements in the measuring devices, thereby lowering their error, the date of the stalagmite of SRA will become even more precise and establish another adjusted minimum age for the site, which will inevitably be even older.

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