- Comparative Anatomy
Sierra de Atapuerca, or the Atapuerca Mountains, are located approximately 14 kilometers (km) east of the city of Burgos in north central Spain. Known simply as Atapuerca, the mountains are home to a series of caves that preserve huge numbers of hominin fossils spanning much of the Pleistocene. Hints about the paleontological potential of Atapuerca first appeared in a vague announcement about human remains in an 1863 Spanish newspaper1. Near the end of the 19th century, a short-lived railroad company excavated a large trench in the Cretaceous limestone that makes up the Atapuerca Mountains. The walls of this trench, known as the “Trinchera del Ferrocarril” contained fissures filled with Pleistocene hominin fossils and archaeological artifacts, none of which were discovered at the time of the railroad construction. Hidden high on the walls of the trench, these sites remained unexplored until 1962, when spelunkers from the Edelweiss Speleological Group informed the museum in Burgos that the cave fillings contained fossils1. Several prospecting missions were subsequently carried out resulting in the discovery of abundant faunal remains.
In 1976 a mining engineer and graduate student named Trinidad Torres was searching for cave bear fossils in the Trinchera for his dissertation research2 . The Edelweiss caver group informed Torres that cave bear bones could be located within the unexposed portion of the cave system known as the Cueva Mayor (or “Older Cave”), located to the southeast of the Trinchera. Torres sampled these sediments at a site called Sima de los Huesos, or “Pit of Bones”, and found many cave bear fossils as well as a human-like mandible, the first hominin remains discovered at Atapuerca. Systematic excavation of the site in the 1990s uncovered thousands more fossils and brought international attention to Atapuerca.
Of the many sites at Atapuerca, the most important are: Sima del Elefante (“Pit of the Elephant”), Gran Dolina (literally “Big Sinkhole”), and Sima de los Huesos. Sima del Elephante is best known for a hominin mandible which, with a date of approximately 1.1-1.2 million years ago (Ma), may represent the earliest known fossil hominin specimen from Europe3. A little younger in age, hominin fossils from the site Gran Dolina that date to greater than 780 thousand years ago (Ka)4, have been attributed to the species Homo antecessor, a hominin species some researchers argue may represent the last common ancestor between modern humans and Neanderthals5. The importance of Sima de los Huesos lies in the extraordinary number of Homo heidelbergensis remains that have been recovered from the site, which represent at least 28 individuals and have all be dated to approximately 530 Ka6. Since the preservation of so many hominin fossils from a single place and time is unique, Sima de los Huesos offers researches a fascinating and detailed snapshot of Middle Pleistocene human evolution.
Atapuerca thus provides an amazing record of hominin evolution in Western Europe spanning much of the Pleistocene. Ongoing excavations at Atapuerca continue to produce new fossils every year, giving evidence that Atapuerca is a critical site for understanding several big questions in paleoanthropology regarding the peopling of Europe, and perhaps the origin of the Neanderthals.
Atapuerca is home to a number of Pleistocene hominins, two of which may be ancestral to either Neanderthals or Homo sapiens. Homo antecessor, which has been found in at least two Atapuerca localities (i.e. Gran Dolina and Sima del Elephante) may represent the oldest known hominin occupation of Europe. Homo heidelbergensis, which some consider to be an early for of Neanderthal, was discovered in an intriguingly large cache of hominin and cave bear material.
Scientific consensus prior to the discovery of hominin fossils at Gran Dolina was that hominins did not occupy Europe before 500Ka7. However, the fossil material found at Gran Dolina dating to >780Ka4 indicates that hominins had been in Europe much earlier and longer than previously realized. These ancient hominin remains were recovered from the Aurora Stratum in level TD6 of Gran Dolina, dating to approximately 800 Ka8; they have been assigned to a newly proposed species called Homo antecessor5.
Of the more than 90 hominin fossils from this level, one of the most important fossils is specimen ATD6-69, a partial facial skeleton of a juvenile that exhibits derived features of the midface that resembles the modern human condition. In particular, the anterior surface of the maxilla bears a vertical depression just below the orbit (i.e., eye socket) known as the canine fossa. The presence of a canine fossa is a derived feature that may link H. antecessor to modern humans5, while at the same time differentiating H. antecessor from Homo erectus, a species that lacks a canine fossa. H. antecessor also possesses a number of traits shared by both modern humans and Neanderthals such as an anterior placement of the incisive canal9. The presence of a canine fossa and reduced mid-face projection, which is also seen Homo sapiens, but not Neanderthals, are two of the traits that have been used to establish the Gran Dolina specimens as a separate species8. Features that are shared between H. antecessor, Neanderthals, and H. sapiens (i.e, the anteriorly placed incisive canal) suggest to many researchers that that H. antecessor was the last common ancestor of both Neanderthals and modern humans5.
One of the most fascinating aspects of the TD6 fossil assemblage concerns the question of how the hominin fossils came to rest with other faunal remains, namely cave bear, at the same locality. The conditions under which the hominin bones were deposited remain undetermined. Just as enigmatic as the assemblage itself is the presence of cut marks and percussion marks seen on most of the animal and hominin fossils. These marks resemble the same type of breakage that results from the use of a hammerstone, a tool utilized to access marrow by breaking open the bone. Some researchers have interpreted the cut and percussion marks on the TD6 fossil assemblage as evidence for routine, nutritional cannibalism among the hominins from TD610.
A recently discovered mandible known as ATE9-1 from the nearby site of Sima del Elephante dates to approximately 1.2 Ma3. Provisionally assigned to H. antecessor, this find is without a doubt the oldest hominin ever discovered in Europe. Its discovery implies that hominins migrated into Europe not long after leaving the African continent. However, the scarcity of fossils from this early date, from Atapuerca or other European sites, limits researchers’ knowledge about the earliest European hominins.
The site of Sima de los Huesos (literally translated at “Pit of Bones”) is an amazing assemblage that has yielded approximately 4000 hominin fossils attributed to the species Homo heidelbergensis. In fact, the Sima de los Huesos sample is so large that it currently constitutes nearly 80% of the entire global Middle Pleistocene hominin fossil record2. The date for the H. heidelbergensis remains in question. Initial studies based on paleomagnatism and biostratigraphy suggested dates between 200 – 320 Ka for the H. heidelbergensis material. Recent work, however, using high-resolution uranium-series dating of speleothem from Sima de los Huesos yields a minimum date of 530Ka6.
Nearly every anatomical region is represented in the Sima de los Huesos hominin assemblage. Arguably the most important fossil from the site is known as Atapuerca 5, an extremely well preserved cranium with associated mandible. As exemplified by Atapuerca 5, the Sima de Los Huesos hominins exhibit certain primitive features of the skull that are also seen in the older species Homo erectus but not shared with the relatively more derived Late Pleistocene Neanderthals. Both H. erectus and H. heidelbergensis exhibit brain sizes well below that of Neanderthals, a large mastoid process, a well-developed centrally-placed occipital torus, and a cranial breadth largest near the base of the skull8. H. heidelbergensis also shares a number of derived traits associated with Neanderthals including a double-arched rather than single-arched supraorbital torus, and the presence of a retro-molar space (i.e., a gap between the third molar and the ascending ramus of the mandible).
Brain size estimates for the most complete H. heidelbergensis specimens range from 1,125 cm3 (Atapuerca 5) to 1,390 cm3 (Atapuerca 4), which is comparable to Neanderthal and modern human estimates (between 1350 cc and 1450 cc), but exceeds that seen in H. erectus (~850-1250 cc). In fact, the Atapuerca specimens, along with other ancient European fossils of H. heidelbergensis (e.g., Petralona from Greece and Arago from France) share a number of traits with later Neanderthals. The shared derived features between these two species are the basis for arguments that H. heidelbergensis is the anagenetic ancestor to Neanderthals, and that Neanderthal features slowly evolved over time in an “accretion” process of Neanderthalization11. If so, this would imply that Neanderthals and H. heidlebergensis would not be considered separate species by some criteria. However, most researchers continue to refer to these distinct groups of fossils using two separate species names.
Little to no non-hominin fossils have been recovered from Sima de los Huesus, prompting some anthropologists to argue that the assemblage result from the purposefully deposition of hominin remains in the cave by other hominins, perhaps in a ritualistic manner12. An accumulation of both hominin and other animal bones would be expected if the assemblage was the result of carnivore activity. If the Sima de Los Huesos assemblage was accumulated ritualistically by hominins, this would extend the evidence of funerary practices in the worldwide fossil record by several hundred thousand years. A careful taphonomic study by other researchers13 concluded that there was a lack of evidence to uniquely support ritualistic accumulation by hominins as the source of the hominin assemblage. Thus, the source of the massive collection remains mysterious.
A large lithic assemblage, comprised of at least 268 pieces, has been recovered along with numerous Homo antecessor remains from the Aurora Stratum in Level TD6 in the Gran Dolina cave site at Atapuerca14. The Gran Dolina lithics appear similar to Oldowan tools found at many African sites, but with a greater frequency in the number of small tools, such as scrapers and notched tools. Previously, the earliest evidence of lithic industry in Europe dated to approximately 600 Ka, and was characterized by Acheulean tools. The Gran Dolina lithics resembling the more primitive African Oldowan industry are therefore consistent with the assemblage’s ancient age of approximately 780 Ka.
Some of the Gran Dolina lithics bear evidence of wear that is typical of butchering and cutting wood14. Another interesting point is that several of the raw materials used to make the lithics (e.g., flint and quartzite) would not have been available in the immediate vicinity, and must have been transported into the site by the hominins. The TD6 assemblage that contains the Oldowan like-tools may represent an extended period of cave occupation, as there is evidence for the complete stone tool manufacturing process at the site (i.e., core reduction, flake production, and debris). Meanwhile, most of the fossils (animal and hominin) from level TD6 exhibit cut marks and percussion marks which reflect the use of a hammerstone, a tool often used to break open bone in order to access marrow. Some researchers have taken the controversial position that these breaks are evidence for routine, nutritional cannibalism among the Gran Dolina hominins10. The cannibalism interpretation remains hotly debated.
Despite the more than 4000 hominin fossils found at Sima de los Huesos (“Pit of Bones”), only 1 stone tool has ever been recovered. In 1998, a hand ax was discovered at Sima de los Huesos, representing the first and only archaeological evidence for lithic tools use in the cave. The hand ax appears similar to an Acheulean handaxe, a tool-making tradition found in Europe after 600 Ka8. As there is no evidence that hominins lived at Sima de los Huesos for any length of time, the presence of a single enigmatic handeaxe may suggest that the tool and the hominin fossil accumulation represents early symbolic funerary practices12. This interpretation remains controversial however, as other researchers maintain that the current evidence does not uniquely support any interpretation regarding the significance of the handeaxe or the fossil assemblage13.
A modest lithic assemblage dating to approximately 1.2 million years has been recovered from level TE9 at Sima del Elephante3. This is the same level which produced a single hominin mandible, provisionally assigned to H. antecessor. The significance of this assemblage lies in its great age; it is the earliest evidence for hominin occupation of Europe. However, the archaeological assemblage is quite small, consisting only of four flakes, five waste flakes, and several pieces which are highly weathered and are difficult to classify. The small size of the archeological assemblage combined with the scarcity of hominin fossils from this time period seriously limits our understanding of the biology and behavior of these first Europeans.
In addition to the hominin fossils and artifacts from Atapuerca, several sites preserve abundant faunal remains as well. The name Sima de los Huesos (“Pit of Bones”) results from the large assemblage of bones that accumulated at the bottom of a shaft. This pit has produced a bounty of H. heidlebergensis fossils, along with a multitude of carnivore fossils. Many of the taxa found at Sima de los Huesos have been identified, including several medium to large size felids (cats), wolves, foxes, and a relative of the weasel known as a marten15. A large proportion (nearly 160 individuals) of the Sima de los Huesos faunal specimens belongs to a single species, Ursus deningeri, an ancestral form of cave bear.
Despite the high taxonomic diversity of the Middle Pleistocene carnivores recovered from the site, not a single herbivore fossil has been found. In fact, some researchers have argued that the lack of herbivores may indicate that the cave was a natural trap for predators. In other words, carnivores entered the cave accidentally by falling down a shaft and were unable to escape15. If this were true, then carnivores were not likely responsible for the accumulation of the hominin fossils. This fails to account for the absence of herbivore remains, as natural traps are not usually taxonomically selective. Another explanation for the hominin assemblage is that the hominin remains were ritually deposited in the Sima12. The presence of felid tooth marks combined with the lack hyaena or wolf tooth marks on the hominin bones suggests yet another explanation; that felids accumulated the hominin carcasses in the cave in an effort to prevent more aggressive scavengers from accessing them13.
Faunal remains, some associated with stone tools, are found throughout several depositional layers of Gran Dolina. Specimens found within TD6 are typical of the Early/Middle Pleistocene boundary species, including various deer, bears, and wild cats. Larger mammalian remains include the spotted hyena, a type of extinct horse, and even an extinct rhinoceros species16. Fossil remains suggests the small animals were brought into the cave in their entirety by hominins, while only partial skeletal material is present for carcasses of large ungulates, suggesting a butchering site outside the cave16.
Fauna have been recovered from several levels of Sima del Elefante (“Pit of the Elephants”). Level TE9, the same level that yielded a small Mode I archaeological assemblage and a hominin jaw attributed to H. antecessor, has a diverse collection of both large and small vertebrates17. Bison, horses, rhinos, European jaguar, lynx, macaques, and beavers are among the larger mammals recovered from the site. Smaller animals are represented by shrews and various rodents17. Certain small mammals, including voles of the genus Mimomys and Microtus are useful as biostratigraphic indicators because different forms of these genera occur through time and can be used to determine the relative age of different sites. The appearance of Mimomys in TE9 and the lack of more advanced forms of Microtus are consistent biostratigraphically with the 1.2 Ma date assigned to this level (TE9) which represents the earliest human occupation of Europe3.
Atapuerca is a low mountain encompassing an area roughly 12 km2 and rising approximately 100 m above the river flats surrounding it18. Fossils from Atapuerca have been found in karstic caves, a type of cave that forms when water dissolves large holes in ancient Cretaceous limestone formations. At Atapuerca, the Pleistocene sediments, bones, and artifacts that accumulated within some of these cavities (e.g., Sima de los Huesos and Gran Dolina) were preserved until their discovery hundreds of thousands of years later by modern paleontologists and archaeologists.
The fossil locality known as Gran Dolina was first exposed when, in the late 1800s, a railroad company cut a trench through the Cretaceous limestone and exposed the Pleistocene infilling sediments. Today, the once filled cavity is no longer covered by a limestone roof, and is open to the air. The Gran Dolina site preserves an 18 m section of sedimentary infill which has been divided into 11 different lithological units19 identified as T1 – T11. The fine grained clays and speleothem sediments in the lowest levels all originated from within the cave. Layers T3 – T11 consist largely of external deposits, or sediments that were washed into the cave from outside. Most of the Gran Dolina levels preserve either fossils, archaeological material, or both. One of the most notable units, TD6, is comprised of 2 to 2.5 m of breccia, a type of sediment comprised of limestone chunks from the cave walls that is cemented together in a red-colored calcite-rich matrix. Within TD6 (~780 Ka) is a subunit known as the Aurora Stratum, an approximately 20 cm thick bed of fine-grained sediment that appears yellowish-red in color19. The Aurora Stratum has yielded fossils of H. antecessor as well as Mode I lithic tools.
The Sima del Elefante cave is approximately 18 m deep and completely filled with sedimentary layers which have been divided into 16 distinct units3. Most of these units represent debris flows that contain gravel and boulders of varying sizes3. Unit TE9, which dates to ~1.2 Ma and has yielded the earliest known hominin of Europe, is sometimes referred to as “toblerone”20. Just like the popular chocolate candy, TE9 consists of fragments suspended a brown matrix. In the case of TE9, the fragments are angular rocks, and the matrix is a dark brown clayey matrix.
The site of Sima de los Huesos is located in the Cueva Mayor, a portion of the cave system not exposed by the railway construction in the 1800s. Sima de los Huesos, (literal translation: “pit of bones”) is located approximately half a kilometer into the modern entrance of the Cueva Mayor. To approach the fossil site, excavators must negotiate a shaft approximately 2-4 m in diameter that plunges 13 m down to the locality known as Sima de los Huesos21, and terminates at the top of a steeply inclined ramp. The ramp descends down to the pit in which hominin and carnivore bones are entombed, giving the cave its name, “The Pits of Bones”. Excavation of the fossil bearing layers is extremely tedious and difficult given the location of the pit at the base of the ramp.
A lack of clearly distinct sedimentary bedding at Sima de los Huesos makes any interpretation of the fossil stratigraphic context difficult and complex. The hominin bones are found within a muddy breccia (rock cemented by calcite, common in caves), which is subsequently covered with another muddy sedimentary layer that contains a huge number of cave bear bones. Finally, the fossiliferous deposits are capped with layers of flowstone and bat feces (guano) in which no fossils have been recovered.
Analyses of floral and faunal remains from Atapuerca hold information regarding the environments faced by H. antecessor and H. heidlebergensis in Pleistocene Spain. Sima del Elephante, Gran Dolina, and Sima de los Huesos are three site at Atapuerca that preserve the most information regarding ancient environments. Taken together, these sites offer important clues regarding the influences of climate on Pleistocene human evolution.
Insectivore faunal remains recovered from Sima del Elephante, dating to approximately 1.2 Ma, point to a relatively warm and humid climate in Northern Spain3.
Nearly 400 Ka later, northern Spain continued to experience a relatively mild climate compared to other European areas north of the Pyrennes Mountains. More species have been recovered from level TD6 at Gran Dolina (~780 Ka) than at any other site at Atapuerca18. Such high taxonomic diversity suggests the presence of a relatively warm and productive environment. Furthermore, cold adapted species are absent from the TD6 ungulates assemblage22. Remains of waterfoul from TD6 (e.g., ducks, crakes, and dippers) suggests that bodies of water once existed nearby; and pollen analysis indicates that Gran Dolina’s TD6 was deposited under moist conditions similar to the conditions of present-day Mediterranean forests, abundant with flora such as cypress and oak trees16. Overall, the flora and fauna from Gran Dolina represent a temperate and humid climate at approximately 780 Ka.
Atapuerca was relatively colder and wetter by at ~530Ka15. Pine, oak, birch, and beech pollen have been identified in sediments associated with the hominin remains from Sima de los Huseos15. In addition to the many specimens of H. heidlebergensis known from the site, Sima de los Huesos is notable for the diversity of carnivores found at the site. Most carnivores have wide habitat tolerances, and thus are not terribly useful for paleoecological reconstruction. However the extremely high abundance of carnivore species in the Sima de los Huesos assemblage implies that the environment must have been relatively productive to support so many different kinds of predators15.
When compared to contemporaneous hominin localities farther north, relatively warm conditions appeared to have dominated the area around Atapuerca between 1.2 and 530 Ka. As Atapuerca provides the earliest evidence for human occupation in Europe, the climatic conditions at the site may prove an important factor for understanding human European origins. Perhaps the relatively warm and stable conditions south of the Pyrenees Mountain in Spain may have eased the transition of tropical-adapted hominins shortly after they left Africa to colonize Europe.
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