67S. Condemi and G.-C. Weniger (eds.), Continuity and Discontinuity in the Peopling of Europe: One Hundred Fifty Years of Neanderthal Study, Vertebrate Paleobiology and Paleoanthropology, DOI 10.1007/978-94-007-0492-3_6, © Springer Science+Business Media B.V. 2011

Abstract We present a revision of the main features with phylogenetic interest observed in the human fossil remains recovered from the Aurora Stratum of the TD6 level, Gran Dolina site (Sierra de Atapuerca, Spain) that have been assigned to Homo antecessor. Our aim is to test the hypothesis of a possi-ble relationship between this species and the European Middle and early Late Pleistocene hominins, the so-called Neanderthal lineage. Some cranial, postcranial, and dental features are ple-siomorphic for the genus Homo and thus, they are not useful for our purpose. Other morphologies are derived with regard to H. ergaster/H. erectus, and TD6 hominins share those traits with modern humans, with Neanderthals or with both lineages. In this context we hypothesize either that there exists a phylo-genetic continuity between Homo antecessor and Neanderthals or that both species shared a common ancestor.

Keywords Human evolution • Pleistocene • Homo antecessor • Phylogenetic analysis.

Introduction

The origin of Neanderthals has been a matter of interest in Paleoanthropology, from the classical publications made by Boule (1911–1913), Howell (1951), Le Gros Clark (1955), or Boule and Vallois (1957), among others, to the most recent papers on ancient DNA recovered from fossil remains (Green et al. 2006; Noonan et al. 2006). There is a general agree-ment that the Neanderthals have deep roots in the Middle Pleistocene of Europe and they have been firmly related to Homo heidelbergensis (Stringer 1993a; Arsuaga et al. 1997). This species was named in 1908 by Otto Schoetensack to include the human fossil jaw found 1 year before at 25 m depth in the alluviate sand levels of the Neckar river, near the German village of Mauer, and 16 km SE from the city of Heidelberg. This jaw belonged to an individual who probably lived during the early Middle Pleistocene, between 640,000 and 735,000 years ago (Hambach 1996).

During the 1980s, some authors understood the necessity of grouping some fossil specimens with a morphology that was more evolved than that of the Early Pleistocene homi-nins (mainly H. erectus and H. ergaster) but still less derived than that of our own species. Thus, some Middle Pleistocene African and European specimens like those from Arago, Binzingsleben, Bodo, Kabwe, Petralona, or Swanscombe, which exhibited a combination of archaic and specialized traits not found in H. erectus and known as “archaic H. sapiens,” were included in H. heidelbergensis taxon (Stringer 1985, 1993b; Rightmire 1988), with the Mauer mandible as the holotype of the species. Furthermore, this species was con-sidered the Middle Pleistocene common ancestor for Neanderthals and modern humans (Stringer and McKie 1996; Tattersall 1996; Rightmire 1996, 1998a).

The study of the exceptional hypodigm of more than 5,600 human fossil remains (belonging to a minimum of 28 individuals) recovered so far from the Sima de los Huesos site of the Sierra de Atapuerca in northern Spain strongly suggests that Neanderthals appeared as a result of a local evolution of the Middle Pleistocene populations (e.g. Arsuaga et al. 1993; Arsuaga et al. 1997; Bermúdez de Castro 1993; Rosas 2001; Martinón-Torres 2006). The Sima

J.M. Bermúdez de Castro (*), M. Martinón-Torres (*), A. Gómez-Robles, A. Margvelashvili, and S. Sarmiento Centro Nacional de Investigación sobre la Evolución Humana (CENIEH), Burgos, Spain e-mail: josemaria.bermudezdecastro@cenieh.es; maria.martinon.torres@gmail.com; aida.gomez@cenieh.es; susanasarm@gmail.com

J.L. Arsuaga Departamento de Paleontología, Universidad Complutense de Madrid- Instituto de Salud Carlos III, Madrid, Spain e-mail: jlarsuaga@isciii.es

J.M. Carretero Universidad de Burgos, Edificio I + D, Burgos, Spain e-mail: jmcarre@ubu.es

I. Martinez Instituto de Salud Carlos III, Madrid, Spain e-mail: imartinezm@isciii.es

Chapter 6The Gran Dolina-TD6 Human Fossil Remains and the Origin of Neanderthals

José María Bermúdez de Castro, María Martinón-Torres, Aida Gómez-Robles, Ann Margvelashvili, Juan Luis Arsuaga, José Miguel Carretero, Ignacio Martinez, and Susana Sarmiento

68 J.M. Bermúdez de Castro et al.

de los Huesos hominins have been included in H. heidelber-gensis by Arsuaga et al. (1997). The last radiometric studies (U-series) of a 14-cm thick in situ speleothem overlying the mud-breccia containing the human bones of the Sima de los Huesos site indicate an age of about 600,000 years for these hominins (Bischoff et al. 2006). These results confirm the deep roots of Neanderthals in the European Middle Pleistocene, and support the notion that H. heidelbergensis should be considered, together with H. neanderthalensis King, 1864, as a chrono-species of the same “evolutionary” species (Arsuaga et al. 1997). In fact, except on rare occa-sions (e.g. the occipital from Vértesszöllös), most European Middle Pleistocene fossil specimens, such as those of Arago Petralona, Swanscombe, Steinheim and, of course, those from Sima de los Huesos site, exhibit one or more derived cranial and mandibular traits shared exclusively with Neanderthals (Neanderthal apomorphies). Furthermore, the dental proportions of the Mauer mandible, specially the buc-colingual dimensions with respect to the total dental size, are similar to those of Neanderthals (Rosas and Bermúdez de Castro 1998; Bermúdez de Castro et al. 1999).

The fossil record suggests that at least from the middle of the Middle Pleistocene Europe was characterized by an endemism. This process was favored by the peculiar paleo-geographical and paleoclimatological conditions of the European Peninsula during this long period (Hublin 1990). European hominins from this period would have evolved in isolation, probably without interbreeding with other non-European populations. Thus, the last common ancestor of Neanderthals and modern humans could not be represented in the European Middle Pleistocene fossil record (Arsuaga et al. 1997) currently available. The common ancestor of Neanderthals and modern humans should be more primitive and should also lack the specialized features characterizing each of these hominin lineages. Thus, we could track back in the fossil record to look for a more ancient hominin, who could be recognized by the presence of some features shared with the Neanderthal and modern human lineages.

Between 1994 and 1996, a rich Early Pleistocene assem-blage of fossils and lithic industry was found at the so-called Aurora Stratum of the TD6 level of the Gran Dolina site, in the Railway Trench of the Sierra de Atapuerca. The assem-blage, which initially included a total of 86 human fossil remains, has been significantly increasing during the 2003–2006 field seasons. These fossils exhibit a unique combina-tion of a modern face and a primitive dentition that led to the Atapuerca research team to name a new species, H. antecessor (Bermúdez de Castro et al., 1997). We further concluded that this species might represent the last common ancestor to both the Neanderthal and modern human lineages, as an alternative to H. heidelbergensis.

In 2003, we made a comparative study of the TD6 and Sima de los Huesos human dental samples. The clear morpho-logical differences found between both populations led us to

propose a possible discontinuity between the Early Pleistocene European populations (represented by the TD6 hominins) and the European Middle Pleistocene populations (represented by the Sima de los Huesos hominins) (Bermúdez de Castro et al. 2003, and see also Carbonell et al. 2005). However, additional studies of the dentition (Martinón-Torres 2006; Martinón-Torres et al. 2007; Gómez-Robles et al. 2007) compelled us to reconsider this hypothesis. The aim of this report is to exam-ine the present TD6 human sample in order to test the hypothe-sis of a possible phylogenetic relationship between H. antecessor and the heidelbergensis/neanderthalensis lineage.

The Age of theTD6 Level and the Human Fossil Sample

The first paleomagnetic investigation of the Gran Dolina site was performed by Parés and Pérez-González (1995). They found a paleomagnetic inversion at the TD7 level, 1 m above the Aurora Stratum, which they identified with the Matuyama–Brunhes boundary, based on the information furnished by the Gran Dolina fossil assemblages. The TD6 macromammals sample includes Vulpes sp., Canidae indet., Mustelidae indet., Panthera sp., Felis silvestris, Ursus, sp., Proboscidea indet., Equus sp., Stenoniano, Stephanorhinus etruscus, Sus scrofa, Dama dama vallonetensis, Cervus elaphus cf. Acoronatus, Megalocerus cf. verticornis, and Capreolus capreolus (van der Made 1999; García and Arsuaga 1999). This fossil assem-blage is characteristic of the end of the Early Pleistocene and the beginning of the Middle Pleistocene. Among the microver-tebrates, the presence in TD6 of Mimomys savini, also repre-sented in TD7 and TD8, is noteworthy for the age determination of the Aurora Stratum (Cuenca-Bescós et al. 1999). Another study by Parés and Pérez-González (1999) confirmed that the Gran Dolina lower levels (TD1–TD6) displayed reversed polarity, whereas the upper levels (TD7–TD11) were normal. At the bottom of the TD section, these authors reported evi-dence of a short normal polarity event, which they interpreted as Jaramillo or Kamikatsura event. The electron spin reso-nance and U-series results obtained by Falguères and co-workers (1999) suggest an age range between 780,000 and 857,000 years for the Aurora Stratum.

The TD6 human hypodigm consists of more than 100 fragmented bones belonging to the cranial and postcranial skeleton. The sample includes more than 50 parts of clavi-cles, radii, femora, vertebrae, ribs, patellae, metacarpal and metatarsal bones, pedal, and manual phalanges. Fragments of frontal, parietal, temporal, occipital, maxillary, zygomatic, and sphenoid bones, as well as four mandibles, comprise the cranial sample. The dental sample consists of 5 deciduous and 37 permanent teeth. The human remains recovered until the 2005 season have been assigned to a minimum of nine individuals, identified by the maxillae, mandibles, and the

696 The Gran Dolina-TD6 Human Fossil Remains

teeth (Bermúdez de Castro et al. 2006). Some of the pieces recovered during 2003–2006 period are unpublished. Two mandibles found in 2006 are currently in the process of restoration.

Cranial and Mandibular Features

Arsuaga et al. (1999) made the study of the neurocranial and facial bones from TD6. We refer to these authors for a detailed anatomical description of the specimens and their particular features, and we will quote only those traits with taxonomical interest for the aim of this report.

The superior border of the temporal squama in the tempo-ral bone fragment ATD6-20 is high and arced. This feature is a derived trait present in European and African Middle Pleistocene populations, modern humans, and Neanderthals, as well as in the Early Pleistocene Ceprano calvaria and later Asian Middle Pleistocene specimens. A high and arced squa-mosal suture is related to an increase in cranial capacity, which in TD6 hominins could be greater than 1,000 cc, according to the dimensions of the ATD6-15 frontal bone (Carbonell et al. 1995).

The facial skeleton is well represented in TD6 by five frag-ments, the most complete being ATD6-69 (Fig. 6.1). This spec-imen shows a fully modern pattern of midfacial morphology,

therefore bearing no resemblance to the derived face of Neanderthals, who exhibit a characteristic midfacial prog-nathism (Rak 1986). This feature is also present in some of the European Middle Pleistocene specimens, such as Arago 21, Petralona, and those of Atapuerca-Sima de los Huesos. Although ATD6-69 belonged to an adolescent with incom-plete facial growth, the specimens ATD6-19 (a small adult right zygomaxillary fragment), and ATD6-58 (an adult left large zygomaxillary fragment, lacking only the zygomatic process) also exhibit “modern” traits. ATD6-58 shows some expansion of the maxillary sinus that reduces the expression of the canine fossa.

The internal nasal cavity of ATD6-69 lacks the three Neanderthal apomorphies described by Schwartz and Tattersall (1996): development of an internal nasal margin bearing a well-developed and vertically oriented medial pro-jection, swelling of the posterior-lateral wall of the nasal cav-ity as a result of a medially expanded maxillary sinus, and lack of an ossified roof over the lacrimal groove. Furthermore, nasal crests of ATD6-69 are similar to those of modern humans and lack the typical Neanderthal sharp lower margin formed by the lateral crest (Arsuaga et al. 1999). The lateral nasal crest is vertical and slightly concave, with its lower extremity behind the rhinion. The orientation of the lateral nasal walls in ATD6-69 clearly suggests that the nasal bones were elevated and forward sloping, a derived condition that H. antecessor shares with modern humans and Neanderthals.

Fig. 6.1 ATD6-69 maxilla of Homo antecessor

70 J.M. Bermúdez de Castro et al.

ATD6-69 also shows an anteriorly located incisive foramen, 5 mm behind the anterior alveolar margin. This incisive fossa opens up in the floor of the nasal cavity and it is also anteri-orly placed. As a consequence, the incisive canal is nearly vertical. In H. erectus/H. ergaster, the incisive foramen is placed well behind the anterior alveolar margin and the inci-sive canal lies obliquely (Rightmire 1998b). ATD6-69 shares this trait with Neanderthals and modern humans, although we have also noticed a vertical incisive canal in the Buia cranium from the Early Pleistocene of Northern Danakil Depression in Eritrea.

The small mandibular fragment ATD6-5, which belongs to an adolescent, and the left half of a gracile adult mandible ATD6-96 (Fig. 6.2) exhibit a primitive structural pattern that is shared with all African and Asian Pleistocene specimens. Furthermore, none of the mandibular features considered apomorphic by Rosas (2001) in the European Middle and early Late Pleistocene hominin lineage are present in any of the two TD6 specimens. The position of multiple mental foramina at the level of P3–P4, the position of the lateral prominence at the level of M2, the low position of the mylohyoid line in relation to alveolar margin at the M3 level, the parallel trajectory of the mylohyoid line in relation to alveolar margin, the shallow relief of the pterigoyd fossa,

and the lateral intersection between the mandibular notch and condyle are plesiomorphic traits observed in the TD6 specimens (Rosas and Bermúdez de Castro 1999; Carbonell et al. 2005). Some features of ATD6-96, such as the position of the M3 in relation to the ramus, the oblique inclination of the retromolar area, or the regular profile of the gonion are slightly derived with regard to the primitive status, ascer-tained in some H. ergaster specimens, as well as in some African (Tighenif) and Asian (Zhoukoudian) Middle Pleistocene mandibles. The absence of alveolar prominence in ATD6-5 and ATD6-96 is noteworthy, which contributes to their low corpus thickness and higher gracility in relation to the African Pleistocene specimens.

Dental Evidence

Most dental features of the TD6 hominins are plesiomorphic for the genus Homo and do not help to solve the question posed in this report. Thus, the upper lateral incisors are shovel shaped, the cingulum is present in the mandibular canines and premolars, the crown of the mandibular P3 is strongly asym-metrical with a well-developed talonid, the root morphology

Fig. 6.2 ATD6-96 hemimandible of Homo antecessor

716 The Gran Dolina-TD6 Human Fossil Remains

of the lower premolars is complex with two (MB + DL) roots, the hypoconulid is present and well developed in M1 and M2, the upper and lower first molars are, respectively, larger than the upper and lower second molars (M1 > M2), and taurodontism is expressed.

Regarding upper incisors, H. antecessor shares with H. erectus and H. neanderthalensis high degrees of labial con-vexity (Martinón-Torres 2006). H. erectus and H. antecessor also present incipient forms of the “triangular shovel shape,” typical of the Neanderthal populations (Martinón-Torres 2006). The thickened marginal ridges invade the lingual sur-face and define a deep and narrow longitudinal fossa, giving the occlusal surface a characteristic V-shape.

Lower canines show a derived morphology when com-pared to Plio-Pleistocene specimens like those assigned to Homo habilis and Homo georgicus, which resembles that of the Sima de los Huesos specimens. Lower canines in this species present an incisor-like conformation, smooth lingual surface, and long parallel marginal ridges. These traits, along with the P3 > P4 sequence, might be an evidence of the relation-ship between the TD6 hominins and the European Middle Pleistocene populations. Still, TD6 lower canines still express cingulum, which is lost in later Homo populations.

Similarly, second lower premolars are more evolved in TD6 hominins than in their African counterparts (see Fig. 6.3). They are closer to the Neanderthal morphology displaying a particular combination of plesiomorphic traits (mesial metaconid, multiple lingual cusps, transverse crest, and assymetrical contour) in association with a reduced occlusal polygon (Martinón-Torres et al. 2006). Eventually, modern human morphology could originate from the TD6 con-formation.

Finally, recent studies have shown that H. antecessor shares a derived conformation in their upper first molars (Gómez-Robles et al. 2007) with the European Middle Pleistocene populations and H. neanderthalensis. This shape

consists of a rhomboidal occlusal polygon (consequence of the relative distal displacement of the lingual cusps), associ-ated to a skewed outline with a protruding hypocone (Fig. 6.4). This conformation clearly differs from the H. sapiens shape, since this species keeps the primitive morphology observed in the African Pliocene species.

Postcranial Remains

Lorenzo et al. (1999) have made a comparative study of a sample comprising 22 hand and foot remains from TD6. They conclude that the morphology and dimensions of these remains are more similar to those of modern humans than to those of Middle and early Late Pleistocene hominins.

The adult clavicle ATD6-50 displays a set of quantitative and morphological traits shared with Neanderthals (Carretero et al. 1999). It is absolutely very long, has a low robusticity index, a pronounced shaft curvature, and relatively small epiphyses. These authors consider that ATD6-50 may present the primitive pattern (relatively longer clavicle) and, there-fore, this bone is not useful for phylogenetic analysis. The same situation applies for the ATD6-43 radius. This speci-men shows an absolutely and relatively long radial neck, a primitive feature that ATD6-43 shares with the Middle and early Late Pleistocene hominins, and differs from the derived condition (short neck length) observed in modern humans. In contrast, the diaphysis of ATD6-43 is long and straight, being more similar in these features to modern humans than to Neanderthals (Carretero et al. 1999). These authors conclude that the large absolute radial length of ATD6-43 suggests a high brachial index for H. antecessor and upper limb propor-tions more similar to H. ergaster and modern humans than to Neanderthals. Similarly, the right ATD6-22 and left ATD6-56 patellae are relatively narrow, with high patellar indices,

Fig. 6.3 Morphological comparison of the lower second premolars of Gran Dolina and Sima de los Huesos hominins. (a) ATD6-4 (Gran Dolina); (b) AT-221 (Sima de los Huesos)

Fig. 6.4 Morphological comparison of the upper first molar of Gran Dolina and Sima de los Huesos hominins. (a) ATD6-11 (Gran Dolina); (b) AT-406 (Sima de los Huesos)

72 J.M. Bermúdez de Castro et al.

similar to those of modern humans, and well above the Sima de los Huesos and Neanderthal values. Carretero et al. (1999) speculate that the Middle Pleistocene hominins, represented by the Sima de los Huesos sample, and the Neanderthals show the derived condition of an absolutely and relatively wider patellae (Table 6.1).

Discussion and Concluding Remarks

Bermúdez de Castro et al. (1997) hypothesized that H. anteces-sor could represent the last common ancestor to Neanderthals and modern humans. In order to test the hypothesis of a pos-sible phylogenetic relationship between H. antecessor and the Neanderthals, we have described in this report some of the most important features of the available TD6 fossil evi-dence, including data of some new findings in the Aurora Stratum and new studies of the material recovered during the nineties of the last century. The TD6 mandibles are a good evidence of the evolutionary status of H. antecessor. They show a generalized morphology, with some plesiomorphic traits, and lack the robusticity that characterizes the African Early and Middle specimens, generally attributed to H. ergaster, the Javanese H. erectus mandibles, and some European specimens, such as Mauer and Arago 13. The TD6 mandibles lack the features considered apomorphic of the Neanderthal lineage, but there are no evidences against the possibility of an evolutionary continuity between the TD6 hominins and the European Middle Pleistocene populations.

Another important element for discussion is the presence of a modern midfacial topography in the TD6 hominins. Now we have proofs that the modern human face appeared in the Early Pleistocene, since it is present at least in the European fossil record (TD6). Obviously, this evidence points to a phylogenetic relationship between H. antecessor and the modern human lineage. In this context, it is necessary to know if the derived Neanderthal face can originate from morphologies similar to those observed in TD6 hominins.

Since the specimen ATD6-58 exhibits a reduced expression of the canine fossa, Arsuaga et al. (1999) think that speci-mens such as AT-404 from Sima de los Huesos and Steinheim could present an intermediate morphological facial pattern between that of theTD6 hominins and that of Neanderthals.

Concerning teeth, most of the dental features of the TD6 hominins are plesiomorphic for the genus Homo. TD6 homi-nins share some sinapomorphies with H. erectus and H. neanderthalensis, particularly referred to the upper incisors morphology. The differences between the Early and Middle Pleistocene dental samples are strong, but the particular mor-phologies of the lower second premolars and upper first molars shared between these two groups could be pointing to a possible phylogenetic continuity between H. antecessor and H. neanderthalensis. Furthermore, Arago hominins show a suite of dental traits that deserves mention. Most Arago permanent teeth are large, especially those of the Arago 13 mandible. In this specimen the crown of the P3 is symmetrical and lacks cingulum and talonid. However, the apical fourth part of the root is divided in two components, MB and BL, each one with a single canal (seen by CT-scan). The crown of the P4 exhibits a well-developed talonid and the apical third of the root is also divided into two compo-nents, MB and DL, like in the TD6 sample. Also, the MB component has two root canals, and therefore, this tooth shows root morphology similar to that of Hominid 1 from TD6. On the other hand, Arago 13 and Arago 21 present a clear M1 < M2 size sequence, and the M2 and M3 of Arago 13 are hypo- and mesotaurodonts, respectively. In conclu-sion, Arago 13 shows a combination of the “Gran Dolina and Sima de los Huesos” dental traits. If the Arago hominins are related to the Sima de los Huesos hominins and both groups can be referred to H. heidelbergensis (in the sense of Arsuaga et al. 1999), then the dental evidence could support a phylo-genetic link between H. antecessor and H. heidelbergensis/ H. neanderthalensis (but see below).

Although the postcranial evidence from TD6 is limited, it seems that the preserved elements support better a relation-ship between H. antecessor and the modern human lineage than with the Neanderthal lineage.

In sum, from the dental and mandibular evidence, we realize that H. antecessor is a species that has preserved a certain number of primitive traits but, at the same time, is clearly derived with regard to H. ergaster/H. erectus. In the fragmentary evidence from the neurocranium, we can also perceive this evolution, probably due to a significant increase in the cranial capacity and associated features, such as the convexity of the superior border of the temporal squama. Concerning the middle and lower facial skeleton, it is note-worthy that the appearance of a “sapiens” pattern, which is clearly derived in relation to H. ergaster/H. erectus as well. Thus, it seems that H. antecessor could represent an event of

Table 6.1 Summary of some of the morphological traits that are derived in the TD6 fossil and their presence/absence in the Neanderthal (NEA) and modern humans (MH) lineage

TD6 NEA MH

Temporal squama high and arced + + +“Modern” midfacial morphology + − +Orientation of nasal lateral walls + + +Vertical incisive canal + + +Triangular shovel shape + + −Upper M1 morphology + + −Lower P4 morphology + + −General morphology of the hands + − +General morphology of the feet + − +

736 The Gran Dolina-TD6 Human Fossil Remains

speciation occurred in the Early Pleistocene very probably from H. ergaster/H. erectus or a related species.

An important additional evidence to this discussion is the adult calvaria recovered near Ceprano in Southern Latium, Italy (Ascenzi et al. 1996). Unfortunately, none of the human fossil remains recovered from TD6 is directly comparable with the Ceprano calvaria. Manzi et al. (2001) suggest that Ceprano specimen represents a unique morphological bridge between the clade H. ergaster/H. erectus and the African and European Middle Pleistocene hominins. In other words, Ceprano could be a representative of the last common ances-tors of Neanderthals and modern humans as well. According to this idea, Manzi et al. (2001) consider the possible attribu-tion of the Ceprano calvaria to H. antecessor or, alternatively, to another unnamed species. Mallegni et al. (2003) name the specimen from Ceprano as Homo cepranensis sp. nov., a spe-cies related to the African Middle Pleistocene hominins, often referred as H. rhodesiensis.

At this point of the discussion it is important to mention that Hublin (2001) and Stringer (2003) considered that the human fossils recovered from Aurora Stratum of the level TD6 could be included in H. mauritanicus (considering the priority of the name assigned by C. Arambourg) together with those of Tighenif (Arambourg 1954), Rabat (Thomas and Vallois 1977), Thomas Quarry, and Sidi Abderrahman (Rightmire 1990). Hublin (2001) and Stringer (2003) have not presented a formal study to test their hypothesis which, however, has been tested by Bermúdez de Castro et al. (2007). These authors conclude that the Tighenif hominins, together with other contemporaneous (Thomas Quarry and Oulad Hamida 1), and perhaps later North African speci-mens (Sidi Abderrahman, Salé, and Rabat [Kebibat]) should be considered as a subspecies of the H. ergaster species, i.e. H. ergaster mauritanicus, and may be the result of an evolu-tion in isolation in this African area. Thus, the TD6 hominins could belong to an exclusive Eurasian lineage, different from the North African group.

In 1996, Rightmire proposed that H. heidelbergensis was the result of an episode of speciation occurred in Africa or western Eurasia during the late Early or early Middle Pleistocene. The subsequent dispersal of H. heidelbergensis through Africa, Western Eurasia, and may be East Asia during the Middle Pleistocene gave rise to a wide-ranging species, which overlaps in time with late H. erectus/H. ergaster. We could agree with Rightmire’s idea of a speciation during the Pleistocene, even though we think that this event may have probably occurred earlier, perhaps around 1 million years ago. As we stated above, H. heidelbergensis should be considered, together with H. neanderthalensis, as a chrono-species of the same “evolutionary” species. Most European Middle Pleistocene specimens exhibit clear evi-dences of their relationship with the Neanderthal lineage and, therefore, they cannot be ancestors of modern humans

as well. As we have also stated above, the common ancestor of Neanderthals and modern humans should be more primitive and should also lack the specialized features characterizing each of these hominin lineages. Furthermore, the African Middle Pleistocene populations could be assigned to another species (i.e. H. rhodesiensis).

From the options presented by Rightmire for the geographic scenario of speciation event, we prefer the area of Western Eurasia. This area represents a true crossroads between Africa, Asia, and Europe, where we could expect to find a more gen-eralized morphology instead of a specialized one. The most parsimonious hypothesis for the origin of the common ances-tor to modern humans and Neanderthals would point to a region halfway to Africa (the origin of modern humans) and Europe (the origin of Neanderthals). From the morphological evidence observed in H. antecessor, this species would be related in some way to the speciation event occurred in this area during the Early Pleistocene. H. antecessor would repre-sent either the true ancestor to the Neanderthal lineage or a dead evolutionary lineage replaced or genetically absorbed during the Middle Pleistocene by another population coming from the “mother area” (Carbonell et al. 2005, and see also Manzi et al. 2001). In this scenario, Homo antecessor and Neanderthals would have shared a common ancestor.

Acknowledgments The authors acknowledge the Gran Dolina field team, and in particular Eudald Carbonell, their dedication and effort made during the excavation of the TD6 level. This research was sup-ported by funding from the Dirección General de Investigación of the Spanish Ministerio de Educación y Ciencia (MEC), Project Nº CGL2006-13532-C03/BTE, and the Cátedra Atapuerca from the Fundación Atapuerca and Fundación Duques de Soria. Fieldwork at Atapuerca is supported by the Consejería de Cultura y Turismo of the Junta de Castilla y León. Aida Gómez-Robles has the benefit of a predoctoral FPU grant of the Spanish MEC.

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