Given its chrono-stratigraphic context, is highly likely that the parietal ATD6-100/168 belongs to H. antecessor, a taxon that was described on the basis of a large set of facial, dental, cranial and postcranial elements (Bermúdez de Castro et al., 1997). Regarding the cranial vault, the temporal squama of H. antecessor was described as high and convex, a synapomorphy shared with H. sapiens and H. neanderthalensis (Arsuaga et al., 1999 and Bermúdez de Castro et al., 1997). However, the morphology of the parietal was not known. Taking into account the absence of comparable specimens with similar geochronological position, this specimen is currently the only consistent evidence we have on the parietal bone of the species living in Europe during Lower Pleistocene.

The fragment suggests small cranial dimensions, and it can be interpreted as belonging to a juvenile, as it happens with other human remains from the same site (Bermúdez de Castro et al., 2015a). Unfortunately, it is not possible to connect physically ATD6-100/168 with other cranial remains from subadult individuals recovered from TD6-2 (see Arsuaga et al., 1999). The frontal squama ATD6-15 (subadult) could belong to the same individual. However, the absence of physical connection between the two fossils hampers the confirmation of this hypothesis. The bone shows several fractures, which can provide information on the individual history of the specimen. Fracture is a type of trauma and consists of the disruption of the bone structure, normally involving the adjacent soft tissues (Bennike, 2008 and Ortner, 2003). The determination of the timing of the injury is crucial to suggest a possible cause of death. In general, signs of bone healing are indicative of a pre-mortem event. However, the distinction between peri- from postmortem fractures is a more complex task (Bennike, 2008, Boylston, 2000, Buikstra and Uberlaker, 1994 and Sauer, 1998). The pattern of fracture is the main criteria to determine the timing of the event. Fresh bone is more liable and tends to fracture in irregular edges. On the contrary, dry bone is more brittle, due to the lack of collagen, grease and fluid, and fractures in more regular fragments. Another criterion is the coloration of the edges. Changes in coloration would indicate a postmortem event, whether same coloration could indicate that the lesion occurred around the time of bone deposition (Bennike, 2008, Sauer, 1998 and Waldron, 2009). The edges of the fracture affecting the parietal boss present the same brownish (sediment) coloration that the rest of the bone. This contrasts with the described whitish edges exhibited by the fracture that affect the tip of the parietal eminence. Fractures have being classified depending on the type of force exerted on the bone (Ortner, 2003) or its morphology (Waldron, 2009). Love et al. (2011) classified the fractures of the skull based on their morphology as linear, depressed, diastatic, basilar and crushing. Within these the most frequent types are linear (non-displaced fractures that cross part or the whole skull resulting from low energy blunt force to a wide area of the skull), followed by depressed. The latter results from high-energy forces, and the range of severity determines their morphology: from depression to inward fractures with displaced fragments. The parietal is the common location for this type of fractures.

Considering the signs recorded (ragged and beveled edges, the most-likely inward displacement/loss of the inner table, location, radiating lines and coloration) in ATD6-100 and ATD6-168, the lesion could correspond to a depressed fracture resulting from a high-energy force to the parietal bone. According to Love et al. (2011) in violent deaths the cranial bone most frequently involved is the parietal, followed by the occipital, frontal and temporal. The signs are also consistent with fresh bone fracture and suggest that it occurred around the time of the individual's’ death. A cannibalistic activity was hypothesized to be associated with the fossil collection recovered in Gran Dolina (Carbonell et al., 2010, Fernández-Jalvo et al., 1999 and Saladié et al., 2012), in agreement with the peri-mortem time of the event. In the context of a cannibal event, a violent trauma would be highly possible, but the evidence is too fragmentary to make a diagnosis. Evidences of cranial fracture and trauma in the fossil record are numerous (e.g., Brain, 1993 and Wu et al., 2011). However, only in a very few cases it was possible to unequivocally associated the trauma with the cause of death (see Cranium 17 from Sima de los Huesos; Sala et al., 2015). Future discoveries may help to clarify the nature of the fracture in the H. antecessor parietal.

From an evolutionary point of view, the parietal bone is relevant because it underwent different changes within the radiation of the human genus (Bruner et al., 2015 and Holloway, 1981). Pleistocene specimens assigned to Homo ergaster and H. erectus s.s. display a shared plesiomorphic parietal morphology, characterized by a parasagittal flattening of the upper surfaces (Bruner et al., 2015). In contrast, Neandertals show a lateral bulging of these areas, and modern humans expand further the superior parietal lobules through a notable longitudinal enlargement (Bruner, 2004, Bruner et al., 2003 and Bruner et al., 2011a). This parietal growth, specific of H. sapiens, is associated with an early postnatal stage only present in our species, and not in Neandertals or living apes (Gunz et al., 2010, Neubauer et al., 2009, Neubauer et al., 2010 and Scott et al., 2014). Therefore, after the first year of age the parietal morphology can reveal its general species-specific final shape. Although incomplete, the parietal bone from ATD6-100/168 suggests an archaic phenoptype: the marked bossing is associated with the inferior parietal lobule (namely the supramarginal gyrus) and the superior parietal areas are flat and depressed. This parietal morphology is generally associated with human species such as H. ergaster and H. erectus (Bruner et al., 2003, Bruner et al., 2015 and Grimaud-Hervé, 1997).

The parietal foramina of ATD6-100/168 are interesting because the information available on these features is scarce, and any additional record adds to the development of a more extended database. Nonetheless, the feature per se cannot add much to the analysis of single specimens. ATD6-100/168 shows a large parietal foramen and a smaller accessory one approaching the lambda. Multiple parietal foramina with variable position and number, and presence of intracranial and extracranial accessory canals of inconstant diameter communicating with the diploe have been previously described in modern humans (Boryslawski et al., 2002, García Gil et al., 2015, Panteado and Santo Neto, 1985, Schelling, 1978, Stallworthy, 1932 and Zenker and Kubik, 1996). However, multiple parietal foramina are rare, having a prevalence of 2.5% in modern populations (Boyd, 1930). Lang and Brückner (1981) mentioned that parietal foramen develops during the first year of life and its dimensions remain constant (in Braga and Boesch, 1997). Therefore, we must assume that the foramen in ATD6-100/168 has reached its final size. Parietal emissary foramina generally contain a common emissary vein connecting the intra and extracranial venous space (Boyd, 1930), and may supply anastomoses between the middle meningeal artery and the occipital artery, or with other pericranial vessels (Alvernie et al., 2006, Knott, 1881, Schelling, 1978 and Yoshioka et al., 2006). Endocranially, the connections of anastomosing arteries are often clearly visible on the grooves of middle meningeal pathways leading towards the parietal foramina (Stallworthy, 1932). In ATD6-100/168, there is an ectocranial trace of a vessel from the foramen to the sagittal suture, although the pathways of the scalp arteries (occipital or pericranial) towards the parietal foramen are generally not visible (Yoshioka et al., 2006). At present, there is no information on the possible functional or structural meaning of these passages, and of their variations. The concentration of minor vascular passages around the lambda between the endocranial surface and the diploe is also a character common in humans, although quantitative data are lacking. The absence of arachnoid granulations may remark the young age of the subject, although also in this case data on these traits, supposed to be associated with aging, are incomplete (Grossman and Potts, 1974 and Le Gros Clark, 1920).

The traces of the middle meningeal artery are frequently analyzed in paleoneurology, because of their variations among hominids (Bruner and Sherkat, 2008). Modern humans display a complex vascular network, mostly at the parietal surface, while non-modern species show simpler vascular systems (Bruner et al., 2005 and Grimaud-Hervé, 1997). During morphogenesis, the complexity of these networks increases, mostly before 2 years of age (Saban, 1995). Although also in this case the exact functions of these vessels are not known, they may be involved in endocranial thermoregulation (Bruner et al., 2011b and Rangel de Lázaro et al., 2015). Archaic human species tends to have larger posterior branches, while modern humans and Neandertals display larger anterior vasculature (Grimaud-Hervé, 1997). In ATD6-100/168 the middle (obelic) branches probably originate from the posterior network, so it would fit the archaic pattern. Such scheme is named Adachi II pattern, a terminology based on the reciprocal relationships among the arterial branches (Falk, 1993 and Falk and Nicholls, 1992). There is no evidence of a clear association between this vascular pattern and cranial morphology (Bruner et al., 2009), although its high frequency in Asian H. erectus suggests that it can be associated with braincases characterized by relatively large posterior districts, namely with large and projecting occipital areas and flat and small parietal areas (Bruner, 2015). Nonetheless, although in this case the parietal vasculature has a posterior origin, also the anterior branches show large traces and ramifications, and the overall vascular system looks equally distributed between the anterior and posterior districts.

Cranial thickness is another feature largely investigated in paleoanthropology, because of its variations among hominids (e.g., Balzeau, 2013, Curnoe, 2009, Gauld, 1996, Kennedy, 1991, Marsh, 2013 and Nawrocki, 1991). Cranial thickness shows a positive allometry and marked correlation with body size in primates, with modern humans showing thicker frontal and parietal bones, and thinner temporal bones (Gauld, 1996). Although there are differences associated with sex and race, the large individual variation, the subtle differences, and the environmental influences, hamper a clear understanding of the role of these factors (e.g., Adeloye et al., 1975, Hwang et al., 2000, Jung et al., 2003, Lynnerup, 2001, Peterson and Dechow, 2002 and Torres-Lagares et al., 2010). After a longstanding debate contrasting genetic and mechanical hypothesis, today is generally accepted that bone thickness is largely influenced by hormones and physiological responses, including physical activity and life-style with their consequent release of systemic factors (Anzelmo et al., 2014, Baab et al., 2010, Lieberman, 1996 and Menegaz et al., 2010). Most of the bone thickness growth occurs rapidly before the 6 years of age, with a following gradual increase until the age of 20 (Anzelmo et al., 2014, Hatipoglu et al., 2008, Li et al., 2011 and Tsutsumi et al., 2013). The thickness of ATD6-100/168 is at the lower range of the modern humans’ adult variation, but it is below the adult range in fossil species. According to the Neandertal age curve, its thickness is compatible with that of a juvenile specimen. Although all fossil hominids show thicker vault elements, a thin bone thickness can be also observed in the Mojokerto individual, an Asian specimen dated approximately to a period between 0.8 and 1.8 Ma and with an age between 1 and 8 years old (Balzeau et al., 2005). Following these general figures, the age of ATD6-100/168 can be tentatively bracketed between 4–12 years. Nonetheless, taking into account the limited information on thickness variations and the limited fossil record such estimation is largely speculative.

A special mention must be done for the diploic thickness. Actually, thick vault bones in archaic humans were largely due to the diploic layers (Copes and Kimbel, 2016). Although its value can be sensitive to special physiological and pathological conditions, preliminary analyses suggest age-dependent patterns (García Gil et al., 2015). According to these results, young ages are associated with a vascularized diploe, and adolescent bones are characterized by a reduction of the diploic space, which further increases in terms of thickness and vascularization in later adult age. However, in adults there is no apparently correlation between parietal thickness and vascularization (Eisová et al., 2016). Diploic vessels are also supposed to be relevant for thermoregulation, and their channels are probably more developed in modern humans than in extinct human species or in non-human primates (Hershkovitz et al., 1999 and Rangel de Lázaro et al., 2015). In the case of ATD6-100/168, the diploic space occupies a percentage of the thickness (40%) comparable with low modern adult values (39–52%; Eisová et al., 2016), and no large diploic channels can be evidenced. Both features are in agreement with a young age. In the case of Mojokerto, the extension of the diploe was compared with a modern pattern with more than 3 years of age (Balzeau et al., 2005).