The Sterkfontein site lies in northeastern South Africa, 10 km NNW of Krugersdorp in the Gauteng province (Fig. 1a). Located within a complex endokarstic system formed by the dissolution of Precambrian dolomite limestone along bedding planes and vertical joints via the action of weakly acidic groundwater (Kuman and Clarke, 2000, p. 829), the cavities at the site became filled with calcareous bone-bearing breccia. As a consequence of its land-use history and the repeated collapse-fill sequences involved in its genesis, great efforts were required to ascertain the spatial and temporal sequences of the site (see Brain, 1957, 1958; Robinson, 1962; Robinson and Manson, 1962; Howell, 1967; Tobias and Hughes, 1969; Wilkinson, 1973, 1983; Partridge, 1978; Stiles and Partridge, 1979; Tobias, 1979; Clarke, 1985, 1994; Partridge and Watt, 1991; Kuman and Clarke, 2000). Six lithostratigraphic members were originally identified (Partridge, 1978; Partridge and Watt, 1991), numbered 1 to 6 (oldest to youngest). Post Member 6, the Stw 53 Infill and the Oldowan Infill were described at a later date (Kuman and Clarke, 2000).
Since 1936 about 500 hominid fossil specimens have been recovered from this site (Partridge et al., 2003), the bulk coming from Member 4 (MNI = 87 individuals; Pickering et al., 2004). Among the most famous of these fossils are the cranium Sts 5 (commonly known as “Mrs. Ples”; Fig. 1b) and the partial skeleton Sts 14 (Fig. 1c). Both specimens were discovered during the 1947 fieldwork season within the same lithostratigraphic unit (Member 4C). After having originally been attributed to different species, by the early 1950s the entire Sterkfontein collection (Member 4) was commonly assigned to Australopithecus africanus (Schwartz and Tattersall, 2005).
Member 4 broadly corresponds to the Type Site but its geological age has been widely debated. According to Clark (2002), the stratigraphic position and the archaeological and palaeontological content of Member 4 indicate it to encompass the age interval of 2–3 My. Partridge (2005) proposed an age for the Sts 5 cranium of between 2.14 and 2.15 My.
According to the original description of Sts 5 by Broom et al. (1950), the degree of closure of the cranial sutures is in agreement with an individual who was “no longer young” (Broom et al., 1950, p. 14). However, Thackeray et al. (2002a) later identified immature traits in the dentition. In particular (thanks to the use of high-quality CT scan techniques) they indicated that the apical ends of the roots of the right third molar (RM3) were not fully closed. Similarly, Sts 14 was also originally considered to represent an adult (Robinson, 1972), but again, subsequent studies revealed signs of immaturity in the iliac crest, the ischial tuberosity, the pubic spine, the anterior superior iliac spine (ASIS), the synchondrosis between the first and the second sacral vertebrae, and the epiphyseal plate of the sacro-iliac joint (Berge and Gommery, 1999; Häusler and Berger, 2001; Thackeray et al., 2002b). All these latter studies consider Sts 14 to be a subadult individual. In particular, Berge and Gommery (1999) established its owner's age-at-death to be little more than 12 years from the ossification stage of the sacrum. On the basis of their very similar developmental stages and matching sex attributions, Thackeray et al. (2002b) and Gommery and Thackeray (2006) proposed that Sts 5 and Sts 14 might belong to the same individual, a gravitational slump explaining their relative spatial positions within the site.
The aim of this study was to reassess the developmental stage and the biological and chronological ages-at-death of the Sterkfontein Sts 5 and Sts 14 fossils, using developmental information for chimpanzees and extant humans. The results were then used to test whether these specimens are developmentally compatible, and therefore might represent a single individual. Finally, the consistency of the encephalization quotient (EQ) of this hypothetical individual with that currently estimated for A. africanus was investigated. To date, no other Australopithecus skeletons recovered have been well enough preserved to make an accurate assessment of this key aspect of the biology of extinct hominins.
Development of Australopithecus africanus
Numerous studies have addressed the maturation pattern and growth rate of the extinct species A. africanus, focusing mainly on the calcification and incremental histological patterns of the dentition and the sequence of tooth emergence and facial growth, but also on the development of the postcranial skeleton. Although some authors (Mann, 1975, 1988; Mann et al., 1987, 1990; Lampl et al., 1993) have supported the idea that the developmental pattern for South African Australopithecus is essentially human-like, it is currently widely accepted that the pattern and timing of postnatal maturation in this species is essentially ape-like (Bromage, 1985, 1986, 1987; Bromage and Dean, 1985; Beynon, 1986; Smith, 1986, 1987, 2004; Conroy and Vannier, 1987, 1988, 1991; Beynon and Wood, 1987; Dean, 1987a, b, 1988, 2000; Beynon and Dean, 1988; Berge, 1998; Dean et al., 2001; Anemone, 2002).
Relative Maturation of the Cranial and Postcranial Systems
Sts 5 is edentulous; only indirect information on the length of the roots can be gleaned from the depths of the alveoli (see later). Therefore, to test the consistency between Sts 5 cranial and Sts 14 postcranial development, skeletal traits were examined. In particular, by taking into account the preservation of these specimens and making use of information in the literature, the slightly earlier closure of the spheno-occipital synchondrosis (SOS) relative to the ossification of the epiphysis of the iliac crest was confirmed (Stewart, 1934; Zilhman, 2007). However, the time span between these maturational events is quite small and reversals of this sequence are not uncommon in modern human populations (Stewart, 1934). Therefore, it was considered that both events take place at roughly the same time, and that small advances or delays of one relative to the other are within the normal range of variation. These advances/delays occur because certain sets of bones are more sensitive to factors affecting skeletal development (see Tanner, 1962), causing some degree of variability in the ossification sequence.
MATERIALS AND METHODS
The Sts 5 specimen is a near complete cranium with some parts of the cranial vault and the dentition missing (Fig. 1b). Sts 14, on the other hand, is a partial skeleton composed of six lumbar (Sts 14a, Sts 14b, Sts 14c, Sts 14d, Sts 14e, Sts 14f, caudo-cranially oriented) and nine thoracic (Sts 14g, Sts 14h, Sts 14i, Sts 14k, Sts 14l, Sts 14m, Sts 14n, Sts 14o, Sts 14p, caudo-cranially oriented) vertebrae (in different states of preservation), several rib fragments (MNE = 9, Pickering et al., 2004), both innominate bones (Sts 14r, Sts 14s; quite complete in their preservation), a partial sacrum (Sts 14q), and the proximal half of the left femur (Sts 14t) (Robinson, 1972) (Fig. 1c).
Data were collected from the Sts 5 and Sts 14 original fossils housed in the Transvaal Museum in Pretoria. Morphological descriptions, photographs of these specimens, and a high-resolution computer tomography (CT) (0.2-mm slice thickness, 120 kV, 299 mA, 512 × 512 matrix, 0.152 mm pixel size) scan of Sts 5 (conducted at the Little Company of Mary Hospital, Pretoria) were made. A commercial tomography scan of Sts 5 (Weber et al., 1997a) was also used as a data source.
The comparative sample was composed of a set of CT scans from nine specimens of Pan troglodytes from Equatorial Guinea but of unknown age-at-death (all held at the Estación Biológica de Doñana [EBD], Spain) (0.5 mm slice thickness, 170–160 kV, 4.0–3.75 mA, 1024 × 1024 matrix, 0.217–0.253–0.289 mm pixel sizes) (Table 1). The original specimen of the partial adult cranium Sts 71 (A. africanus) from Sterkfontein, held at the Transvaal Museum in Pretoria, was also examined. A commercial CT scan of this specimen (Weber et al., 1997b) was also used as an information source. The comparative sample was completed with information from the literature on the pattern and schedule of development of the traits studied in chimpanzees and modern humans. The reason for including extant humans was the incomplete data regarding the maturation of certain features in the chimpanzee essays. Tables 2 and 3 show most of the information used, pertaining to chimpanzees and humans. The patterns and timing of closure of the cranial sutures were obtained from Krogman (1930), Schultz (1940) and Ashley-Montagu (1935, 1937 in Chopra, 1957) for chimpanzees, and from Todd and Lyon (1924, 1925a, b, c), Krogman (1939), Wood Jones (1946 in Chopra, 1957), Abbie (1950 in Chopra, 1957), Singer (1953 in Chopra, 1957), Acsádi and Nemeskéri (1970), Meindl and Lovejoy (1985), Mann et al. (1991), Gruspier and Mullen (1991) and Key et al. (1994) for humans.
Table 1. Developmental stages of the cranial features of the chimpanzee sample (EBD)
Stage 1—No evidence of ossification. Stage 2—Partial ossification and/or evidence of osseous bridging on one side. Stage 3—Complete ossification on one side. Stage 4—Complete ossification on both sides.
Based on the posterior extension of the sinus in the sagittal plane. Stage 1—Sella turcica. Stage 2—Dorsum sellae. Stage 3—First half of the clivus. Stage 4—Second half of the clivus.
Most of the basisphenoid and occipital bone are missing.
Part of the occipital bone is missing. The jugular synchodronsis score was assessed with reference only to the left side.
From the attainment of adulthood, that is, just before the eruption of all teeth (Krogman, 1930), to young adult, that is, early after the eruption of all teeth and around the time of first reproduction (Schultz, 1940; Zilhman, 2007) (Table 1), or older.
From the end of the adolescent growth period, that is, before, but more commonly after, M3 eruption (Stewart, 1934; Melsen, 1969, in Scheuer and Black, 2000), or later.
From 13 to 25 years, as determined by direct inspection or from CT scans studies (Hrdlička, 1920, in Chopra 1957; Madeline and Elster, 1995; Sahni et al., 1998; Veschi and Facchini, 2002; Coqueugniot and Weaver, 2007). Probably at the end of the second decade or in the first half of the third decade, when high percentages of the population would normally show full closure (Krogman and İşcan, 1986; Sahni et al., 1998; Veschi and Facchini, 2002).
No precise age can be obtained; no data on unilateral fusion are available. Bilateral fusion ranges from late adolescence-young adulthood through to fully adult individuals (Maat and Mastwijk, 1995; Hershkovitz et al., 1997).
No precise age can be obtained; no data on unilateral fusion are available. Bilateral fusion occurs from mostly above 20 years of age (this occurs in 90%–93% of the population by this time; Hershkovitz et al., 1997), or 22–36 years according to Maat and Mastwijk (1995).
Juvenile or older. Most probably fully adult due to nondetectable SOS (Scuderi et al., 1993; Spaeth and Krugelstein, 1996; Weiglein, 1999; Yonetsu et al., 2000; Barghouth et al., 2002).
From 15 years (Scuderi et al., 1993; Spaeth and Krugelstein, 1996) to the end of the third decade of life (this occurs in 93% of the population by this time; Yonetsu et al., 2000)
Sts 14 skeleton
Vertebrae: annular epiphyses
Ranging from the beginning of union to the achievement of almost complete union
From late adolescence to young adulthood (Albert and Maples, 1995).
From 17 (youngest age of early Stage 2; Albert and Maples, 1995) to 27 years (oldest age of late Stage 2; Albert and Maples, 1995).
Ribs: articular region epiphyses
Almost completely fused
Just older than the latter stages of adolescence, according to first rib ossification (Kunos et al., 1999).
From above 17 to around 25 years according to first rib ossification: fusion occurs from 14 to 17–19 years. The rounded edges and lenticular profile of the facet might be expected until 21 years. The dense surface with smooth irregularities suggests 20–25 years (Kunos et al., 1999). Remaining ribs: poorly documented, the onset of fusion to the tubercle suggests an age ≥ 18 years (Fawcett, 1911; Stevenson, 1924, in Scheuer and Black, 2000).
Iliac crest epiphysis
From the adolescent period to the onset of full adulthood (see adjoining chronological ages).
From 15 to 26 years (Galstaun, 1937, Lurie et al., 1943, Mckern and Stewart, 1957, Jit and Singh, 1971, Birkner, 1978, Webb and Suchey, 1985, in Scheuer and Black, 2000; Veschi and Facchini, 2002; Coqueugniot and Weaver, 2007), but most probably in the 19–22 years range (80% of the population would be expected to show this from data in Veschi and Facchini, 2002).
Ridge and furrow structure attenuation. Well-limited borders
Around mid-late 20s: moderate to slight billowing and active rampart formation suggests 24–37 years. Appearance of distinct lower extremity suggests 25 to about 30 years. Reduced billowing together with full lower extremity definition suggests an age older than 29 (Meindl et al., 1985) to 25–30 years (Phase IV/V; Todd 1920, 1921a, b, c)
Sacrum: S1-S2 costal elements
Late adolescence to full adulthood or later (see adjoining chronological ages).
≥19 years (first occurrence of complete fusion; Coqueugniot and Weaver, 2007).
Sacrum: S1-S2 annular epiphysis
Juvenile to young adult (see adjoining chronological ages).
Likely under 27 years (Mckern and Stewart, 1957, in Scheuer and Black, 2000; Coqueugniot and Weaver, 2007).
Sacrum: sacro-iliac joint epiphysis
Juvenile to full adulthood (see adjoining chronological ages).
From 16 to 30 years (Johnston, 1961; Szilvássy 1988 in Berge and Gommery, 1999; Berge and Gommery, 1999; Rogers and Cleaves, 1935, Bollow et al., 1997, in Scheuer and Black, 2000).
Taking into account the degree of preservation of Sts 5 and Sts 14, the features studied are those of developmental significance. These features were scored in the chimpanzee comparative sample (for stage definitions see footnote in Table 1) and for the Sts 5 and Sts 14 fossils. Using the chimpanzee data, a modal pattern was established for the development of the petro-exoccipital synchondrosis and the pneumatization of the sphenoidal sinus relative to the dentition (tooth calcification and dental emergence) and the SOS. However, the chronological age-of-death of the chimpanzees studied was unknown; thus, only the relative development of these features was recorded.
The gathered data allowed the developmental status of both fossil specimens to be established, followed by the biological age-at-death of the individual(s) represented. This procedure was also used to attempt an approximation of the chronological age-at-death. To show that Sts 5 and Sts 14 belong to a single individual requires that the corresponding ages-at-death be compatible and that the relative development between the iliac crest epiphysis of Sts 14 and the SOS of Sts 5 be consistent within the framework of the chimpanzee and human patterns (as mentioned earlier).
Table 1 reveals a common pattern of relative development between the cranial traits of the chimpanzee sample. Once the dentition is erupted and completely calcified, the SOS appears fused (or about to fuse) (Fig. 2a). With respect to the sphenoidal sinus, the pneumatization of the greater wings and the area beyond the dorsum sellae occurs with the complete fusion of the SOS and the fully developed dentition (Fig. 2). These statements are in agreement with the pattern described by some authors for chimpanzees and humans (see references in Tables 2 and 3). Finally, the petro-exoccipital synchondroses show partial or complete ossification on one side at the time of completion of the dentition and SOS closure.
External inspection of the Sts 5 maxilla reveals no evidence of dentition (Tables 2 and 3). The commercial axial CT images (Weber et al., 1997a) show some rounded and homogeneous structures with a brighter signal than the surrounding areas in what appears to be the position of the dental roots within the maxilla (Fig. 3a). In contrast, the CT scan images of the teeth of Sts 71 (Weber et al., 1997b) show one lingual and two oral roots in the maxillary molar series to have rounded and structured morphologies, that is, a bright external layer (cementum), a darker internal area (dentine), and a central white area (root canal) resembling the cementum in shade (Fig. 3b). In comparison with the tomography of the roots of Sts 71, the apparent tooth cavities of Sts 5 have an irregular outline and are poorly structured internally (Fig. 3). The formations seen in Sts 5 are therefore probably casts of the alveolar sockets, formed as a result of mineralization and/or sedimentary filling after the complete loss of the dentition. Sts 5 was therefore concluded to be edentulous (contrasting with Thackeray et al., 2002a).
Alveoli are perfect natural casts of the root of each tooth, and their size and shape change as the roots grow. In the sagittal tomogram of Sts 5, the suggested natural casts of all the alveoli cut through most of the alveolar bone of the maxilla (Fig. 4a). Further, when compared to Sts 71, these alveoli show similar development relative to the maxillary sinus (Fig. 4b). Although the alveoli of Sts 5 could have been broken, allowing the loss of the teeth, their deep ends are not likely to have been affected. This would imply that the roots of the third molars were completely developed.
The basisphenoidal region shows no sclerotic remnants of this synchondrosis in either the CT scans or on the external surface of the fossil, indicating that its full closure was far from being recent (in agreement with Broom et al., 1950) (Fig. 5a)
Sphenoidal air sinus
The sphenoidal aeration extends to the border between the dorsum sellae and the clivus, the sphenoidal wings and the anterior pterygoid processes. It is partially filled with what are probably calcite crystals, since the brain case is lined with this material (Broom et al., 1950). The pattern of pneumatization of this sinus in humans and chimpanzees and the absence of traces of the SOS in Sts 5, suggest that the sinus of Sts 5 had reached this synchondrosis and exceeded it dorsally (Fig. 5a,b)
This stage was only assessed on the left side, where the occipital and temporal bones are continuous, signifying at least unilateral complete fusion (Fig. 5c)
The dynamite blast that uncovered Sts 5 left portions of bone adhered to the calcified matrix of the breccia, making it difficult to observe most of the sutures (Broom et al., 1950; Thackeray, 1997; Prat and Thackeray, 2001). Most of the facial and palatal group of sutures are unidentifiable to the naked eye. However, the right side of the fronto-zygomatic—the left side is affected by a fracture that prevents observation of the suture—and the naso-premaxillary sutures are closed although still recognizable on the ectocranial surface (in agreement with Broom et al., 1950). In the cranial vault, only a small ectocranial portion of the sagittal suture within the bregma-lambda region is visible; it is closed (in agreement with Broom et al. 1950; Thackeray, 1997; Prat and Thackeray, 2001). The parieto-squamosal, the spheno-temporal, and the spheno-frontal sutures (belonging to the circum-meatal group of sutures) are fused but remain noticeable ectocranially on both sides (in agreement with Broom et al., 1950). Finally, in basal view, most of the occipito-mastoid suture is somewhat perceptible endocranially on the right side, and identifiable but closed ectocranially on both sides (in agreement with Broom et al., 1950), whereas in the splanchnocranium, the anterior and posterior median palatines and the spheno-palatine sutures are closed but still recognizable
Developmental stage and age-at-death of Sts 5
The lack of signs of recent closure of the SOS and the high degree of obliteration of the cranial sutures, especially those of the vault, are indicative that Sts 5 represents a fully grown cranium. Irrespective of whether the chimpanzee or human pattern is used as a model for Sts 5 (Tables 2 and 3), the state of these features signify that this cranium was part of an adult individual with a fully developed dentition. Moreover, the aerated portions of the sphenoidal sinus, the fusion status of the petro-exoccipital synchondrosis, and the probable completion of the M3 roots, are all consistent with this statement. Thus, the Pan and modern human schedules of maturation support ages-at-death of 14.5 years or older, and older than around 20 years, respectively (Tables 2 and 3)
The annular rings of the spine of Sts 14 show a variety of stages of ossification (Fig. 6a) (Tables 2 and 3). Following the scoring system of Albert and Maples (1995), these epiphyses appear to range from the early (beginning of union; Sts 14b, Sts 14h) to late (progressing union; Sts 14d, Sts 14e, Sts 14f, Sts 14g, Sts 14i, Sts 14k, Sts 14l, Sts 14m, Sts 14n, Sts 14o, Sts 14q) phases of Stage 1, through to the early (almost complete union; Sts 14a, Sts 14c) phase of Stage 2
Those ribs with a preserved tubercle region (none of which were identified as a first rib) show the articular epiphyseal plate with nearly complete fusion, though the suture is still identifiable (Fig. 6b). In addition, the profile of the tubercle is no longer elliptical but distinctively lenticular, and its boundaries are mostly rounded and slightly raised above the shaft. The surface texture of the tubercle is dense with smooth depressions and ridges
The iliac crest is mostly fused on the left side (Sts 14r), showing only traces of the suture, while the right coxal (Sts 14s) bone shows partial loss of theanterior portion of the epiphysis, probably due to incomplete ossification (in agreement with Berge and Gommery, 1999; Häusler and Berger, 2001) (Fig. 7a)
The sacrum shows incomplete fusion of the S1-S2 annular epiphysis. Further, even though the epiphyseal plate of the left sacro-iliac joint is fragmentary, the suture with the sacral alae remains discernible (in agreement with Berge and Gommery, 1999) (Fig. 7b).
The pubic symphysis of Sts 14 appears to have a characteristic ridge and furrow structure on its ventral margin, albeit mostly attenuated along the dorsal border. Additionally, its dorsal, superior and inferior margins are well limited by a continuous rim, whereas its anterior wall shows the beginning of the formation of the ventral rampart (Fig. 7c).
This lacks most of the greater trochanter and the most basal part of the lesser trochanter; they cannot, therefore be used for age diagnosis
Developmental stage and age-at-death of Sts 14
The several signs of immaturity in the ossification of the epiphyses of Sts 14 show that this skeleton had not completed its development. With respect to both the Pan and Homo sapiens patterns (Tables 2 and 3) these epiphyses are among the last to complete ossification. The individual to whom this skeleton belonged was likely fully grown in terms of height (as shown by the ossified acetabulum, and the probably-fused long bone epiphyses) and close to—thought not completely—grown in terms of body breadth (as shown by the incomplete iliac crest and sacrum epiphyses) (in agreement with Berge and Gommery, 1999). Therefore, this fossil assemblage represents part of a young adult probably between 14.5 years to 17 years according to chimpanzee standards, or somewhere in its early mid-20s according to the modern human schedule of maturation (Tables 2 and 3)
One and the Same Individual?
According to the present results, the biological and chronological ages-at-death of both specimens are partially compatible. However, Sts 5 shows no sign of having recently reached adulthood. Since the SOS of Sts 5 is no longer detectable, its closure does not appear to have occurred recently before death. Even when considering the intrapopulation variation of chimpanzees and modern humans, such a degree of the SOS closure in Sts 5 does not seem to be consistent with the still fusing iliac crest in Sts 14.
Although cranial vault closure may occur over an extended period of time in extant human populations, severe obliteration of this vault is common in fully adult individuals (Acsádi and Nemeskéri, 1970, Meindl and Lovejoy, 1985; Key et al., 1994). Therefore, against the human standard, the high degree of vault obliteration shown by Sts 5 does not suggest a young adult age.
In conclusion, no evidence supporting the idea that the studied fossils belong to a single individual was found; in fact, some data clearly suggest they belong to different individuals.
Using previous estimates for the body mass for Sts 14 (27.6 kg by McHenry, 1976 and 34.8 kg by Jungers, 1988) and an endocranial volume for Sts 5 of 485 cm3 (Falk et al., 2000), the method of Ruff et al. (1997) was used to calculate the brain mass and EQ for a hypothetical association between Sts 5 and Sts 14. This resulted in an EQ value of 2.9–3.4, depending on the body mass estimate used. This interval lies between the uppermost estimates for A. africanus (2.4–3.0 as derived from data in McHenry and Coffing, 2000) and the lowermost limits for Homo habilis (3.4–3.8 as derived from data in McHenry and Coffing, 2000). The result for the Sts 5-Sts 14 fossil assemblage is therefore only partially compatible with our current knowledge of the encephalization of A. africanus. If these fossils do represent a single specimen, the range for the relative brain capacity of this species would be necessarily increased.
A thorough re-evaluation of the developmental stage and age-at-death of the landmark Sts 5 and Sts 14 A. africanus fossils has been attempted in this work.
Contrary to the conclusions of a previous assessment (Thackeray et al., 2002a), Sts 5 was found not to belong to an adolescent specimen but to a fully grown adult whose estimated chronological age-at-death was 14.5 years or above, or more than around 20 years, according to the chimpanzee and modern human schedules of development respectively. This conclusion rests upon the following evidence: the mostly obliterated cranial vault sutures, the complete closure of the spheno-occipital and petro-exocccipital synchondroses (as observed on the left side), the pneumatization of the sphenoidal sinus to the dorsum sellae, and the adult size of the alveolar sockets in the maxilla.
Sts 14 shows signs of immaturity in the pelvis, ribs, and vertebrae. On the basis of data for chimpanzees and modern humans, the ossification stage of the studied features in Sts 14 suggests this partial skeleton belonged to a young adult that died between 14.5 to around 17 years of age according to chimpanzee standards, or within the first half of its third decade of life according to extant human standards. Previous studies on the pattern of development in A. africanus suggest that the chronological ages of both Sts 5 and Sts 14 are best determined using the chimpanzee developmental pattern.
These results also show the age-at-death of these two specimens to be partially consistent which one another. However, no evidence was found to support the idea that Sts 5 had just attained full growth, nor, therefore, that Sts 5 is the cranium of the young adult Sts 14 partial skeleton. Further, the stage of development of some of the Sts 5 features suggest it unlikely that this be the skull of a young adult.
Finally, the lower value of the EQ range resulting from a hypothetical association between Sts 5 and Sts 14 lie at the top end of that currently supposed for A. africanus. Should future research confirm such an association, the relative brain capacity of this species would necessarily increase, shortening the distance separating Homo and Australopithecus.
The authors thank the staff of the Transvaal Museum (Pretoria, South Africa), especially Francis Thackeray, for allowing us to study the original fossils Sts 14, Sts 5, and Sts 71. They are also indebted to the Estación Biológica de Doñana, Spain, for granting access to its chimpanzee collection, and to the University of Burgos, Spain, where some of the CT scans were performed. Their thanks also go to A. Gracia, I. Martínez, F. Gracia, and A. Alcazar de Velasco for their helpful commentaries and guidance, to E. Poza, R. Quam and E. Santos for their help, and to the staff at the Centro UCM-ISCIII de Evolución y Comportamiento Humano.