Proximal Humerus

To the degree that it can be determined, the head of all known early hominin humeri is elliptical rather than spherical in shape as in apes, and the intertubercular groove is somewhat shallow, rather than deep and tunnel-like as in African apes. Broom et al. (1950) describe the STS 7 proximal humerus as basically humanlike, although the tubercles are somewhat distinct, with a more prominent lesser tubercle than typically seen in modern humans or great apes. The Chemeron proximal humeral fragment KNM-BC 1745 and the A.L. 288-1r proximal humerus are also described as displaying relatively large lesser tubercles (Pickford et al., 1983; Johanson et al., 1982). Robinson (1972) notes that, unlike most modern human humeri, STS 7 displays a prominent ridge on the greater tubercle separating the facets for attachment of supraspinatus and infraspinatus. Although the area of attachment for supraspinatus is not preserved in A.L. 288-1r, there is a ridge that would have separated it from the clearly defined ovoid depression for the attachment of infraspinatus (Johanson et al., 1982). Lovejoy et al. (1982) describe similar separation of the facets for attachment of the dorsal rotator cuff muscles on the greater tubercle of A.L. 333-107.

Humeral torsion refers to the orientation of the humeral head relative to the distal articular surface. Since the degree of humeral torsion is high in both modern humans and African apes (Evans and Krahl, 1945; Krahl and Evans, 1945; Krahl, 1947), this condition is often cited as a shared derived feature of apes and humans (e.g., Le Gros Clark, 1959; Andrews, 1985; Martin, 1986; Harrison, 1987), in which case early hominin humeri should also display a high degree of torsion. Unfortunately, all known early hominin humeri are incomplete, making direct measurement of humeral torsion impossible. However, Larson (1996) has developed methodologies for estimating the degree of humeral torsion on incomplete humeri using regression analysis and a set of alternative references axes. Contrary to expectations, she reports modest to low levels of torsion for A.L. 288-1r, STS 7, Omo 119-73-2718, and KNM-ER 739 (Fig. 7.1). Larson (1996) concludes that the high torsion of modern humans is therefore a more recently acquired characteristic, and its similarity to other hominoids is due to convergence.

In sum, the pectoral girdle/shoulder of early hominins appears to have retained many features of the presumed ancestral condition. The scapula was positioned high on a funnel-shaped thorax (see also Schmid, 1983), the clavicle was

Torsio Humeri

Fig. 7.1 Box and whisker plots of humeral torsion for comparative samples of apes, modern humans, and fossils. Comparative data (except Khoe-San sample) and early hominin fossil torsion estimates are from Larson (1996). Fred Grine and Louise Jacqui Friedling collected the African Khoe-San humeral data. Torsion data for Neandertals (Lezetxiki 1, Regourdou 1, Neanderthal 1, La Chapelle 1, Tabun C1, Kebara 2) and Early Modern Homo (Skhul IV, Qafzeh 9, and humeri from 15 Early Upper Paleolithic sites) are from Churchill (1994). Torsion estimates for the juvenile (D2850) and adult (D4507) humeri from Dmanisi are from Lordkipanidze et al. (2007). The humerus II (H. heidelbergensis) torsion value is from Carretero et al. (1997). The lesser apes have a fairly low degree of humeral torsion, orangutans display an intermediate amount of torsion, and the African apes have the highest torsion values among hominoids. Modern human samples also display a high degree of torsion. Estimated torsion for early hominins is much lower than modern human values.

Fig. 7.1 Box and whisker plots of humeral torsion for comparative samples of apes, modern humans, and fossils. Comparative data (except Khoe-San sample) and early hominin fossil torsion estimates are from Larson (1996). Fred Grine and Louise Jacqui Friedling collected the African Khoe-San humeral data. Torsion data for Neandertals (Lezetxiki 1, Regourdou 1, Neanderthal 1, La Chapelle 1, Tabun C1, Kebara 2) and Early Modern Homo (Skhul IV, Qafzeh 9, and humeri from 15 Early Upper Paleolithic sites) are from Churchill (1994). Torsion estimates for the juvenile (D2850) and adult (D4507) humeri from Dmanisi are from Lordkipanidze et al. (2007). The humerus II (H. heidelbergensis) torsion value is from Carretero et al. (1997). The lesser apes have a fairly low degree of humeral torsion, orangutans display an intermediate amount of torsion, and the African apes have the highest torsion values among hominoids. Modern human samples also display a high degree of torsion. Estimated torsion for early hominins is much lower than modern human values.

oriented obliquely, and the glenoid fossa was cranially directed. However, the intertubercular groove was relatively shallow, the humeral head was elliptical, and the humerus displayed only modest torsion. Putting aside any implications that this morphology may have regarding reconstructing the lifestyle of early hominins, this configuration can be taken as the starting point from which the shoulder of early Homo was derived.

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