Claviculohumeral Ratio

Because the KNM-WT15000 skeleton includes all of the pectoral girdle/shoulder elements, it is possible to further explore this region to try and determine how a shoulder with such limited humeral torsion could have functioned. The claviculohumeral ratio is a commonly used measure of relative clavicular length (e.g., Schultz, 1930, 1937; McCown and Keith, 1939; Martin and Saller, 1959; Marquer, 1972). The estimated total length of 319 mm for the KNM-WT 15000 humerus reported by Walker and Leakey (1993) yields a claviculohumeral ratio of 40.89 for the Nariokotome boy. Figure 7.2 presents comparative data on claviculohumeral ratios for samples of apes and humans including Euro-Americans and several African populations. KNM-WT15000 falls at the lower fringes of modern human populations and overlaps with apes. However, both clavicular and humeral length change with age, and since the Nariokotome boy is immature, would he have had a more human-like claviculohumeral ratio as an adult? Jungers and Hartman (1988) report that humeral length displays isometric growth allometry in great apes and slight positive growth allometry in humans, while clavicular length displays negative growth allometry in all taxa. Therefore, no matter whether KNM-WT15000 followed a great ape or human growth trajectory, if the Nariokotome boy had reached adulthood these scaling patterns would have resulted in an even shorter relative clavicular length, and the 40.89 claviculohumeral ratio reported in Fig. 7.2 is likely to be an overestimate. Although Lordkipanidze et al. (2007) do not report a claviculohumeral ratio for any of the Dmanisi hominins, they describe the nearly complete subadult clavicle D2724 as being comparatively short, supporting the interpretation of early H. erectus displaying a relatively short clavicle.

A relatively short clavicle is surprising for early H. erectus since relative clavicular elongation is another trait that is considered to be a shared derived feature of humans and extant hominoids (Le Gros Clark, 1959;

Ciochon, 1983; Andrews, 1985; Martin, 1986; Harrison, 1987). It is therefore worth considering whether differences in claviculohumeral ratios between apes and humans could be due to differences in humeral length. Jungers (1994) has shown that humerus length has a conservative scaling relationship to body mass, with relative humeral length being virtually identical in different sized human populations as well as in African apes. To further explore what the primitive condition for relative clavicle length might be, mean clavicular and humeral lengths were collected from the literature and are displayed in Fig. 7.3. Regression analysis of the comparative nonhuman primate data reveals a linear relationship (r = 0.97) passing through the origin, which indicates an isometric scaling relationship between clavicular and humeral length (Mosimann, 1970; Jungers et al., 1995). The close adherence of most nonhuman taxa to this scaling relationship suggests that it may represent the primitive condition for primates. Among the outliers are baboons and Ateles, both of which fall well below the line, which could be due to reduced clavicular length in the former and elongation of the humerus in the latter, but this is pure speculation. However, even though the lesser apes also have elongated forelimbs, they appear to follow the isometric scaling relationship quite closely. Among great apes, chimpanzees fall slightly above the line while

Gibbon(n=30) Siamang(n=14) Orangutan(n=28) Gorilla(n=35) Chimpanzee(n=57) . Bonobo(n=18) Euro-American(n=18) ■ African Kikuyu(n=28) -AfricanNiiotic(n=10) African Pygmy(n=25) ■ KhoeSan(n=18) KNM-WT15000 Early Mod Homo(n-7) -Neanderthal(n=7)

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Claviculohumeral Ratio

Fig. 7.2 Box and whisker plots of claviculohumeral length ratios for comparative samples and fossils. Comparative ape, African pygmy and Euro-American data were provided by William Jungers. Fred Grine and Louise Jacqui Friedling collected clavicular and humeral data for the African Khoe-San sample. African Kikuyu and Nilotic data were provided by Chris Ruff. Early modern Homo sample includes: Abri Pataud 5 (Churchill, 1994), Jebel Sahaba, Wadi Kubbaniya (Angel and Kelley, 1986), Doni Vestonice 13 & 15 (Sladek et al., 2000), and Skuhl IV & V

(McCown and Keith, 1939). Neandertal sample includes: Kebara 2 (Churchill, 1994), Shanidar 1 & 3, Régourdou 1, Tabün C1, La Ferrassie 1 (Trinkaus, 1983), and Neanderthal 1 (McCown and Keith, 1939). The samples of modern humans vary in average in stature yet all have similar claviculohumeral ratios, which are consistently higher than those of apes except for orangutans. Relative clavicular length for KNM-WT 15000 is more similar to apes than modern humans. Neanderthals appear to have the longest clavicles among hominins.

Fig. 7.3 Scatter plot of mean clavicular length against mean humeral length in nonhuman primates, modern human populations, and homi-nin fossils. Squares indicate data derived from Mivart (1868); circles indicate data from Schultz (1930); triangles represent data provided by William Jungers (apes and African pygmies), Chris Ruff (African Kikuyu and Nilotics), and Fred Grine and Louise Jacqui Friedling (Khoe-San). Early modern Homo sample (grey star) includes: Abri Pataud 5 (Churchill, 1994), Jebel Sahaba, Wadi Kubbaniya (Angel and Kelley, 1986), Doni Véstonice 13 & 15 (Sládek et al., 2000), and Skuhl IV & V (McCown and Keith, 1939). Neandertal sample (grey star) includes: Kebara 2 (Churchill, 1994), Shanidar 1 & 3, Régourdou 1, Tabün C1, La Ferrassie 1 (Trinkaus, 1983), and Neandertal 1 (McCown and Keith, 1939). Black star indicates KNM-WT 15000.

Fig. 7.3 Scatter plot of mean clavicular length against mean humeral length in nonhuman primates, modern human populations, and homi-nin fossils. Squares indicate data derived from Mivart (1868); circles indicate data from Schultz (1930); triangles represent data provided by William Jungers (apes and African pygmies), Chris Ruff (African Kikuyu and Nilotics), and Fred Grine and Louise Jacqui Friedling (Khoe-San). Early modern Homo sample (grey star) includes: Abri Pataud 5 (Churchill, 1994), Jebel Sahaba, Wadi Kubbaniya (Angel and Kelley, 1986), Doni Véstonice 13 & 15 (Sládek et al., 2000), and Skuhl IV & V (McCown and Keith, 1939). Neandertal sample (grey star) includes: Kebara 2 (Churchill, 1994), Shanidar 1 & 3, Régourdou 1, Tabün C1, La Ferrassie 1 (Trinkaus, 1983), and Neandertal 1 (McCown and Keith, 1939). Black star indicates KNM-WT 15000.

Clavicular and humeral lengths for Epipliopithecus were measured on casts. An estimate of clavicular length for Oreopithecus was provided by Terry Harrison, and humeral length is from Harrison (1986). Ratios for the latter two taxa are included to offer an indication of relative clavicular length in Miocene hominoids. Regression line (with 95% confidence intervals) is for nonhuman primates only and has a correlation coefficient of 0.97. Since it passes through the origin, it indicates an isometric scaling relationship across primate species. Assuming that this linear relationship represents the primitive condition for primates, orangutans and all modern human populations as well as later fossil hominins display relative clavicular elongation. However, KNM-WT 15000 appears to retain the primitive condition.

both bonobos and gorillas lie somewhat below. At the same time, orangutans are highly divergent above the line indicating that they have elongated clavicles relative to their humeri, despite the fact that they too have elongated forelimbs. All of the means for the modern human populations are also above the line, as are the means for samples of early modern Homo and Neandertals. If the common isometric scaling relationship seen across nonhuman primates does indeed represent the primitive condition, then modern humans and recent fossil hominins all exhibit the derived condition of relative clavicular elongation. KNM-WT 15000, however, falls very close to the line suggesting that it retains the primitive condition, as do the African and lesser apes.

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