Proximal radius variation in hominoids

Patel (2005) examines the morphology of the proximal radius in different species of apes. He sets the work into the context of earlier work on hominid positional behavior and locomotion based on the radius; in particular, the work by Richmond and Strait (2000) suggesting knuckle-walking adaptations for early hominid distal radii. A question arising from this work is whether radial morphology specifically indicates functional correlates such as knuckle-walking or climbing, or whether instead it is more generalized in its anatomy. However, the proximal radius reflects not the wrist joint but the elbow, and may not be expected to reflect the same locomotor or positional constraints as the distal end.

The analyses are admirably complex. My favorite is the one where a ball bearing is allowed to roll freely in the proximal fovea in order to measure its shape.

But the results show that the proximal radius is not a strong indicator of function:

[W]hen hylobatids were included in the comparative analysis, they were not clearly distinguishable from African apes, and early hominins resembled both African apes and hylobatids. Because African apes and hylobatids have different locomotor behaviors (see Tuttle 1986; Fleagle 1999), the results of this study suggest that determining specific locomotor behaviors from the proximal radius may not be possible -- it cannot be determined whether the bony morphology is indicative of terrestrial quadrupedal locomotion or acrobatic suspensory behaviors (i.e. brachiation). Thus, with reference to the proximal radius, it is difficult to determine whether early hominins may have had the ability to utilize any form of terrestrial locomotion similar to extant African apes, a conclusion that is similar to those of previous studies of the distal humerus (Feldesman 1982; Senut and Tardieu 1985) and proximal ulna (Aiello et al. 1999) (Patel 2005:426, references therein).

Patel does draw a contrast between monkey-like quadrupedalism and potential suspensory locomotion for the elbow joint:

Although most early Miocene hominoids [or proconsuloids (e.g. Harrison 2002)], such as Proconsul, Afropithecus, and Turkanapithecus, were quadrupedal, with a monkeylike elbow morphology (Napier and Davis 1959; Fleagle 1983; Rose 1988; 1993b; 1994; 1997; Richmond et al. 1998), the elbow region of later hominoid taxa, such as Oreopithecus, Dryopithecus, and Sivapithecus, resemble both African apes and hylobatids. This suggests that these taxa may have utilized suspensory behaviors (e.g., Begun 1992; Rose 1993b; 1997; Richmond et al. 1998) (Patel 2005:428, references therein).

This places emphasis on the question of the evolution of suspensory posture. It is quite clear that early hominoids that are probable relatives of both Asian apes and the European and African clade of apes were suspensory, such as Dryopithecus and Pierolapithecus. Gibbons are obviously also suspensory. Were the common ancestors of all living hominoids suspensory, or were they independently derived from Proconsul-like quadrupeds?

What is unsatisfying about this study is the lack of biomechanics. Consider the following:

African apes and hylobatids have relatively small radial foveae resulting from an expanded proximal articular surface. A smaller fovea results in a smaller area of contact between the radius and the capitulum, indicating an emphasis on stability (e.g. Godfrey et al. 1991).... Although the depth and the curvature of the radial fovea were not measured in this study, it would be expected that the fovea in African apes and hylobatids would be deeper and more curved to promote increased stability in the elbow joint (e.g., Hamrick 1996) (Patel 2005:429, references therein).

Why? How does the specific form of this joint promote stability? What is the contrary force making stability less desirable in species with less extensive bony articular surfaces? Patel notes that it is a bit of a mystery why orangutans should not have proximal radii more like the other apes and speculates that they accentuate mobility rather than stability. Is this true? Are chimpanzee elbow joints less mobile than humans? Are gibbons really like the African apes in function, or is there an allometric difference that leads to the appearance of similarity between them? Gorillas, chimpanzees, and orangutans are different on average, but there is overlap in some features. Do individuals in this overlap region have similar biomechanical properties, and if so, why does this degree of variability persist?

Answering these questions requires proper biomechanical modeling. One must explore the relationship between radius shape and forces acting on the elbow joint. Statistical comparisons of similarity among hominoid species are interesting, but they do not replace this process.

References:

Patel BA. 2005. The hominoid proximal radius: re-interpreting locomotor behaviors in early hominins. J Hum Evol 48:415-432.