One of the,
more many, interesting debates in paleoanthropology concerns the question of whether our human ancestors went through a knuckle-walking stage at some point in our history. In the recent past, the idea that human ancestors went through a knuckle-walking stage is most closely associated with the work of Richmond and Strait.
In a number of papers Richmond and Strait have argued that knuckle-walking is a derived feature of the African ape/human clade and that humans evolved from a knuckle-walking ancestor. There are a number of morphological traits that have been associated with knuckle-walking (you can find an overview of this morphology here) and a recent paper in PNAS takes a new look at the subject. The new paper, by Kivell and Schmitt, looks at traits in the capitate, scaphoid, and hamate in a wide variety of primate species – both terrestrial and arboreal. The included both males and females, as well as adult and juvenile specimens. The researchers looked specifically at the scaphoid dorsal concavity and scaphoid beak, the capitate distal concavity and dorsal ridge, the waisting of the capitate, the hamate dorsal ridge and the hamate distal concavity. The distribution of these traits across species proved to be interesting. Gorillas, the most terrestrial of the primates, had the fewest traits specifically for knuckle-walking:
This research shows that none of the carpal features discussed here that have traditionally been used to diagnose knuckle-walking can be considered clear functional adaptations to knuckle-walking behavior in all African apes. Contrary to functional predictions, the pattern of development, expression, and frequency of the putative knuckle-walking features listed in Table 1 are not the same in all African apes; they are not more frequent or accentuated in gorillas and are not correlated with increased knuckle-walking behavior or body size. The difference between Pan and Gorilla in the presence or absence of carpal bone morphology thought to limit wrist motion is confirmed by reported patterns of joint flexibility in the 2 genera. Gorilla have a much larger range of wrist extension (58°) (40) compared to that of P. troglodytes (30–42°) (19, 40). It remains challenging to explain this difference if it is assumed that knuckle-walking in Pan and Gorilla is biomechanically similar. The morphological and range of motion data demand a new perspective on knuckle-walking that leads to a reevaluation of long established models of human evolution.
Kivell and Schmitt argue that instead, what we are seeing reflects habitual loading of the wrist in an extended posture. They argue that gorillas and chimps display two biomechanically different types of knuckle-walking – something I find intriguing. This leads Kivell and Schmitt to propose that knuckle-walking evolved independently in gorillas and chimps.
For me, two things stand out. First, the authors looked at variation of the traits within species as well as across species – although Kivell and Schmitt overstate their case on this (Richmond and Strait’s 2000 Nature article, for example, looked at within and among species variation). Second, the morphology was examined in juveniles as well as adults.
Am I convinced? Not really. Kivell and Schmitt examined only a few of the traits that have been used to support the hypothesis that humans had a knuckle walking ancestor. All in all, an interesting and suggestive paper.