LOVEJOY, C.O.; RENO, P.L.*; MCCOLLUM, M.A.; HAMRICK, M.W.; MEINDL, R.S.; COHN, M.J.: The Evolution of Primate Hands: Growth scaling registers with posterior HOXD expression
The hands of great apes and humans are very similar morphologically but differ systematically in digital proportions. This implies a modular basis of size. Since Abdominal-B-related Hox genes have been shown to act as growth regulators in the autopod, we explored the size relationships among elements which share their expression. Our measurements, taken from a balanced sample composed of all 5 hominoid genera (N=120), include the lengths of the proximal and distal radius (defined by the nutrient foramen), all 5 metacarpals, and the proximal 2 phalanges of each ray. Geometric means of joint dimensions were used for size adjustment. An ontogenetic series was also collected for Homo, Pan and Gorilla. We also monitored the expression of posterior HOXD genes in developing murine autopods using whole-mount in situ hybridization of riboprobes. In situ hybridization demonstrates that the growth plates of the distal radius and posterior 4 digits collocate with Hoxd11 expression. ANCOVA shows a strong relationship between the posterior rays and distal radial length, but none with its proximal segment. These data suggest that hominoid digit and distal forearm proportions are modulated by responses of growth promoting targets downstream of posterior HOXD genes. Similar, relatively simple, changes in gene expression can wholly explicate the probable pattern of evolution in the early hominid hand and forearm skeleton, including the emergence of unusually short posterior digits and forearms in the time period between Australopithecus garhi and Homo habilis/erectus. Such changes are likely to be related to developments in lithic technology and to be completely unrelated to locomotion.