(TOP) Predictions of peak joint loading during the habitual hand postures used by apes. Hand postures are shown for (A) arboreal and (B) knucklewalking locomotion and (C) human manipulation [adapted from (2)]. Straight arrows indicate the predicted direction of peak joint reaction force. Curved arrows in (C) help to illustrate opposition of the thumb and fingers during precision grasping.
(BOTTOM) Pattern of trabecular bone distribution in A. africanus, H. neanderthalensis, and early H. sapiens. The A. africanus composite includes StW 418 (left Mc1, mirrored), StW382 (left Mc2, mirrored), StW394 (left Mc3,mirrored), and StW 552 (right Mc4), as well as (*) Swartkrans specimen SK(W) 14147 (left Mc5, mirrored) associated with either A. robustus or early Homo. Arrows point to regions of highest BV/TV (red), indicating a recent H. sapiens–like distopalmar and asymmetrical peak loading of the metacarpal heads in all fossil hominins that is distinctly different from the pattern found in Pan. Trabecular bone volume fraction is scaled to 0 to 45%.
Abstract: The distinctly human ability for forceful precision and power “squeeze” gripping is linked to two key evolutionary transitions in hand use: a reduction in arboreal climbing and the manufacture and use of tools. However, it is unclear when these locomotory and manipulative transitions occurred. Here we show thatAustralopithecus africanus (~3 to 2 million years ago) and several Pleistocene hominins, traditionally considered not to have engaged in habitual tool manufacture, have a human-like trabecular bone pattern in the metacarpals consistent with forceful opposition of the thumb and fingers typically adopted during tool use. These results support archaeological evidence for stone tool use in australopiths and provide morphological evidence that Pliocene hominins achieved human-like hand postures much earlier and more frequently than previously considered.
Matthew M. Skinner, Nicholas B. Stephens, Zewdi J. Tsegai, Alexandra C. Foote, N. Huynh Nguyen, Thomas Gross, Dieter H. Pahr, Jean-Jacques Hublin, Tracy L. Kivell
Science 23 January 2015: Vol. 347 no. 6220 pp. 395-399 DOI: 10.1126/science.1261735
http://www.sciencemag.org/content/347/6220/395.abstract
