Early Pleistocene third metacarpal from Kenya and the evolution of modern human-like hand morphology

Abstract

Despite discoveries of relatively complete hands from two early hominin species (Ardipithecus ramidus and Australopithecus sediba) and partial hands from another (Australopithecus afarensis), fundamental questions remain about the evolution of human-like hand anatomy and function. These questions are driven by the paucity of hand fossils in the hominin fossil record between 800,000 and 1.8 My old, a time interval well documented for the emergence and subsequent proliferation of Acheulian technology (shaped bifacial stone tools). Modern and Middle to Late Pleistocene humans share a suite of derived features in the thumb, wrist, and radial carpometacarpal joints that is noticeably absent in early hominins. Here we show that one of the most distinctive features of this suite in the Middle Pleistocene to recent human hand, the third metacarpal styloid process, was present ∼1.42 Mya in an East African hominin from Kaitio, West Turkana, Kenya. This fossil thus provides the earliest unambiguous evidence for the evolution of a key shared derived characteristic of modern human and Neandertal hand morphology and suggests that the distinctive complex of radial carpometacarpal joint features in the human hand arose early in the evolution of the genus Homo and probably in Homo erectus sensu lato.

Investigating interrelationships between Lower Palaeolithic stone tool effectiveness and tool user biometric variation: implications for technological and evolutionary changes

“…Rather, it is possible to state that hominins would likely have displayed manual dimensions that were increased relative to those required purely for 'typical' Oldowan flake tool use, and that these dimensions are likely to be nearer the modern human range. Recent discoveries of fossils and handaxes are broadly supportive of such a statement, indicating that the emergence of both this new tool type and novel anatomical condition may have been broadly coincident (Beyene et al, 2013;Ward et al, 2014;Domínguez-Rodrigo et al, 2015;Diez-Martín et al, 2015;Lorenzo et al, 2015). This is particularly noteworthy since our results show that individuals with smaller hands may have more profitably employed the use of flakes as opposed to handaxes.…”

“…Hence, it appears that the efficient use of handaxes as a cutting tool is linked to the manual dimensions of the tool user, with increased hand sizes resulting in greater levels of cutting efficiency. It is, therefore, possible that the use of large cutting tools (such as large flakes and handaxes) may potentially have provided a selective pressure contributing towards the large manual dimensions seen in some later Homo species (e.g., Churchill, 2001;Ward et al, 2014;Lorenzo et al, 2015) relative to earlier Homo or australopithecines. Consideration of any evolutionary scenario must, however, be examined within an evolutionary context of increasing body sizes and changes observed in a suit of anatomical features that contribute to modern human-like manipulative capabilities.…”

“…Hence, Homo species with relatively large manual dimensions (i.e. after ~1.8-1.4 Mya [Ward et al, 2014;Domínguez-Rodrigo et al, 2015]) may have found the use of handaxes and other large cutting tools to be a more worthwhile endeavour than other recent, but more diminutive, hominin species (e.g. Larson et al, 2009).…”

“…The recent discovery of a 1.42-million-year-old third metacarpal from West Turkana, Kenya, suggests that modern human-like hand anatomy is likely to have evolved in an Oldowan/early Acheulean behavioural context (Ward et al, 2014). Specifically, it identifies the earliest known appearance of a prominent styloid process, which is a morphological feature linked to the effective production and use of stone tools (Marzke and Marzke, 1987;Napier, 1993;Marzke, 1997Marzke, , 2013Kivell, 2015).…”

A complex adaptive system may be essential for cumulative modifications in tool design

“…The third metacarpal styloid process in modern humans is an adaptation for an enhanced power grip when making and using tools (Ward et al, 2014). This feature is not in the hands of H. habilis or earlier hominins and nonhuman apes, but was recently found in a H. erectus hand from 1.42 Mya (Ward et al, 2014).…”