The prehistory of handedness: Archaeological data and comparative ethology

Abstract

Homo sapiens sapiens displays a species wide lateralised hand preference, with 85% of individuals in all populations being right-handed for most manual actions. In contrast, no other great ape species shows such strong and consistent population level biases, indicating that extremes of both direction and strength of manual laterality (i.e., species-wide right-handedness) may have emerged after divergence from the last common ancestor. To reconstruct the hand use patterns of early hominins, laterality is assessed in prehistoric artefacts. Group right side biases are well established from the Neanderthals onward, while patchy evidence from older fossils and artefacts indicates a preponderance of right-handed individuals. Individual hand preferences and group level biases can occur in chimpanzees and other apes for skilled tool use and food processing. Comparing these findings with human ethological data on spontaneous hand use reveals that the great ape clade (including humans) probably has a common effect at the individual level, such that a person can vary from ambidextrous to completely lateralised depending on the action. However, there is currently no theoretical model to explain this result. The degree of task complexity and bimanual complementarity have been proposed as factors affecting lateralisation strength. When primatology meets palaeoanthropology, the evidence suggests species-level right-handedness may have emerged through the social transmission of increasingly complex, bimanually differentiated, tool using activities.

Introduction

Human hand use patterns can be characterised as complementary role differentiation (CRD). The CRD model of bimanual action derives from the Kinematic Chain model, which was proposed by Guiard (1987) and applied by Uomini (2006a) to the prehistoric activities that offer evidence for handedness (Steele and Uomini, 2009). In this model, one hand executes high frequency tasks (involving finer temporal and spatial resolution) while the other hand performs low frequency tasks (such as supporting an object). Rather than one hand being “dominant,” the CRD model recognises that both hands have different but equally important roles (Corbetta and Thelen, 1996). “Right-handers” are thus defined as people who prefer to adopt the high frequency role with the right hand and the low frequency role with the left hand, as shown by experiments on humans for a complementary bimanual task requiring precision versus support (Hinckley, 1996, Hinckley et al., 1997). A hand role dichotomy appears to emerge between seven and thirteen months of age (Bresson et al., 1977, Ramsay et al., 1979, Michel et al., 1985, Kimmerle et al., 1995, Michel, 1998) and is well established by age three (Ingram, 1975, Gaillard, 1996), yet its genetic determinants are still unknown (Crow, 1998, Van Agtmael et al., 2001).

Beyond the individual, Homo sapiens sapiens displays lateralised hand preference at the species level. This means that a bias to the right-handed CRD pattern is found in all human populations around the world (reviewed in Llaurens et al., 2009), with the frequency of right handed persons in any given population varying between 74% and 96% (Hardyck and Petrinovich, 1977, Porac and Coren, 1981, McManus, 1991, Connolly and Bishop, 1992, Perelle and Ehrman, 1994, Annett, 2002, Raymond and Pontier, 2004, Faurie et al., 2005). There has never been any report of a human population in which left handed individuals predominate (Llaurens et al., 2009). In contrast to the human bias, it is clear from observations of experimental and spontaneous hand actions in captive and wild subjects that the non human primates do not show a species wide consistency in hand use patterns (Colell et al., 1995, Papademetriou et al., 2005). While group level biases can occur in some populations of chimpanzees (e.g., a rightward bias at Yerkes [Hopkins et al., 2007]) and for certain manual actions in some great apes (e.g., gorillas feeding on plants [Corp and Byrne, 2004]), there is no consistent pattern across populations at the species level, since some populations are claimed to show a leftward bias (e.g., for termite fishing in Gombe's wild chimpanzees [Lonsdorf and Hopkins, 2005]).

Furthermore, these biases do not extend to all actions, nor do they reach the extreme degree of consistency seen in humans across all tasks including unimanual actions (McManus, 1985, Hopkins, 2006). As discussed below in more detail, the compilation of research led by various authors suggests that a species-wide group level manual preference across all tasks is not the norm in primates. Therefore, human handedness is unique in both its direction (rightward CRD pattern) and its strength (species wide preference), and remains to be explained in evolutionary terms.

The timing and context for the emergence of species handedness in hominins is therefore of much interest to palaeoanthropologists. Combining palaeoanthropology with primatology can help us decide whether human and nonhuman hand preference patterns are part of the same continuum, or are qualitatively different. A selection of archaeological and primatological findings for hand preference is discussed in a comparative framework, with a special focus on the population- versus species-level distinction.