Digit ratios predict polygyny in early apes, Ardipithecus, Neanderthals and early modern humans but not in Australopithecus.

Social behaviour of fossil hominoid species is notoriously difficult to predict owing to difficulties in estimating body size dimorphism from fragmentary remains and, in hominins, low canine size dimorphism. Recent studies have shown that the second-to-fourth digit ratio (2D : 4D), a putative biomarker for prenatal androgen effects (PAEs), covaries with intra-sexual competition and social systems across haplorrhines; non-pair-bonded polygynous taxa have significantly lower 2D : 4D ratios (high PAE) than pair-bonded monogamous species. Here, we use proximal phalanx ratios of extant and fossil specimens to reconstruct the social systems of extinct hominoids. Pierolapithecus catalaunicus, Hispanopithecus laietanus and Ardipithecus ramidus have ratios consistent with polygynous extant species, whereas the ratio of Australopithecus afarensis is consistent with monogamous extant species. The early anatomically modern human Qafzeh 9 and Neanderthals have lower digit ratios than most contemporary human populations, indicating increased androgenization and possibly higher incidence of polygyny. Although speculative owing to small sample sizes, these results suggest that digit ratios represent a supplementary approach for elucidating the social systems of fossil hominins.

Can hominin 'handedness' be accurately assessed?

The expression of a strong, population-level right hand preference has, to date only been unequivocally identified in Homo sapiens and is often considered to be unique to this species. For this reason, and because of purported co-evolutionary links between this trait and language capabilities in modern humans, the identification of hand preference in the hominin fossil record has long been of interest to researchers studying the evolution of 'handedness' within the genus Homo. Identifying hand preference in skeletal samples, however, is not straightforward. Problems arise from difficulties in determining the precise nature of the relationship between hand use and bone morphology, the methods by which hand preference is assessed, the paucity of material available for study, and even what is meant by the term 'handedness'. Various attempts have been made to address these issues, encompassing a range of methodological approaches, such as comparisons of osteological techniques, studies of prehistoric material culture and ethnographic analysis of hand use behaviours in modern hunter-gatherer societies and non-human primate groups. What such research suggests is that hand preference is a complex phenomenon, in both extant and extinct groups, and in order to assess its expression in extinct populations care must be paid to the questions asked of the available material and the methodologies used to answer them.

The evolution of handedness in humans and great apes: a review and current issues.

Population-level right-handedness is a defining characteristic of humans. Despite extensive research, we still do not know the conditions or timing of its emergence in human evolution. We present a review of research into the origins of handedness, based on fossil and archaeological data for hand preference and great ape hand-use. The data show that skeletal asymmetries in arm and hand bones supporting a rightsided dominance were present at least in the genus Homo, although data are more robust for Neanderthals. The evidence from tool-use, production, and cave art confirms that right-hand preference was established in Neanderthals and was maintained until the present. The great apes can provide real-life models for testing the conditions that facilitate or enhance hand preference at both the individual and group levels. The database on great ape hand-use indicates that they do exhibit hand preferences, especially in complex tasks. However, their preferences vary between tasks, and while group-level biases have occasionally been reported, no human-like handedness bias has been found. We discuss the methodological problems encountered in these approaches. Shared problems include a lack of agreed terminology both within and between disciplines, small sample sizes, interpretation biases and a failure to replicate experiments. In general, there is a paucity of fossil material, with poor preservation hampering traditional metric methods. The archaeological data are often founded on unreliable methods. The primate database is plagued by the use of measures that could be inappropriate for revealing hand preference, and by methodological inconsistencies between studies. We emphasise the need to standardise the methods to allow between studies and species comparisons. We propose that when referring to "handedness" it is more appropriate to use the terms "hand preference" and "hand use", to avoid confusion with each discipline's own definition of handedness.