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In light of recent genetic and archaeological research, the debate concerning genetic mixing between Neanderthals and anatomically modern humans (AMHs) has moved from whether interbreeding occurred to when, where and for how long it took place. The idea of Neanderthals being direct ancestors of Homo sapiens has been considered since the hominid was first discovered, however until recently the scientific consensus has largely been that as AHMs left Africa and colonised the globe, resulting in the complete replacement (CR) of all other hominids (Stringer & Andrews, 1988). According to that theory, Neanderthals – once widespread and ecologically dominant – were rendered extinct by the presence of AHMs approximately 30,000 year ago. Despite early genetic evidence supporting a CR scenario, subsequent archaeological and genetic data is increasingly establishing a very different sequence of events (Green et al., 2010; Condemi et al., 2013). Genetic studies have established that Neanderthals contributed 1-4% to the present day genetic diversity of all people except populations originating from sub-Saharan Africa (Sankaraman, 2012). Of this gene flow, East Asians carry 40% more Neanderthal DNA than Europeans, indicating the nature of AMH/Neanderthal interaction is more complex than previously thought (Wall et al., 2013) and involved prolonged interbreeding and/or successive waves of AMHs migrating out of Africa. Archaeological evidence supporting an interbreeding hypothesis has been found across multiple sites in Europe and Asia, indicating stabilised populations of hybrid hominids (Condemi et al., 2013; Smith, Jankovic & Karavanic, 2005; Duarte et al., 1999). It is argued that in light of mounting evidence, Neanderthals were a subspecies of Homo sapiens rather than a separate species of hominid. Though contributing a relatively small amount of genetic material, it is clear Neanderthals did not go extinct in the classical sense. Rather, Neanderthals were likely genetically swamped by their AMH cousins and absorbed into the invading populations of AMHs as they colonised Europe and Asia.
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Considering the highly derived nature of human bipedalism and its associated skeletal characteristics there must have been strong selection pressures that specifically required prolonged periods of upright walking (Harcourt-Smith 2007, p. 1507). Of the many proposed explanations for the selection of bipedalism (e.g. tool-carrying, male provisioning, thermoregulation) an energy efficient mode of locomotion, possibly associated with foraging, appears the most robust. Given the palaeoenvironmental trends during the Plio-Pleistocene, it may be easier to argue for an increased efficiency hypothesis for the shift to obligate bipedalism. However, as Rodman & McHenry (1980, p. 105) argued in their paper on the relative efficiency of human bipedalism, It is not necessary to posit special reasons such as tools or carrying to explain the emergence of human bipedalism, although forelimbs free from locomotor function surely bestowed additional advantages to human walking.