Paleoanthropology's uses of the bipedal criterion.

Bipedalism is one of the criteria that paleoanthropologists use in order to interpret the fossil record and to determine if a specimen belongs to the human lineage. In the context of such interpretations, bipedalism is considered to be a unique characteristic of this lineage that also marks its origin. This conception has largely remained unchallenged over the last decades, in spite of fossil discoveries that led to the emergence of bipedalism in the human lineage being shifted back by several millions of years. In this paper, I analyze the uses of this criterion in paleoanthropology and demonstrate that interpretative biases (such as underdetermined inferences and circular reasoning) are at play in interpretations of hominin remains. By discussing Darwin's hypotheses about the evolution of bipedalism, I identify major theoretical issues that need to be addressed in the current debates on hominin evolution. First, the assumption that "man alone has become a biped" (Darwin in The descent of man, and selection in relation to sex, John Murray, London, 1871) is analyzed in the light of recent empirical data. Three major issues are discussed: the definition of "man", i.e. "human", the uniqueness of human bipedalism, and the equivocal meaning of being a "biped". Then, I highlight some of Darwin's remarks that may be helpful for current debates in paleoanthropology, regarding natural selection in locomotor evolution, as well as taxonomic and phylogenetic significance of functional features. Finally, I analyze two examples of how fossil discoverers referred to Darwin in the recent years and discuss his role as an intellectual support.