Stride lengths, speed and energy costs in walking ofAustralopithecus afarensis: using evolutionary robotics to predict locomotion of early human ancestors

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Abstract
This paper uses techniques from evolutionary robotics to predict the most energy-efficient upright walking gait for the early human relativeAustralopithecus afarensis, based on the proportions of the 3.2 million year old AL 288-1 ‘Lucy’ skeleton, and matches predictions against the nearly contemporaneous (3.5–3.6 million year old) Laetoli fossil footprint trails. The technique creates gaitsde novoand uses genetic algorithm optimization to search for the most efficient patterns of simulated muscular contraction at a variety of speeds. The model was first verified by predicting gaits for living human subjects, and comparing costs, stride lengths and speeds to experimentally determined values for the same subjects. Subsequent simulations forA. afarensisyield estimates of the range of walking speeds from 0.6 to 1.3 m s−1at a cost of 7.0 J kg−1 m−1for the lowest speeds, falling to 5.8 J kg−1 m−1at 1.0 m s−1, and rising to 6.2 J kg−1 m−1at the maximum speed achieved. Speeds previously estimated for the makers of the Laetoli footprint trails (0.56 or 0.64 m s−1for Trail 1, 0.72 or 0.75 m s−1for Trail 2/3) may have been underestimated, substantially so for Trail 2/3, with true values in excess of 0.7 and 1.0 m s−1, respectively. The predictions conflict with suggestions thatA. afarensisused a ‘shuffling’ gait, indicating rather that the species was a fully competent biped.