Meeting Abstract
Humans have relatively short arms compared to their hominin ancestors. Traditionally, the evolution of shorter arms relative to the legs has been interpreted as a gradual behavioral transition from generalist locomotion (including climbing) to near-obligate bipedalism. This interpretation is supported by R.M. Alexander’s Dynamic Similarity hypothesis, which states that two animals of different sizes have similar gaits if they travel at the same dimensionless speed AND are isometrically scaled. However, I challenge the notion that inter-limb isometry is a condition for similarity in bipeds. Instead, I argue that similarity is conditioned on having the same mass-specific joint stiffness at the shoulder during arm swing, and that longer legs require relatively shorter arms for similarity. I derived a simple quantitative arm swing similarity model using walking data from humans, and show that the model reasonably predicts fossil hominin arm lengths for all but the earliest species. I conclude that a behavioral transition is not necessary to explain the evolution of limb proportions in hominins, and that the modern human walking gait may have evolved much earlier in the hominin lineage than thought. When the same model is applied to non-avian theropod dinosaur limbs, they too fall along an arm swing similarity scaling relationship. The similarity model outperforms isometric models in both taxa, and has the strength of being derived from first principles. Furthermore, the swing similarity model is generalized and can be applied to leg swing in the future. Surprisingly, insights into why human arms are relatively short may have led to an answer to the age-old question of why T. rex arms were so short: bigger bipeds require relatively shorter arms for similar arm swing mechanics.
