The evolution of bipedal running in lizards suggests a consequential origin may be exploited in later lineages.

The origin of bipedal locomotion in lizards is unclear. Modeling studies have suggested that bipedalism may be an exaptation, a byproduct of features originally designed to increase maneuverability, which were only later exploited. Measurement of the body center of mass (BCOM) in 124 species of lizards confirms a significant rearward shift among bipedal lineages. Further racetrack trials showed a significant acceleration threshold between bipedal and quadrupedal runs. These suggest good general support for a passive bipedal model, in which the combination of these features lead to passive lifting of the front of the body. However, variation in morphology could only account for 56% of the variation in acceleration thresholds, suggesting that dynamics have a significant influence on bipedalism. Deviation from the passive bipedal model was compared with node age, supporting an increase in the influence of dynamics over time. Together, these results show that bipedalism may have first arise...