Speech is a byproduct of the respiratory adjustments associated with walking upright on two legs. With bipedalism came a secondary and unrecognized consequence, the respiratory plasticity necessary for speech. Quadrupedal species must synchronize their locomotion and respiratory cycles at a ratio of 1:1(strides per breath), a coupling required by the shared, rhythmic use of the thoracic complex (sternum, ribs, and associated musculature), and the need to endure impacts of the forelimbs during running. Without such sychronization, running quadrupeds would fall face first into the dust because their thorax would be only a floppy air-filled bag that could not absorb the shock of forelimb strikes. Human bipedal runners free of these mechanical constraints on the thorax employ a wide variety of phase-locked patterns (4:1, 3:1, 2:1 [most common], 1:1, 5:2, and 3:2), evidence of a more plastic coupling between respiratory rhythm and gait. The relative emancipation of breathing from locomotion permitted by bipedality was necessary for the subsequent selection for the virtuosic acts of vocalization we know as speech.
The contribution of bipedality to speech evolution has been neglected because linguists typically focus on higher-order cognitive and neurobehavioral events that occur from the neck up and overlook the neuromuscular processes that produce the modified respiratory movements known as speech.