Robert Provine: University of Maryland

After an introduction to the nature and use of laughter in daily life, discussion turns to the

evolution of laughter and its implications for the emergence of speech.

Contrasts between human and chimpanzee laughter reveal that laughter is the ritualized

signal of labored breathing of rough-and-tumble, with the vocalization representing

the playful context of its origin. Laughter is primate onomatopoeia that signals, “this

is play, I’m not attacking you.” The ancestral primate laugh as performed by extant

chimpanzees is a noisy, unvoiced “pant-pant” in which one vocalization is uttered per

inward and outward breath. Naïve human observers do not recognize this utterance as

laughter, volunteering that it sounds like panting, perhaps of a dog, or even sawing or

sanding. In contrast, humans laugh as they speak, by modulating (parsing) an exhalation

into voiced bursts of “ha-ha.” Chimpanzee laughter suggests why they cannot speak–

they are confined to one utterance per inward and outward breath. The breakthrough in

human respiratory control necessary for speech came with evolution of bipedal running.

Chimpanzees and other quadrupeds have a 1:1 ratio between stride and respiratory cycle,

a necessary adaptation for forelimb impacts during running. (Without inflated lungs,

the thorax is a floppy, air-filled bag.) An unappreciated consequence of the evolution

of bipedal human locomotion is the freeing the thorax of its support function during

running, and the associated uncoupling of respiration and locomotion. Unlike the 1:1

strides per breath ratio of quadrupeds, humans runners have highly variable ratios of

4:1, 3:1, 5:2, 2:1, 3:2, or 1:1, with 2:1 being most common. With the uncoupling of

respiration and locomotion during bipedal running, there is more voluntary control of

breathing and greater opportunity for the natural selection for longer and more complex

vocalizations, including human laughter and speech. This is the bipedal theory of speech

evolution.