Laughter in Chimpanzees and Humans - Biology 342 Fall 2012By Yuan Xue, Soso |
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What Is the Evolutionary Relationship Between Human and Chimpanzee Laughter Behavior?Common Chimpanzees and humans are relatively closely related; our most recent common ancestor dates to be about 6 million years ago. Genetically speaking, the sequence divergence of humans and chimpanzees in DNA is estimated to be about 4.85% (Britten, 2002). Most studies suggested that humans and chimpanzees share many similar social behaviors and patterns. In fact, a study on primate laughter behavior revealed strong evidence that laughter, especially when induced by tickling, is homologous between great apes (orangutans, gorillas, chimpanzes, and bonobos) and humans in terms of source of vocalization and social context; it supports the more general postulation of phylogenetic continuity from nonhuman displays to human emotional expressions of laughter-like behavior (Ross et. al, 2009). Distinctively human laughter characteristics such as predominantly regular, stable voicing and consistently egressive airflow are found to be traceable to characteristics of shared ancestors with great apes.
Frequency Spectra of human (top) and chimpanzee (bottom) reveals similarity in voicing production via stable and consistent airflow exchange. Human laughter is, however, distinguished from chimpanzee laughter by its sharply defined onset and offset of the voiced, vowel-like notes.
Bipedalism makes the difference The evolution of bipedalism permitted flexibility in the co- ordination of breathing, running, and vocalizing. This is the basis of the bipedal (‘‘walkie-talkie’’) theory of speech evolution (Provine, 2000). A bipedal human runner, for example, may employ a variety of strides per breath: The ratio can be 4:1, 3:1, 5:2, 2:1, 3:2, or 1:1, with 2:1 being the most common (Bramble & Currier, 1983). Freed of the necessarily rigid 1:1 link between stride and breath characteristic of quadrupeds, humans evolved a vocal system in which individual sounds were no longer tied to single breaths, permitting the subsequent natural selection for speech and, incidentally, our species’ characteristic ‘‘ha-ha’’ laugh.
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