June 4, 2009
Apes Help Scientists Discover Origins Of Laughter
When researchers set out to study the origins of human laughter, some gorillas and chimps were literally tickled to assist.
The scientists tickled 22 young orangutans, chimpanzees, gorillas, and bonobos, as well as three human infants, then acoustically analyzed the laughing sounds they produced.
Although the vocalizations varied, the researchers found that the patterns of changes fit with evolutionary splits in the human and ape family tree.
"This study is the first phylogenetic test of the evolutionary continuity of a human emotional expression," said Marina Davila Ross of the University of Portsmouth in the United Kingdom.
"It supports the idea that there is laughter in apes."
A quantitative phylogenetic analysis of the acoustic data produced by the tickled infants and apes revealed that the best "tree" to represent the evolutionary relationships among those sounds matched the known evolutionary relationships among the five species based on genetics. The researchers said that the findings support a common evolutionary origin for the human and ape tickle-induced expressions.
They also provide evidence that laughter evolved slowly over the last 10 to 16 million years of primate evolutionary history.
Nevertheless, human laughter is acoustically distinct from that of great apes and reached that state through an evident exaggeration of pre-existing acoustic features after the hominin separation from ancestors shared with bonobos and chimps, about 4.5 to 6 million years ago, Ross said.
For example, humans make laughter sounds on the exhale. Although chimps do that as well, they can also laugh with an alternating flow of air, both in and out. Humans also use more regular voicing in comparison to apes, meaning that the vocal cords regularly vibrate.
Ross said the researchers were surprised to find that gorillas and bonobos can sustain exhalations during vocalization that are three to four times longer than a normal breath cycle -- an ability that had been thought to be a uniquely human adaptation, important to our capacity to speak.
"Taken together," the researchers wrote, "the acoustic and phylogenetic results provide clear evidence of a common evolutionary origin for tickling-induced laughter in humans and tickling-induced vocalizations in great apes. While most pronounced acoustic differences were found between humans and great apes, interspecific differences in vocal acoustics nonetheless supported a quantitatively derived phylogenetic tree that coincides with the well established, genetically based relationship among these species. At a minimum, one can conclude that it is appropriate to consider 'laughter' to be a cross-species phenomenon, and that it is therefore not anthropomorphic to use this term for tickling-induced vocalizations produced by the great apes."
The research was reported online on June 4th in Current Biology, a Cell Press publication.
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