Monday, June 8, 2009

Exploring the million-year origins of laughter by tickling apes

Laughing seems to be older than humanity itself, according to new research published this week by a team of international researchers, including a Georgia State University scientist.

Michael Owren, associate professor of psychology and neuroscience, was part of a team led by Marina Davila Ross, of the University of Portsmouth in England, with Elke Zimmerman of the University of Veterinary Medicine Hannover in Germany, which analyzed the recorded sounds made by young apes, as well as human infants, when tickled.

The study was published online last week in Current Biology.

Davila Ross went to zoos in Europe and Asia to record the vocalizations made from orangutans, gorillas, bonobos and chimpanzees when tickled. The results were then taken to Georgia State, where she and Owren analyzed the acoustics of the samples.

The vocalizations of the species — humans’ closest evolutionary relatives — have different characteristics. For example, while humans make a continuously egressive sound in the form of a “haa haa haa,” without breathing in, chimpanzees make a pant-like sound, breathing in and out, Owren said.

The scientists conducted a phylogenetic analysis based on the acoustics of all of the vocalizations to reconstruct an evolutionary “family tree.” Usually, such an analysis is based on genetics, where scientists try to reconstruct the likely divergence in the evolution of species. Scientists believe that humans and the other great apes share a common ancestor.

“We already know what the phylogenetic tree looks like for these five species,” Owren explained. “This gave us the opportunity to ask, ‘if we make the same phylogenetic reconstruction based on acoustics, would we come out with the same tree?’”

The acoustical data independently produced a tree identical to the tree produced by genetic data, suggesting that the sounds derive from a common ancestor living somewhere between 10 to 16 million years ago.

“This then makes us comfortable to call these vocalizations laughter because they seem to have a common origin,” he said.

In examining the five species, human and chimpanzee laughter stand out from the other primates. Owren said it seems that during human and chimpanzee evolution, specialization occurred in opposite directions — where humans exhale and form vowel-like sounds, but chimpanzees pant, alternating between ingressive and egressive airflow.

Other species do something in between, as some alternate, and some produce consecutive sounds from egressive airflow.

Also during evolution, humans might have evolved from making very noisy, harsh sounding laughter, to producing a regular and stable vocalization, he said.

The results from this study lead to other evolutionary questions — why each species’ laughter evolved into their current forms, and what advantages the different acoustical features might give.

“The study gives us more specific information on which to begin to think about what the function of laughter was in the common ancestor of all these species, and how the function of laughter may have changed in early humans to produce differences in acoustics,” Owren said.

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