A funny thing happened way back in the mysterious prehistory of humanity: Hominids started to walk on two legs, and then somebody laughed.
Nobody knows just what brought about the first characteristically human ``ha-ha-ha.'' Unlike questions about the health benefits of laughing, or why some things are funny and others are not, research into the physiology of laughter -- the mechanics and evolutionary origins of what has been called ``a luxury reflex'' common in all cultures -- has not been given much scientific scrutiny.
Robert Provine, a psychologist at the University of Maryland-Baltimore County, has waded into the breach.
He has spent the better part of the past 10 years studying when and how people laugh, why laughter is contagious and, most recently, the significance of the fact that humans laugh differently than our closest primate cousins.
His serious book on the subject, ``Laughter,'' an outgrowth of a 1996 article he published in the magazine American Scientist on the science of laughter, is expected to be out within the year.
Provine draws a sharp distinction between humor studies and his own ``naturalistic'' approach.
``Humor is a different problem,'' he said. ``Laughter is really not about humor. It's about social situations. And it's one of the more common, but neglected, human behaviors.''
Provine maintains that a close look at laughter can open a window into much more complicated matters, including the origins of human speech and bipedality, and why efforts to teach chimpanzees verbal communication skills have met with little success.
Laughter as Fossil
He is not the first to propose the notion that walking upright was a precondition for speech in humans. What sets Provine apart is his use of laughter as a kind of living fossil record to demonstrate why that might be the case.
All agree that laughter is ancient, instinctual behavior, one of the few universal forms of sound communication.
``It's part of human nature,'' said psychologist Harvey Mindess, a professor emeritus at Antioch University who has studied laughter and mental health.
``There is no human society we know of that has ever been without humor,'' he said. ``It's a form of release, something that reconnects us to what people nowadays call the `free child' in us. It's definitely part of being a human being.''
It's also part of being a chimpanzee.
Provine has spent years looking into just what is different about chimp laughter and human laughter. Funny as that may sound, he maintains it's a way to gain insights into ``a very difficult point in evolutionary history'' -- the possible links between the way we talk and the way we get around.
He presented a summary of this laughter-linked ``walkie-talkie'' theory, as he likes to call it, at the recent annual meeting of the Society for Neuroscience.
It goes something like this: One reason chimpanzees don't laugh the same as humans is because chimps almost always walk on all fours. For anatomical reasons, it's difficult for the animals to use their breath and vocal cords to make complex sounds.
They can hoot and holler and make a play face, chasing one another around with obvious glee. But a laughing chimp will never sound like a laughing person, no matter how long you tickle it.
Instead, chimps make a loud sort of panting noise when they laugh, which Provine has likened to ``the sound and cadence of a handsaw cutting wood.'' It's a sound made by rapidly breathing in and out, one ``laugh note'' emitted per breath cycle in a way that humans can mimic only with a little effort.
By contrast, and after intense deconstruction of the signature sound patterns involved, Provine concluded that what makes human laughter distinct is its repetitive bursts of sound -- consisting of multiple chopped-up bits of a single exhalation rattling past the vocal cords.
The ``laugh notes'' may vary from one individual to the next, but whether it's a ``ha-ha-ha,'' a ``hee- hee-hee'' or a ``ho-ho-ho'' does not alter the fundamental observation that ``chimps laugh like chimps, and people laugh like people,'' Provine said.
Previous research by primatologists has posited a need for breathing patterns timed with gait in animals that use their forelimbs to bear their full weight while walking or running. The breathing cycle has to match up with locomotion so that the lungs are full of air when the forelimbs hit the ground.
In bipedal hominids, however, this ``tyranny of gait'' was broken, freeing the lungs to help generate complex sounds with a rhythm all their own. Of course, other things had to happen, too, in the brain and spinal cord and larynx, for human speech to be possible.
But Provine argues that the way we laugh offers an important clue as to why walking upright must have come before complex verbal speech.
Nobody knows exactly when human-style speech began. Based on fossil remains and other clues, most experts agree it's a relatively recent phenomenon -- almost certainly beginning no more than 200,000 years ago or so.
Ancestors of Homo sapiens were walking on two legs almost exclusively some 3 million years ago.
``The evolution of bipedalism made possible the evolution of speech,'' Provine said, adding that the freedom to breath in and out irrespective of gait gave early humans ``the luxury of evolving more virtuosic feats of soundmaking.''
Michael Arbib, a neuroscientist at the University of Southern California who has studied primitive language acquisition and the brain, said there may well be links between walking upright and the advent of verbal speech. But he said laughter may be somewhat beside the point.
Instead, it could be that people started using their hands to communicate routinely only after they got used to walking on two legs. Vocalizations presumably followed, reinforcing the hand signals, or perhaps substituting for hand signals when they could not be seen.
``If we were still quadrupeds, it would be hard to release the hands enough to allow for communication,'' Arbib said.
Human-style speech also required a well-developed larynx, as well as the brainpower and spinal circuitry to drive the voice box. Our first ancestors paid a price for that capability in the form of a much narrower airway, suggesting to Arbib one possibility for the world's first sentence: ``Help, I'm choking.''
Nobody knows whether anybody took that seriously.
ANATOMY OF A BELLY LAUGH
Virtually all forms of human laughter are but variations on a theme -- short, vowel-like notes repeated at regular intervals. Careful study of the universal forms of human laughter, compared with the breathy panting of chimpanzee laughter, has yielded clues to the origin of speech.
Chimps nearly always walk on all fours, which for anatomical reasons makes it hard for them to use their breath and vocal cords to make complex sounds. By contrast, humans ``laugh as we speak,'' says laughter expert Robert Provine, author of an upcoming book on the subject.
CHIMP vs. HUMAN LAUGHTER Using a sound spectrograph, psychologist Robert Provine compared the differences in sound frequency between chimp and human laughter.
Explosively voiced blasts of vowel-like sounds, such as ``ha-ha-ha'' or ``hee-hee-hee,'' with each exhalation chopped into multiple ``laugh notes''
A series of breathy, in-and-out panting sounds, resembling ``ah-ah-ah,'' with just one laugh note for each exhalation or inhalation.
LAUGHTER IS CONTAGIOUS
Our brains seem to be pre-wired to detect laughter and respond in kind, which is the reason ``laugh tracks'' are so effective. Specific neural circuits may be involved in detecting and generating this instinctual social behavior.
1. Laugh Generator Experimenters used a mechanical ``laugh box'' to create a disembodied laugh stimulus.
2. Auditory feature detector
Hearing the unique sound of a laugh may activate a listener's built-in ``auditory feature detector,'' much the same as a bird can pick out the unique song of its own species.
3. Laugh generator The laugh-detecting circuitry taps into other circuits in the brain, larynx and chest responsible for generating the appropriate response. Soon, everybody's laughing even though nobody really knows why.
Source: American Scientist, Jan.-Feb. 1996; Robert Provine, University of Maryland-Baltimore County