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One of the more complex aspects of human behaviour is our universal ability to laugh. Laughter has puzzled behavioural biologists for many years because it is hard to imagine how this strange behaviour has evolved.

Why would laughing individuals be fitter in reproductive terms? And why is this ability is built-in, like sneezing, rather than something we learn, like hunting?

Today, we get an interesting insight into these questions along with some tentative answers from Pedro Marijuán and Jorge Navarro at the Instituto Aragonés de Ciencias de la Salud in Spain.

The evolution of laughter, they say, is intimately linked with the evolution of the human brain, itself a puzzle of the highest order. There is widespread belief that the brain evolved rapidly at the same time as human group sizes increased.

Bigger groups naturally lead to greater social complexity. And it’s easy to imagine that things like language and complex social behaviours are the result of brain evolution. But the latest thinking is more subtle.

Known as the social brain hypothesis, this holds that the brain evolved not to solve complicated ecological problems such as how to use tools, how to hunt more effectively and how to cook. Instead, the brain evolved to better cope with the social demands of living in larger groups.

In chimps, an important aspect of social behaviour is grooming, something they can spend up to 20 per cent of their time doing. Grooming is an activity that takes place in pairs. It is important because it establishes and strengthens bonds between individuals. However, there is a clear practical limit to the number of individuals you can bond with in this way before you begin to starve.

The social brain hypothesis is that language evolved as a way of establishing and strengthening bonds with larger numbers of individuals in a shorter a period of time. Conversation can easily include up to 10 individuals and would have been a skill that dramatically improves the fitness of these individuals for life in the group.

Laughter is simply an extension of this process, say Marijuán and Navarro. Since the act of talking limits the number of individuals who can take part in a conversation, laughter is a method that individuals use to signal their participation in larger group chats. And the result of all this extra bonding is that the larger group, and hence the individuals within it, flourishes.

The idea of the social brain has been around for a few years now. What Marijuán and Navarro bring to the discussion that is new is an explanation for why laughing is built in, rather than something we learn. Their new idea is that the evolution of laughter is analogous to the evolution of blushing.

Blushing occurs when cerebral blood flow is channelled through the facial artery, a branch of the carotid artery that feeds the brain. It serves the important function of relieving the excess flow that occurs during certain social situations. This extra blood flow shows up in the face, not because the face is visible but because that’s where the facial artery goes. The social significance of blushing evolved as a consequence of this.

Laughter is a similar kind of release, say Marijuán and Navarro. The intellectual momentum that builds up during conversation needs to be relieved, either through verbalisation or some other mechanism.

Marijuán and Navarro’s suggestion is that this other mechanism is the channelling of excess cortical excitations to parts of the brain responsible for vocalisation. But without anything specific to say, the result is the kind of panting and cackling that we call laughter. That’s why it is built in. This social significance of this behaviour is the thing that has evolved, not the activity itself.

This interesting idea is a synthesis of ideas from a mind-boggling array of disciplines: neuroimaging, neurophysiology, sound analysis, physiology as well as evolutionary theory and sociobiology, to name just a few.

The question now, of course, is how to test it.

Ref: arxiv.org/abs/1010.5602: The Bonds of Laughter: A Multidisciplinary Inquiry into the Information Processes of Human Laughter


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