In a species with a reputation for cunningly manipulating others to maximise personal gain, blushing is pretty difficult to explain. Why would humans evolve a response that puts us at a social disadvantage by forcing us to reveal that we have cheated or lied? One suggestion is that blushing started out as a simple appeasement ritual: a way to show dominant members of the group that we submit to their authority. Perhaps later, as our social interactions became increasingly complex, it became associated with higher, self-conscious emotions such as guilt, shame and embarrassment.
The primatologist Frans de Waal of Emory University in Atlanta, Georgia, thinks blushing could have emerged as a way to foster trust. "If you were to go hunting with a partner with a face of stone, so you could never tell what he wants, you would feel uncomfortable and wouldn't be sure if you could trust him," he says.
Once blushing became associated with embarrassment, anyone who did not blush might have been at a disadvantage because we are less likely to trust someone who appears never to feel ashamed about anything.
"Do you have a rubber band?" No, it's not a joke, but it was enough to make someone in a Baltimore shopping mall laugh. It is one of more than 2,000 instances of natural laughter recorded by the psychologist Robert R Provine of the University of Maryland, Baltimore, and his team, during their classic 10-year study, the results of which Provine published in his book Laughter: A scientific investigation. Their most striking finding: laughter is more often prompted by banal comments than amusing jokes. That makes it even more mysterious.
Provine thinks laughing began in our pre-human ancestors as a physiological response to tickling. Modern apes maintain the ancestral "pant-pant" laugh when they are tickled during play, and this evolved into the human "ha-ha". Then, he argues, as our brains got bigger, laughter acquired a powerful social function - to bond people.
Indeed Robin Dunbar, at the University of Oxford, has found that laughing increases levels of endorphins, our body's natural opiates, which he believes helps to strengthen social relationships.
"The interpretation of dreams is the royal road to a knowledge of the unconscious activities of the mind." So said Sigmund Freud. Today, most researchers reject his belief that dreams are expressions of our unconscious desires, but the fascination with why we dream is stronger than ever. Dreams are not meaningless, and they are certainly not useless. For a start, they are crucial for processing emotions. "Dreams modulate the emotions - they keep them within a certain range," says Patrick McNamara of Boston University.
REM dreaming also helps other types of memory and problem solving. People are better able to recall lists of related words and the connections between them after a night's sleep than after the same time spent awake during the day. It has recently become apparent that not all dreaming occurs in REM sleep, and there are hints that non-REM dreams have their own special function.
McNamara thinks that by simulating aggressive encounters, REM dreams help us cope with real aggression, whereas non-REM dreams support co-operative behaviour.
The discovery of universal themes could herald a return to the study of the meaning of dreams, this time based on science. "It suggests that a certain kind of dream interpretation may be possible," McNamara says.
Barack Obama, the US president, likes to play basketball on the morning of an election. The golfer Tiger Woods always wears a red shirt when competing on a Sunday. Most of us have our own superstitions, even though we know rationally that they cannot work.
Yet superstition is not entirely nonsensical. Our brains are designed to detect structure and order in our environment, says Bruce Hood at the University of Bristol. We are also causal determinists - we assume that outcomes are caused by preceding events. This combination of sensing patterns and inferring causes leaves us wide open to superstitious beliefs. "But there are very good reasons why we have evolved these capabilities," Hood adds. Spotting and responding to some uncertain cause-and-effect relationships can be crucial for survival.
Our ancestors would not have lasted long if they had assumed that a rustle in the grass was caused by wind when there was even a small chance it was a lion. And it is worth making false-positive mistakes to get these relationships right. Kevin Foster, of Harvard University, and Hanna Kokko, from the University of Helsinki, Finland, used mathematical modelling to show that whenever the cost of believing a superstition is less than the cost of missing a real life-or-death association, superstitious beliefs will be favoured by evolution.
Explaining the peculiar human urge to create works of art in terms of evolutionary survival is a challenge. Darwin suggested art has its origins in sexual selection, and Geoffrey Miller at the University of New Mexico in Albuquerque has run with the idea. He thinks that art is like a peacock's tail - a costly display of evolutionary fitness.
Miller's studies show that both general intelligence and the personality trait of being open to new experiences correlate with artistic creativity. But Miller admits sex alone may not explain the evolution of art. "It might have originated for some other function, and acquired the sexual display function later," he says. So what other purpose might art serve?
Evolutionary psychologists John Tooby and Leda Cosmides at the University of California, Santa Barbara, think the drive to seek out aesthetic experiences could have evolved to push us to learn about different aspects of the world - those that our brain's hard-wiring has not equipped us to deal with at birth.
Kissing - in the amorous, lip-locking sense-- is not practised in all cultures, so the urge to pucker up cannot be in our genes. Still, you have to wonder why so many of us do it. There is no shortage of speculation.
One idea is that our first experience of comfort, security and love comes from the mouth sensations associated with breastfeeding. Added to this, our ancestors probably weaned their babies by mouth-to-mouth feeding of chewed food, as chimpanzees and some mothers do today, reinforcing the connection between sharing spit and pleasure. Another idea is that kissing has its origins in foraging. The story goes that our ancestors were first attracted to ripe, red fruit, then co-opted this attraction for sexual purposes, developing pronounced red colouration on the genitals and lips. "Instead of reinventing the wheel you use the same template for other kinds of attraction," says V S Ramachandran at the University of California, San Diego.
Since red lips are most obvious in Caucasians, he suggests kissing may have started at northern latitudes and then spread culturally around the world. Without solid evidence, however, even Ramachandran is wary of his own idea, acknowledging that kissing may have arisen independently several times throughout human history. www.newscientist.com