But as science probes deeper into these eccentricities, it is becoming clear that behaviours and attributes that seem frivolous at first glance often go to the heart of what it means to be human.
Even Darwin struggled to explain why we would evolve a response that lets others know that we have cheated or lied
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?
It is a question that Charles Darwin struggled with. He pointed out that while all people of all races blush, animals - other primates included - do not. When it came to explaining the evolution of "the most peculiar and the most human of all the expressions", he was at a loss. That has not stopped others from trying.
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. This would seem to put individuals at a disadvantage, but blushing might actually make a person more attractive or socially desirable.
Noting that women blush more than men, neuroscientist V. S. Ramachandran of the University of California, San Diego, suggests that blushing might have evolved as a way for women to demonstrate their honesty to men and so enlist their help in rearing offspring. "Blushing is telling you that I can't cuckold you. If you ask me about infidelity I can't lie - my blush gives me away," he says.
Primatologist Frans de Waal of Emory University in Atlanta, Georgia, also 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.
The discovery that laughter is more often produced at banal comments than jokes prompts the question, why did it evolve?
"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 2000 instances of natural laughter recorded by 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 (Current Directions in Psychological Science, vol 13, p 215). 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.
"But there's a big difference between 'laughing with' and 'laughing at'," Provine notes. Laughing at someone can encourage them to conform socially or it can push them away. There's also a big difference between natural, "emotional" laughter and conversational, embarrassed, nervous and aggressive laughter, points out Matthew Gervais at the University of California, Los Angeles, who has analysed these different types. He thinks the latter forms of "forced" laughter probably evolved later, to fine-tune our manipulation of social situations .
So where does humour fit in? It may be playing more of a role than Provine thinks, according to Thomas Flamson at UCLA. He suspects that laugh-inducing comments like the rubber-band question might not seem like jokes to eavesdropping researchers, but many could still be funny to the right person. Such shared in-jokes would boost laughter's ability to generate camaraderie. Even Provine believes humour is important. He has found that on average, men are funnier than women, and his analysis of personal ads indicates that women generally request humour, while men offer it. This suggests that the ability to make others laugh has evolved at least in part through sexual selection.
Scent radiator, warmth provider, or chafe protection? The answer to why humans have clumps of hair in private places is still open for debate
We may be the naked ape, but on one measure of hairiness humans trump all other primates. While most of them have finer hair around their genitals than on the rest of their body, adult humans sport an impressively thick bush of pubic hair.
It has long been assumed that pubic hair is a remnant of a furrier period in our evolutionary history, and that the real question is why the rest of the body lost its hirsuteness. Earlier this year, though, Robin Weiss of University College London pointed out that our pubic hair clearly became thicker than that on the rest of our bodies at some point in our evolution . And this must have happened for a reason. So what drove the evolution of pubic hair?
There's no accepted explanation, but many potential advantages have been suggested over the years. Perhaps the most popular is that since thicker hair gathers in regions where we have apocrine (scent) sweat glands as well as eccrine (cooling) ones, it may serve to waft odours that signal sexual maturity. It may also act as a visual signal of adulthood, along with growing breasts and widening hips in girls and deeper chests and beards in boys. Various other benefits could have made it worth keeping. A thick bush not only protects the genitals during sex and at other times - reducing chaffing while walking, for example - it also helps keep our most sensitive regions warm and free of draughts.
So when did it evolve? Based on studies of the evolution of pubic lice by David Reed at the Florida Museum of Natural History in Gainesville, Weiss suggests some time around 3.3 million years ago. This is when human lice diverged from a closely related species found on the thick body hair of gorillas. This, Weiss suggests, could signal that while the rest of our bodies were naked by that time, we had evolved thick enough pubic hair for the lice to jump species and take hold.
Even our closest relatives, the great apes, move smoothly from their juvenile to adult life phases – so why do humans spend an agonising decade skulking around in hoodies?
No other species has teenagers. Even our closest relatives, the great apes, move smoothly from their juvenile to adult life phases. So why do humans spend an agonising decade or so skulking around in hoodies? Traditionally, the teenage years have been seen simply as a sort of reproductive apprenticeship, but a better understanding of adolescence has spawned some more interesting explanations.
David Bainbridge of the University of Cambridge, author of Teenagers: A natural history, says there are two big clues. The first is when adolescence evolved. Evidence from growth in the bones and teeth of fossilised hominins indicates that it emerged sometime between 800,000 and 300,000 years ago. This, he notes, pre-dates by a "fascinatingly short period" the great leap forward in human brain size, when our ancestors' brains underwent the last big expansion to reach today's size.
The second clue comes from neurobiology and brain imaging, which show that there is a wholesale reorganisation of the brain during the teenage years. "The brain is roughly the same size at 20 as it is at 12, yet we can do so much more with it," Bainbridge says.
For Bainbridge, adolescence is less about achieving sexual maturity than about developing a mind capable of negotiating the psychological and social landscape that makes human life so different from that of other animals. "Without teenagers we would never have become fully human," he says. "They are the most important part of human life."
Without teenagers we would never have become fully human. They are the most important part of human life
Anthropologist Barry Bogin of Loughborough University, UK, has a slightly different take on it. His explanation stems from the observation that during adolescence, girls and boys undergo their own characteristic patterns of growth and development . For girls, the teenage growth spurt occurs early, so that they look sexually mature several years before they reach full reproductive maturity. "They get into the networks of adult women," says Bogin, which allows girls not only to practise skills they will require later but also, crucially, to build coalitions. Humans, he points out, have evolved a form of cooperative breeding in which success depends on sharing childcare among both family and non-family members.
Boys, by contrast, are sexually mature long before they develop their manly physique (see graph). This, argues Bogin, allows teenage boys to acquire sexually selected attributes that appeal to potential mates, such as linguistic creativity, humour and artistic talent, in a relatively safe environment because their boyish stature means that fully grown men do not consider them a threat.
"I see adolescence as a trade-off," Bogin says. By investing time during the second decade in acquiring greater cognitive, practical and material resources both girls and boys improve their chances of successfully reproducing later. "It's all about value added."
Today, most researchers reject Freud's belief that dreams are expressions of our unconscious desires – but if that's the case, what are they for?
"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. New research has found that naps consolidate emotional memories - and the greater the amount of rapid-eye-movement (REM) dream sleep, the greater the processing of these memories .
One idea is that REM dreams allow us to relive powerful emotional memories, but without the rush of stress hormones that accompanied the real experience . In this way, we keep the memory but the emotion accompanying it gradually lessens.
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. By waking students during both REM and non-REM sleep, McNamara and colleagues have discovered that REM dreams are more story-like, with more emotion, more aggression and more unknown characters than non-REM dreams, which often involve friendly social interactions . McNamara thinks that by simulating aggressive encounters, REM dreams help us cope with real aggression, whereas non-REM dreams support cooperative behaviour.
The content of dreams can be influenced by external factors such as scents, or even the Earth's geomagnetic field, but there seem to be some reoccurring themes. Many REM dreams contain unknown males, usually in aggressive social interactions with the dreamer. 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.
People still debate whether humans are genuinely altruistic by nature, but if we are, most agree it doesn't make evolutionary sense
If you believe there is no such thing as altruism, you are in good company. In The Selfish Gene, Richard Dawkins writes that we must "try to teach generosity and altruism, because we are born selfish". Even if we are nice to members of our family, that doesn't count because there is a pay-off, at least in biological terms: they share some of our genes, so by helping them we indirectly further our own genetic immortality. Meanwhile, other acts of seeming altruism are often just reciprocity. If you scratch my back, then I scratch yours - no matter how much later - that's not selfless either.
This all makes good evolutionary sense, since spending time and energy helping someone without any return puts you at a distinct disadvantage in the survival stakes. The only trouble is that in recent years evidence has amassed that people do commit acts of genuine altruism. In experimental game-playing situations, for example, many people will share money with a stranger even when there is nothing in it for them. This has led biologists to conclude that altruism is a part of human nature. What they cannot decide is how or why it evolved.
People with a certain version of a gene called AVPR1 are more altruistic than the average
According to Robert Trivers of Rutgers University in New Jersey, pure altruism is a mistake. He argues that natural selection favoured humans who were altruistic because in the small, close-knit groups in which our ancestors lived, altruists could expect reciprocity. However, in our globalised world where many of our interactions are with people we do not know and may never meet again, our altruistic tendencies are misguided: they are unlikely to be reciprocated and are therefore maladaptive .
Others disagree. They accept that altruism cannot be a product of genetic evolution, but argue that ever since our ancestors began to shape their own environment through culture, we have evolved by a process of genetic and cultural co-evolution. As well as favouring traits that benefit individuals, this can also select traits that benefit one group over another - and that is how altruism evolved. Altruism is crucial for social cohesion. And groups that are more cohesive are more likely to survive in interactions with other groups.
At a mechanistic level, gene-culture co-evolution makes sense. There are clear social mechanisms that promote altruism: for example, fear of punishment, reputation building, ideas of fairness and inculcation by religious or authority figures. There are also indicators that altruism has biological roots. Brain imaging reveals that it stimulates the reward centres of our brains. What's more, it would appear that people with a certain version of a gene called AVPR1 are more altruistic than the average. Their brains are more susceptible to the effects vasopressin, a feel-good hormone implicated in social bonding . Of course some might argue that if random acts of kindness give us a mental buzz, then this is not pure altruism after all.
Sexual display, learning tool or form of social glue? Art still refuses to be pinned down
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. He has also found that when women are at their monthly peak in fertility, they prefer creative over wealthy men . 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 (SubStance, vol 30, p 6). In a similar vein, Brian Boyd from the University of Auckland in New Zealand believes art is a form of intellectual play, allowing us to explore new horizons in a safe environment .
Another idea is that art is a social adaptation. Ellen Dissanayake at the University of Washington in Seattle suspects that it is all about making an object or event "special" by appealing to the emotions through, say, colour or rhythm. She thinks this process helped increase our ancestors' chance of survival by bonding a group together. This "making special" could have started in magical or supernatural rituals, and later become more aesthetic.
None of this explains where our aesthetic sense comes from. Michael Gazzaniga of the University of California, Santa Barbara, suggests we could be biologically primed to find certain images, such as symmetrical designs, more aesthetically pleasing - more beautiful - simply because our brain can process them more quickly. However, he adds that we respond positively to some art not because it appeals to us aesthetically, but because seeing it or, better still, owning it is an indicator of status. Miller goes even further. "It takes quite a lot of counter-intuitive education to distinguish good from bad contemporary art," he says. "Most people don't have the time to acquire such elite aesthetic taste - which is a form of fitness display in its own right."
Many of us have superstitions – odd, reassuring habits that make no rational sense – but there may be an underlying reason for such behaviourBarack Obama likes to play basketball on the morning of an election. 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, UK. 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 .
Religion offers another possible evolutionary benefit of superstition. "[Religious faith] involves a susceptibility to believe in a spirit world and its efficacy - even if it doesn't actually work," says Dunbar, who is a leading proponent of the idea that religion is adaptive. He thinks religion's main function is to persuade a community to toe the line, so promoting cohesion. This is achieved in part by tapping into our natural propensity to believe in supernatural beings that can influence our fate.
Although it is in our nature to be superstitious, cultural and environmental factors clearly influence how superstitious an individual actually is. For example, when we feel we are losing control over our lives, we tend to become more superstitious. One study found that people living in high-risk areas of the Middle East, such as Tel Aviv, are much more likely to carry a lucky charm than other people. Another showed that the growth rate of evangelical churches in the US jumps 50 per cent with the downturn of each economic cycle. Nobody is immune. "We can all shift our supernatural inclination depending on the circumstances," says Hood. "There are few atheists in a plane that is plummeting from 30,000 feet."
The urge to kiss is not brought about by genes, so why do we find it so pleasurable to share saliva?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 and why it feels so darn good. 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.
When it comes to the physiology of kissing we are on slightly firmer ground. Our lips are among the most sensitive parts of our bodies, packed with sensory neurons linked to the brain's pleasure centres. Kissing has been shown to reduce levels of the stress hormone cortisol and increase the bonding hormone, oxytocin.
There may even be a link between kissing and the way we assess our biological compatibility with potential partners. In recent years it has become apparent that we are most attracted to the smell of sweat from people whose immune system is most dissimilar from our own - with whom we are likely to produce the healthiest children. And of course kissing lets us get up close and personal enough to sniff that out.
Many of us do it, but eating bogeys offers little nutritional reward – could there be a health reason for the unappealing habit?In 2001, Chittaranjan Andrade and B.S. Srihari of the National Institute of Mental Health and Neurosciences in Bangalore, India, won an IgNobel prize for their research into nose-picking. They reported that almost the entire sample of 200 adolescents from four Bangalore schools admitted they engaged in the habit, at an average of four times a day. However, only nine pupils - 4.5 per cent - owned up to eating their nasal debris .
Given the social stigma attached to eating bogeys (also known as boogers), this figure is probably an underestimate. But even if numbers are lower than experience suggests, this still begs the question of why anybody should choose to eat their own snot.
"There isn't any significant nutritional content in nasal mucus," says Andrade. Moreover, he and Srihari found nothing to distinguish the bogey-eaters from the others, and they neglected to ask them why they did it. It is possible that ingesting nasal detritus might help build a healthy immune response - after all, researchers investigating the hygiene hypothesis have built a large body of evidence indicating that lack of exposure to infectious agents can increase one's susceptibility to allergic diseases.
It is possible that ingesting nasal detritus might help build a healthy immune system
Unfortunately, it seems the only research on the matter dates back to 1966, when Sidney Tarachow of the State University of New York found that people who ate their bogeys found them "tasty" .