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 鈥渇ascinatingly short period鈥� the great leap forward in human brain size, when our ancestors鈥� brains underwent the last big expansion to reach today鈥檚 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. 鈥淭he brain is roughly the same size at 20 as it is at 12, yet we can do so much more with it,鈥� Bainbridge says.
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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. 鈥淲ithout teenagers we would never have become fully human,鈥� he says. 鈥淭hey are the most important part of human life.鈥�
鈥淲ithout teenagers we would never have become fully human. They are the most important part of human life鈥�
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 (American Journal of Human Biology, vol 21, p 567). For girls, the teenage growth spurt occurs early, so that they look sexually mature several years before they reach full reproductive maturity. 鈥淭hey get into the networks of adult women,鈥� 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.
鈥淚 see adolescence as a trade-off,鈥� 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. 鈥淚t鈥檚 all about value added.鈥�
Read more: Ten mysteries of you