Teenagers: A Natural History, By David Bainbridge

Should you have a difficult teenager in your life, this book might just help everyone co-exist. Friendly and light-hearted, it attempts to explain why those years are so challenging. David Bainbridge is convinced that the teenage years are something to celebrate. Were it not for teenagers, we oldies wouldn't even be around to complain: "human longevity has evolved because we need to bring up our intensively supported, slowly developing offspring". So adolescence is not "an irritating transitional phase, but... the fulcrum about which the rest of our life turns".

Bainbridge is a vet – he teaches veterinary anatomy at Cambridge University – and reckons that his profession gives him a special perspective on the second decade of life. One of the most distinctive thing about humans, compared to dogs, cats or chimpanzees, is that we take an inordinately long time to grow up. Human teenagers constitute a unique biological phenomenon.

No one knows precisely when delayed maturation evolved in Homo sapiens, but recent analysis of growth rings in ancient tooth enamel suggest that the onset of adulthood in the second decade of life first emerged some 300,000 years ago. If that date is correct, the first teenagers appeared just before the final evolutionary expansion of the human brain, 250,000 years ago, when it swelled to its modern volume.

This timing prompts Bainbridge to speculate that extended adolescence could have been the spur that drove our brain to make its final leap forward. Delayed maturation extends the time available for learning skills, placing a premium on increased brain power. "Teenagers as the cause of increased mental activity may seem like a contradiction in terms, but maybe that is indeed what the ancient teeth are telling us," says Bainbridge.

His story begins with puberty, and the brain centres and hormonal cascades that prime a teenager's body for a reproductive future. He stresses just how long the whole process takes, with puberty stretching over many years: "patterns of male body hair often continue to change throughout adulthood."

If physical transformations are subtle and slow, so too are the mental transformations that "draw the mind into new realms of analysis, abstraction and creativity". This constellation of carefully timed events is all for the good: "in the long run, they help us succeed as individuals." Quite how spots, hair and smelliness boost status and self-esteem may not be obvious, but Bainbridge takes a brave stab at evolutionary justifications for virtually everything that befalls the adolescent.

If you've wondered why the average 15-year-old rolls out of bed on Saturday afternoon in a grumpy mood, Bainbridge offers no-nonsense biological explanations. The "terrible triangle of laziness, risk-taking and anger" reflect consistent re-organisations in the adolescent brain. For instance, shifts in the release of the jet-lag hormone melatonin could account for many cases of adolescent morning sleepiness.

At times, his reductionist explanations seem a mite unconvincing, and he repeatedly stereotypes boys and girls, ignoring the vast differences within each sex. But he powerfully conveys the pressures on today's teenagers - physiologically primed for sex, yet discouraged from reproducing.

As conflicts arise, beleaguered parents may find solace in Bainbridge's central theme: supporting children and teenagers is what adults are for, because that is why humans have evolved to live as long as we do. Teenagers: a natural history offers a fresh and encouraging perspective on a difficult and embarrassing, yet vitally important, stage in the human life cycle.