Why are teenagers so rude? They mope around, make unreasonable demands, are self-centred, stay up too late, sleep in, shirk their responsibilities and spend too much time with equally antisocial friends.

We understood adolescence very poorly until recently. But new discoveries in psychology, anthropology, neuroscience and palaeontology have revolutionised our understanding in the past ten years. Before that we did not know when teenagers first evolved, we could not actually watch the living teen brain develop, and psychologists did not have much to say about teenagers. Remarkably, Freud almost ignored them.

But teenagers turn out to be a phenomenon unique to our species and I believe that important weaknesses in the teenager-as- antisocial-monster idea become clear when we take a long, hard, zoological look at them. I am a vet with a zoology degree, so to me this seems a sensible way to study them.

One reason teenagers come into conflict with parents is a new-found sense of autonomy. Recent psychological studies show that teenagers have to develop a strong sense of autonomy to function as normal adults. They are learning to solve emotional, social and analytical problems for themselves - after all, what sort of an adult will they become if they cannot?

To do this, they must break free from their beloved parents and we think that a degree of family conflict is beneficial in the long run. This can be painful for all concerned, but evolution does not act to make life easy, and teenagers are our best evidence of this.

Another reason why teenagers argue with their parents is the remarkable change going on in their brains. Magnetic resonance imaging allows us to look inside living, maturing teenage brains, and the results are striking. Although the brain is smaller at 20 years old than at 12, we know that humans acquire incredible abilities over that period. The key to these changes turns out to be an elaborate choreography of cerebral restructuring in which connections between brain cells are trimmed and honed, and neural pathways completed and switched on. In those few teenage years, the brain effectively switches from dial-up to broadband, giving us amazing human skills of abstraction, planning, self-analysis, empathy and creativity. The brain is smaller after adolescence, but has become fully human.

The downside is the temporary disruption it causes, which probably underlies many irksome aspects of teenage life: morning laziness, violent emotional reactions, risk-taking and excessive introspection. MRI scans have allowed us to watch in real time how brain activity in regions that process fear, emotion and self-control differs in teenagers. Sleep studies suggest that teenagers' internal clocks may run slower than those of adults, so 8am really does feel like 6am to them.

Studies of cognitive abilities show that teenagers' “working memory” is getting larger - they can hold more ideas in the “front of their mind” at any one time.

As a result, they are skilled at comparing dissimilar concepts, giving them tremendous powers of creativity and lateral thinking. However, it also makes them good at detecting inconsistency and hypocrisy - and this underlies another trigger of teenager-parent conflict. We give them plenty of advice, but we are giving it to people newly able to sniff out contradictions.

We tell teenagers not to take drugs when they know that some people enjoy them and do not become addicts. We tell them that illegal drugs are bad when they can see that alcohol and nicotine cause far more suffering. We tell them about the risks and worries of sex when they are preprogrammed, thankfully, to expect it to be enjoyable. And we expect them to become sexually happy and confident, but we want them to do this without actually having sex.

The final ingredient in this potent mix of conflict is that, as their cerebral cortex matures, teenagers acquire new language skills. For example, they learn to adjust their communication depending on whom they are talking to: they may grunt rudely at their parents, but can be articulate and charming when it suits them, and they cannot get enough of talking to their friends. Their language becomes a rich mix of idioms, slang, irony, in-jokes, insults, flirting and sarcasm, perfect for establishing their exclusive social groups. Is it any wonder that they sometimes seem rude, or even incomprehensible?

So the scene is set for teenager-parent conflict. In the back of their minds, I think that adults resent that these irritating adolescents will one day be running the show. Their free-running quirky creativity and untrammelled view of the world taunts us - it shows us that we are on the way out and they are most definitely on the way in.

Evolutionary theory suggests that the teenage years are the most important of our lives, because they were when we used to select mates and start having babies. In natural selection, reproduction is all important. And, most gallingly, we now think that teenagers were the innovation which allowed mankind to become the all-conquering uberprimates we are now.

Recent discoveries in palaeoanthropology allow us to measure how rapidly our hominid ancestors grew up. As humans mature, tooth enamel is laid down in a regular daily cycle that leaves an imprint in the layered structure of the tooth, so we can “age” fossil teeth by sawing through them and counting the rings, in a similar way to determining the age of trees. The results suggest that the first “numerical teenagers” - the first humans to take that extra decade to become mature - appeared 300,000-500,000 years ago. Tantalisingly, this was just before the human brain made the great leap to its full Homo sapiens size.

This evidence, along with changes we can measure in modern, moping adolescents, shows that there is something special about teenagers. This extra decade of growing up, and its tortuous mix of cerebral, psychological and physical change, is unique to our species, and is the reason we are unique.

Rude teenagers are more important than the rest of us. Just accept it.

David Bainbridge is the Clinical Veterinary Anatomist at Cambridge University. His book, Teenagers: A Natural History, is published by Portobello

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