Hannah only turned eight a couple of months ago – but she has already started puberty. Her mother, Liz, is shocked. "Over the last few weeks her nipples have suddenly become swollen. They're really painful for her – she finds it hard to put on a T-shirt or to hug people. We went to the doctor and he said it was hormonal changes linked to the onset of puberty. I was horrified. Hannah's a skinny little thing and I hadn't anticipated puberty for at least three more years."

Yet puberty is arriving earlier and earlier. Last week it was reported boys' choirs were finding it hard to maintain their standards because they now lose experienced choristers much earlier – trebles who used to sing until the age of 15 or 16 are now having to stop at 12 or 13, because their voices are breaking.

Peter Hindmarsh, professor of paediatric endocrinology at Great Ormond Street Hospital in London, says it's harder to collect data on male puberty, and that reports of puberty starting earlier in boys are anecdotal. But for girls, the evidence is conclusive. "Between 1958 and 2005, breast development – the sign that puberty is starting in girls – moved about nine months ahead of where it used to be," he says. "Interestingly, though, the age at menarche (first period) hasn't moved much – from about 13 to about 12.8."

The average age for the onset of puberty is 10.75 years in girls, and 11.5 years in boys (the first sign for boys is an increase in testicular size). But more children than ever are being referred to specialists after their parents take them to their GPs with worries about the psychological and physical fallout of being ahead of their peer group.

The usually cited reason for the change is the increase in childhood body weight. Better nutrition lowers the age of puberty, partly because the hormone changes that initiate it are triggered by a child's body reaching a certain size. Now that an increasing number of children are overweight or obese, it's likely that this too is playing its part. "It is very likely that there is a link with increasing body weight, but the data is only just beginning to show that," says Hindmarsh. "It makes sense, though. In a time of plenty, a species which always has a strong vested interest in self-preservation tends to reproduce quickly."

One recent piece of research in the Journal of Adolescent Health, for example, found that girls whose biological father didn't live with them were more likely to start breast development sooner. It is not clear why – but some scientists have speculated that it might be to do with unrelated males such as a mother's boyfriend giving off pheromones that affect girls' biological clocks; or it might be the absence of a father sending a "danger – shortage of men" signal to the pre-pubescent girl's brain, triggering the hormone changes that will enable her to reproduce.

As Hindmarsh explains, "The age of sexual maturation is highly adaptable. The drive to continue the species is so strong that we have evolved a system that is extremely sensitive to change. If there's even a hint of change, or circumstances that might threaten the drive to procreate, then the hormone system is designed to adapt to that to create the circumstances most likely to result in offspring."

Other studies have found link-ups that, like the absent father finding, seem totally baffling. For example, an Australian study found that the more older brothers a girl has, the later she hits puberty. Another piece of research found a link between exposure to artificial light and TV screens and early puberty, and another connected early puberty to weak maternal bonding.

So, where our primeval evolutionary instincts meet the complexities of 21st-century living, puberty seems sometimes to be triggered early. Yet while we don't fully understand the mechanisms that are put into play, we are aware of the immense difficulties early puberty can cause. "For the children themselves, the big problem is that they stand out from their peers," says Hindmarsh. Because they are taller than their classmates, he says, "they may have difficulties fitting into the classroom furniture, and primary schools aren't always prepared for girls who are having periods."

"It's confusing, too, because the surge of female sex hormones come at a time when their brain isn't ready for it. Children at that age are normally still in a stage of childhood where they are into fantasy play, and the brain changes that are triggered by puberty are all about reality, and understanding that actions have long-term consequences. It's all very confusing for young children, and, in my experience, they often react by becoming withdrawn. Parents worry that they'll become precocious, but I don't see as much of that."

Dr Anna Symonds, a Midlands-based clinical psychologist, says that while it's tough enough for young girls to handle the mood swings and PMT that come with puberty, handling these while still at primary school and before their friends go through something similar, is even more difficult. "The impact on body image, and self-esteem, can also be significant," she says. "Children in this situation tend to be a lot bigger than the other children around them, so everyone is well aware that something is different. Things like gym lessons, and having to wear a bra before your classmates, make children very self-conscious."

Tam Fry of the Child Growth Foundation says his organisation fields far more calls from parents who are worried about premature puberty than it did five or 10 years ago. "Parents are very concerned, and it's easy to see why," he says. "We've all heard about supermarkets being hauled over the coals for selling push-up bras for eight-year-olds . . . and the fact that more eight-year-olds need bras is only going to make the problem worse.

"But it's not just a psychological and social problem; there are growth problems, too. The irony is that a girl who goes through puberty early will be a lot taller than her peers initially – but as an adult, she'll actually be shorter. That's because a girl's first period is a signal to the brain to slow growth down – so after this stage she might grow another two or three centimetres, but that will be it.

"So a girl who starts her periods early, even though she may seem tall at the time, will invariably go on to be short as an adult."

For Hannah's mother Liz, this is adding insult to injury. "The whole thing seems so unfair," she says. "Childhood goes so quickly anyway, and now Hannah is on the path to being grown up. I worry that growing breasts, and maybe starting her periods, before her friends will make her self-conscious."

Unfortunately, too, the story of early puberty isn't over once that life stage is completed: there's an increasing amount of research linking early puberty to a higher chance of getting breast cancer in later life – and there could be a higher risk of ovarian cancer and heart disease, too. It's not, on any reading of the situation, good news for today's little girls.