09/16/2010 12:35 pm ET Updated Nov 17, 2011

Calories and Consequences

I recently read a news story about girls going through puberty earlier than ever before. While a generation ago the average age was around 14-15, it has now dropped to around 12, and 10% of girls show signs of puberty as early as age seven. This is a concern because the onset of puberty -- the age at which a child is hormonally thrust into the adult world -- has all sorts of consequences, from medical to social. Breast and uterine cancer later in life have been tied to early puberty, for instance, and psychologists warn that looking more mature than you really are can lead to behaviors that are unsafe for a pre-teen. The reason for this mass change in physiology is unknown, though the study's authors cite possible exposure to chemicals such as BPA, sometimes found as a manufacturing by-product in plastic food containers, and increasing rates of childhood obesity.

While the effects of unwanted chemical compounds are easy to decry as 'unnatural,' with calls for increased industrial oversight, a rise in childhood obesity is perhaps less likely to galvanize policymakers. After all, if it's just 'baby fat,' surely the child will simply shed it as their body adjusts to the rigors of adolescent growth? As we learn more about fat, though, we're discovering that it plays a variety of roles in the body's complex web if chemical interactions -- it's not simply a passive storehouse for extra calories, but rather an active organ. Part of its role is to concentrate and synthesize female hormones, which -- in young girls -- can lead to early puberty, with all of the fallout that entails.

This story is a good example of 'unintended consequences' -- where something that we think will have an isolated effect actually produces a complex chain of other effects, often negative. Historically, excess body fat has been desirable -- a sign of higher social status, broadcasting to the world that you didn't have to perform manual labor for a living and could afford to eat well. The glut of calories in today's Western diet, though, has changed this. Cherubic pre-teens, once held up as the picture of health, are becoming the poster children for the obesity epidemic -- and a sign of things to come later in life. More than two-thirds of US states now have adult obesity rates above 25%, and in eight of them the rate tops 30% -- a significant increase since the early 1990s, when none were above 20%. Obesity adds more than $1,400 to the average American's healthcare costs every year, and as the epidemic grows, our healthcare dollars look set to be stretched even further.

The cause of this country-wide bulge, as we've all heard by now, is a double whammy of overeating -- particularly sugary, fatty foods -- and inactivity. Super-sizing while sitting still creates a perfect recipe for the deposition of excess calories on our frames. And unfortunately, we seem to be hard-wired to like these sorts of foods. Long before the era of giant milkshakes, sweet meant safe -- think of ripe fruit -- and fatty foods contained lots of hard-won calories in an era when you had to hunt and gather your dinner. While our distant ancestors would have depended on such cues to guide their dining choices, today they have become false friends preying on our evolutionary Achilles' heel.

The journey from the days of our hunter-gatherer ancestors to the modern obesity epidemic -- and precocious puberty -- is a complex one. The take-home message, though, is that along the way we have developed new technologies or behaviors in order to solve immediate problems, without any thought for the long-term fallout from our actions. The cultivation of crops during the Neolithic period thousands of years ago, for instance, allowed hungry people to thrive in challenging climatic conditions, but the greatly increased population densities later encouraged the spread of infectious diseases. Who would have predicted that back when the first seeds were being planted?

Though our choices usually made sense at the time, they often set in motion longer-term trends that now pop up in the craziest places -- including in the bodies of our daughters today. As we face down the many new challenges confronting us this century, from the obesity epidemic to climate change to how best to utilize revolutionary tools like genetic engineering, it's worth taking lessons from the past as a guide, because today's easy solution could lead to tomorrow's big problem. The march of progress can't be stopped, but it should undertaken with an eye to what might be over the next hill.

Spencer Wells is the director of the Genographic Project and the author of
Pandora's Seed: The Unforeseen Cost of Civilization, published by Random House.