They probably evolved to keep everyone happy.
Many people figure that this uber-feminine characteristic evolved through sexual selection—or because well-endowed women mated more often and with more-virile partners, increasing the likelihood that the trait would be passed along. But Williams points out that this origin theory brings up more questions (like, why are breasts at their fullest and firmest when women are pregnant and lactating, when they're effectively off the market?). She interviewed proponents of a newer hypothesis: that our breasts evolved through natural selection—or because they did more for women than boost their attractiveness. Here's how Williams breaks it down: "We needed to be fatter at puberty and beyond to produce human infants; our fat made estrogen, and estrogen made our breasts grow because the tissues there are so attuned to it." According to this reasoning, breasts are merely "byproducts of fat deposition," but that fat is key to the survival of a species like ours that lacks hair for warmth (and that needs extra fat to grow a big-brained baby). It's tricky to prove which theory is right, says Williams, because the soft tissue of breasts leave no fossil record, but we're partial to the more egalitarian one.

They're arriving earlier than ever before.
Breasts are appearing, on average, a full year sooner than they did in generations past, says Williams. You've probably heard this is due to the rise in obesity, but Williams says that's only part of the problem. "Thin girls are also developing at a younger age," she says. One theory points to the chemicals that mimic estrogen in food and personal-care products. The Breast Cancer Fund suggests avoiding BPA (the plastic additive in the lining of cans) and using paraben-free beauty products whenever possible.

They're brimming with goodness...
Human breast milk starts out as nature's perfect food, says Williams, chockfull of not only the vitamins, minerals, proteins and fat a baby needs to grow but also immunity-boosting antibodies, probiotics and other substances that can protect against salmonella and E. coli as well as diabetes and cancer. It also contains endocannabinoids similar to those found in marijuana, which may help infants chill out and avoid overeating. well as not-so-goodness.
In a cruel twist of fate, the helpfully fatty tissue in the breasts also causes them to absorb "pollutants like a pair of soft sponges," says Williams. Modern breasts are storing potentially toxic chemicals such as flame retardants, pesticides, PCBs, mercury, lead, rocket fuel, gasoline byproducts, fungicides and more. We can hold on to some of them for years—even decades. The high-fat, high-protein content of the milk also attracts heavy metals and other contaminants, and nursing mothers can pass these along to their babies (this doesn't cancel out breast milk's other amazing properties, though). Williams cites a 2011 report by the Institute of Medicine of the National Academies recommending that all women reduce their exposure to gasoline fumes, vehicle exhaust and cigarette smoke.

They're particularly vulnerable to cancer.
More than 1.3 million women were diagnosed with breast cancer in 2008, according to Williams, and that number is expected to increase by 26 percent by 2020. She says that there are three reasons cancer seems to thrive in our breasts: These dynamic organs keep growing new cells throughout our lives; the fatty tissue is like a haven for industrial chemicals; and they're filled with hormone receptors and hormones that can feed cancer cells ("The tumors grow in the presence of estrogen and progesterone," says Williams). She adds that this news underscores the importance of following the Institute of Medicine's advice for minimizing environmental risk factors: exercise, limit smoking and alcohol, and avoid unnecessary radiation.

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