Who gets the thanks for the mammaries?

Part 1 of 2.

We are the only buxom mammal. Others don't have boobs, per se. Most have mammary glands that swell when they're lactating. What sits or hangs or juts out from the human female chest is mostly fat — not just ordinary fat but fat packed with evolutionary mystery and cultural baggage.

Why do men love certain types of breasts? Are women drawn to breasts as well? If not, why is so much cleavage displayed in women's magazines?

To answer those questions, we must go back in time — before Victoria's Secret, before even the breast-shaping influences of Hugh Hefner and Helen Gurley Brown.

Hundreds of thousands of years ago, perhaps in our Homo erectus ancestors, a coordinated burst of evolution transformed the female body and redirected the male sex drive toward that new body.

The details remain sketchy, but evolution can give us a plausible account, says Bobbi Lowe, a biologist at the University of Michigan. At some point, it all goes back to fertility. Women who could pack on fat reserves were more fertile, and men who chose them had more offspring. Men with a taste for fat reserves and their fat-loving genes proliferated. Faced with this, women who could create showy pockets of fat also left more offspring because they attracted more devotion from males. They put fat on the buttocks, hips and breasts.

The process behind all this is called sexual selection, a concept discovered by Darwin and detailed in his "Descent of Man." It explains the tail of the peacock and other sexual ornaments. If enticing enough, such ornaments can evolve even when they detract from a creature's ability to get food or escape predators.

But why do women pay attention to breasts? Because breasts have been subject to the tyranny of fashion for centuries. Your powers of attraction and class status depend on the lacing of your corset, the wiring in your bra or the skill of your surgeon.

Stanford women's studies professor Marilyn Yalom lays this all out in "The History of the Breast."

The earliest Stone Age sculptures depict women with obvious, matronly breasts. Antiquity worshipped breasts as symbols of motherhood. In the early Middle Ages they became eroticized, but men preferred them small. The ideal female form had rounded womanly hips and pubescent breasts.

Women in the upper classes often gave their babies over to "wet nurses," a practice driven in part by the desire to keep their breasts looking pert and fashionably petite, says Yalom.

Henry VIII, according to one account, rejected his fourth wife, Anne of Cleves, because he didn't like her looks, in particular, her breasts. They were large and a little pendulous and made it too hard for him to perform his kingly duty.

The 1920s saw the rise of the flat chest in the flapper girls — an androgynous look that Yalom says coincided with an unprecedented expansion of women's rights. In the 1950s, bras turned breasts into torpedoes. In the 1970s, breasts started to go free from bras as well as strict aesthetic standards. But then came the technology of breast implants and the new ideal — skinny with melon-sized breasts and exposed cleavage.

One of the big issues that might face breasts in the 21st century is the prospect of declassifying them as private parts. It might create a healthier attitude, says Yalom, especially in the United States, where we manage to display grotesque fake cleavage on newsstands while our own John Ashcroft covered two nude female statues that decorate the Justice Department.

There's still hope for us. Breasts could go the way of feminine legs. Leg nudity is now acceptable. Will the liberated breast of the 21st century demand and acquire the right of public nudity? There's only one way to find out. Do we dare?

Faye Flam's Carnal Knowledge column appears Wednesdays in The Seattle Times.