Awhile back, an article popped up on the Comcast home page suggesting 10 things every single girl needs, including tottery high heels, CDs you hate but will make guys think you're cool and a sexy picture of yourself to hang on your fridge to impress dates.

Which made me wonder: How did this sex thing get started?

Humans inherited sex from some ancestor deep in the history of life but not from life's very beginning. For an idyllic billion-some-odd years, early life, which was single-celled, didn't need sex. You just copied your genes and divided in two. Why didn't all life stick with something so simple and efficient?

"We don't have a definite answer," says Matthew Meselson, a molecular and cellular biologist at Harvard. Scientists have about 20 competing theories but don't know which one's right.

Sex is rife with mystery. In "Carnal Knowledge," I'll be exploring the topic of sex every week. Webster's defines sex not just as intercourse, but also gender and reproduction. "Carnal Knowledge" will cover many facets of sex — sex in different life forms, how sexual behavior might have evolved, how men and women differ and even how drugs and other technologies are changing the nature of sex for us. Luckily for me, scientists are learning new things about sex all the time.

But back to the very root of the issue — how sex got started.

The bacteria that long dominated the planet didn't use sex to reproduce — there were no boy and girl bacteria — but their modern counterparts do on occasion sprout little hoses called pilli and shoot a few genes into their neighbors. It's a more direct and intimate contact than our sex — imagine you slept with someone and woke up with his or her gene for freckles, brown eyes and asthma.

In animals, sex requires you to carry two copies of each gene — one from each parent. Humans carry that double set on 46 separate structures, the chromosomes. Then you have to make sex cells — sperm and egg — with just one copy of each gene; in our case, on 23 chromosomes. When two sex cells meet, you get back a complete double set.

Most scientists agree that the birth of sex probably was some kind of accident. A few ancient microbes bumped into one another, a few of those merged, some of the genetic machinery got rewired and 2 billion years later we have match.com and Dating on Demand.

And yet, in many living things, sex isn't necessary. Birds may do it, but bees can have some baby bees without doing it, and dandelions don't do it at all. Some all-girl species of insects, fish and reptiles can reproduce by cloning. Some of these animals, such as whiptail lizards, probably started out sexual, but at some point the females learned to clone themselves and then the males went extinct, says biologist Sarah Otto of the University of British Columbia.

"There are lots of advantages to being asexual," says Otto. You can pass on all your genes to your babies, rather than throwing away half and replacing them with someone else's genes. You don't have to waste energy finding or courting a mate. There's no risk of sexually transmitted disease.

Given its drawbacks, sex must incur some advantage to have become so widespread. One school of thought holds that asexual creatures suffer from copying errors with each generation and can degrade like photocopies of copies of copies. You lose the most advantageous genetic combinations and they're gone forever, says Meselson. "Sex lets you trade genes back and forth, like shuffling cards and handing each player a new deck."

Other scientists have proposed that creatures need to constantly change to keep a step ahead of parasites, which are forever evolving better ways to infest us. And the way to keep ahead is to have children different from you.

Whatever its advantage, sex now dominates. Most animals, many plants, and even fungi practice their own versions of sex. And yet, sex carries the ultimate price.

Barring any mishaps, asexual bacteria don't seem to die, but that potential immortality was lost in the first sexual organisms.

Death, apparently, is the cosmic cost of sex. Perhaps that's why the French refer to orgasm as "le petit mort" — the little death.