If sex is truly universal, then Jupiter just might be in the moon for love

If there is life on other planets, would there be sex?

Do we happen to live on a freaky sex-soaked planet, or is rampant sex normal or even inevitable for any life-supporting world?

It may seem like a silly question, but it gets at the foundation of our understanding of life. And the prospect of extraterrestrial life in our own solar system is looking more plausible as NASA spacecraft deliver starkly detailed images of the moons of Saturn and Jupiter, hinting at tidally heated oceans of water.

Earlier this month, new images of tiny Enceladus appeared to show a geyser spewing through the icy surface, adding this moon of Saturn to the short list of watery places where science can't rule out life.

But life doesn't necessarily imply sex. The first living things on Earth almost certainly didn't have sex as we know it. They just divided in two. A small number of plants, insects, fish and even some lizards reproduce by cloning themselves, showing sex isn't absolutely necessary. And yet, "There is a lot of sex going on out there," says Ricardo Azevedo, a biologist from the University of Houston. The prevalence of sex poses a long-standing puzzle — but two papers published just last month go far toward solving it.

You can multiply a lot faster by cloning, he says. If you had two groups, one of regular people who averaged two children apiece and one of asexual ones who could each create two clones, the clone population would double every generation, swamping their sexual counterparts.

Not only that, says Azevedo, "There are many things sexuals have as baggage. ... You have to waste energy and resources in looking for mates," he says. For some species, that also means competing for mates, keeping mates interested, fighting with mates, divorcing mates and, if you can stand it, repeating the cycle with new mates.

In 1932, Nobel-winning biologist Herman Muller suggested a compelling reason for sex to exist. The downside of asexual cloning, he reasoned, is that you'll inevitably make errors in copying your DNA. Whenever such an error, or mutation, appears, you saddle all your children and grandchildren with it.

"There's no way back," says Indiana University biologist Michael Lynch. "You'd take the population out by a mutational meltdown."

If you have sex to reproduce, you pass down only half your genes, mixing them with those of your mate. So if both of you carry two harmful mutations apiece, some of your kids will inherit all four, but some may get none.

Natural selection weeds out the most mistake-riddled, while those with the least errors go on to survive. So sex gives living things a chance to clean up genetic copying errors.

Lynch, working with Susan Paland, tested the idea by studying the genes of two closely related species of water flea, one sexual and one asexual. They found that indeed the asexual water fleas were accumulating mutations much faster. The results were published in Science magazine.

Azevedo used a computer model to conclude that in a typical living organism, individual mutations get more harmful as they accumulate. In biology this is called negative epistasis, and it makes sex even more critical for long-term survival. His results, published in last month's Nature, help explain why most asexual plants and animals appeared less than a million years ago. They can't last much longer than that.

In light of this new knowledge, it's not totally insane to talk about sex on exotic and distant worlds. It's not easy to define life, but it would probably need some sort of chemical code, like DNA, which would inevitably have mutations, making sex advantageous.

Nobody knows the odds of life originating beyond Earth, but Enceladus and some of Jupiter's moons seem to have the minimal requirements — energy, liquid water and certain chemical building blocks. The energy doesn't have to be sunlight — deep ocean vents support whole communities of microbes, worms, crustaceans and other creatures fueled by chemical power.

Such an environment isn't out of the question on Enceladus, and in some ways this distant moon is more accessible than other potentially habitable bodies. Scientists say it's free of the intense radiation fields that surround Jupiter, and the liquid water appears relatively close to the surface. So knowing whether some strange and exotic sex goes on out there may be within the grasp of some future generation of Earthlings.

Faye Flam writes for

the Philadelphia Inquirer.