Sex has long puzzled evolutionary biologists. It is literally happening everywhere.
As eloquently put by Discover magazine, "The world is rife with sex. Animals do it. Plants do it. Even mushrooms do it".
But why bother, researchers have long asked, perplexed about why asexual reproduction by species that can self-fertilize — avoiding complications of wooing a mate, fighting off competitors and risking exposure to STDs — has not won the evolutionary race.
The answer, according to a study on the sex life of, yes, worms, sheds new light on just why all Earth’s creatures, great and small — humans included — started having sex with each other.
Bizarre but true, as LiveScience explains, kudos must go to ever-evolving pathogens and parasites of this world, for without them, sex as we know it might never have existed.
Put simply, new research by a team of scientists at Indiana University, published in Science on Friday, points to death by parasites as the evolutionary driver behind sexual reproduction.
Indeed, as the BBC and Huffington Post report, sex emerged out of a "genetic arms race" between parasite and host, often referred to as an example of the Red Queen hypothesis which stems from a line in Lewis Carroll's Through the Looking Glass: "It takes all the running you can do, to keep in the same place."
A fitting analogy, notes the BBC, when considering how species must continually evolve to keep up with each other.
LiveScience also refers to the "arms race" analogy, stating: "without the presence of pathogens, the host organism sticks with the tried-and-true method of asexual breeding. When threatened by a continually evolving pathogen that gets better and better at killing its host, however, the host organism starts seeking out sex partners."
In the case of the Indiana University study, scientists faced no easy task, needing to bring together a host able to reproduce sexually and asexually with a parasite, then enabling them to evolve in response to each other, Discover reports.
But the scientific team did just that, setting their sights on the humble round worm Caenorhabditis elegans, and enginnering two types — namely those that could only produce by having sex and others that cloned themselves.
"They mixed together worms and germs in several different arrangements and let them duke it out for 30 worm generations,” writes Discover.
The BBC reports that the worms forced to reproduce asexually succumbed to bacterial infection and died.
"The bacteria got more and more infective, but the [clonal worms] did not get more and more resistant, and that is why they went extinct," study author Levi Morran, a postdoctoral researcher at Indiana University, told the BBC.
He also told LiveScience: "You actually need these pathogens or parasites to be co-evolving for sex to be maintained."
Moral of the story: Sex is good for you and, with the benefit of scientific hindsight, it now seems parasites have some redeeming qualities after all.