Humans love to ponder whether alien life is out there, and what it might look like. So here’s a burning question: Would extraterrestrials have sex?
The question isn’t entirely prurient. The evolution of sex is a tricky subject. Sexual reproduction is costly. It requires finding a mate, convincing that mate to mingle DNA with you, and opening yourself up to the possibility of sexually transmitted disease or predation while you’re busy wooing.
All that considered, and it might not even result in viable offspring. After all, mixing and matching a genome is a crapshoot, said Sally Otto, director of the Biodiversity Research Centre at the University of British Columbia.
Potential parents “know their genome works in the current environment,” Otto told Live Science. “They know they survived to reproduce. And here they are, shuffling their genomes together with another individual. … You have no idea if that combination is going to survive and be fit.”
And yet, sexual reproduction is very common on Earth. And given the conditions in which sex evolved, it’s quite possible that aliens might get busy, too.
Swapping genes
Not all life on Earth requires sex for reproduction. Amoebas, yeast and millimeter-long freshwater hydra all manage to create offspring solo, as do many invertebrates. So do some surprisingly complex animals: Virgin births have been reported in Komodo dragons, pit vipers and sharks.
There are species, like the tiny crustacean Daphnia middendorffiana, that can only reproduce asexually. But sex appears to go way back. There are few very old lineages that are entirely asexual, Otto said. [Photos: Bizarre Sex Lives of Hermaphrodite Sea Slugs]
Amoebas, for example, date back at least a billion years, long before multicellular life evolved. For a long time, scientists thought amoebas were purely asexual. In 2011, however, researchers from the University of Massachusetts announced that they had discovered amoeba sex.
Sexual evolution
It’s possible, Otto said, that deliberate sexual reproduction arose with the evolution of eukaryotic cells because all those internal membranes prevented the frequent accidental uptake of foreign DNA.
So, one question that might determine whether aliens have sex, Otto said, is what their cells look like.
“Do they evolve nuclei or other ways of protecting their DNA inside a series of membranes?” she said. If extraterrestrial life is equipped with nuclei, they might benefit from sex.
Another thing that Planet Xenon might need to prompt the evolution of sex is change.
Sex is very beneficial to organisms because the environment is rarely static, Otto explained. Offspring may have to deal with challenges that are slightly different from those of their parents’ generation. As long as change is a constant, genetic variation is helpful.
If an alien planet had, for some reason, constant weather, temperature and other environmental factors, “sex would have mainly costs, but no benefits,” Otto said.
Best of both worlds
Assuming alien planets aren’t entirely static, extraterrestrials might try to get the best of both worlds. Some aphids (small insects that suck plant juice) clone themselves asexually when food is abundant. In fact, Otto said, these cloning aphids can have not only their babies inside them but also their babies’ babies, “like a set of Russian nesting dolls.”
“That really speeds up reproduction when resources are plenty,” she said.
At the end of the growing season, though, the aphids switch to sexual reproduction. This switch to sex during times of stress is a common pattern. Some water flea species spring for sex when food supplies drop or when the environment becomes hostile, according to a 1981 study in the journal The American Naturalist.
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