Aliens like us? I don’t think so.

Whether alien culture resembles our own depends on one big question: do they have sex?

A few weeks ago, NASA announced that it had discovered 700 new planets in our galaxy, 140 of them apparently “Earth-like.” People immediately went nuts speculating about life on other planets, and many scientists called for a renewed push in the largely moribund search for extraterrestrial intelligence. But before we get too excited about finding E.T., we might ask ourselves a hard question: what’s in it for us?

Stephen Hawking actually brought this up a few months ago, when he said that while he believes aliens are out there, it is probably too dangerous for us to try to interact with them. “I imagine they might exist in massive ships?.?.?.?having used up all the resources from their home planet,” he said. “Such advanced aliens would perhaps become nomads, looking to conquer and colonize whatever planets they can reach.”

Hawking’s worry was that the aliens might be too much like us, backed by the usual clichéd portrayal of humanity as a rapacious and violent species that is nothing more than a cancer on the planet. The far more frightening possibility, though, is that the alien culture would be completely, well, alien.

There are a handful of reasons for wanting to make contact with an extraterrestrial intelligence. We would learn plenty about biology and the nature of life by seeing how it evolved on other planets, and aliens would certainly have new knowledge and technologies to share.

They also might have interesting culture, literature or music. And (if the recurring theme of virtually all sci-fi ever produced is any indication of what we’re really after), there might be an abundance of scantily clad alien females, offering the possibility of the ultimate exotic experience, space sex. But all of these make a fundamental assumption about alien species, which is that they will be pretty much like us. They will have culture and technology, and be made up of co-operative individuals possessed of curiosity, morality and a sense of justice. It also assumes they would be interested in having sex—which, as it happens, is the big question.

Why we have sex is one of the toughest problems in biology. Even on our own planet, most species don’t do it, though the complex, interesting and intelligent ones do reproduce sexually. But why? And why are there always two sexes, and not three or four or nine? Biologists don’t really know. The popular theory is that sex is a mechanism for error correction, preventing genetic errors from piling through the generations. Another theory is that sex is the turbocharger of evolution: the shuffling of genes through sex allows evolution to try out a whole bunch of phenotypic variations, allowing a species to rapidly diversify.

One thing we do know is that sex is the foundation of everything recognizably human. If we were clones, like poplar trees or certain species of ants, there would be no need for morality. Clones are genetically identical, which means from the gene’s-eye view of things, the survival of any given member is as valuable as any other.

But humans are genetic individuals. We are almost all completely unique, sharing at most one half of our DNA with our parents or siblings. And so, unlike clones, which have a deep evolutionary inclination to collective behaviour, humans have no biological reason to co-operate. Our factory setting is self-interest. We’ve had to evolve mechanisms to promote co-operation: enter religion and law, art forms like singing and dancing, and of course morality.

Sex, in a way, underlies all of this. Which means that, if we have no reason to believe aliens have sex like we do, we have no reason to believe that they’ll be anything like us at all. One possibility is that there will be more than two genders, though many biologists think that it is just too complicated a manoeuvre for evolution to stumble upon. The more worrisome possibility is that we would encounter a species of space-faring clones. Star Trek may have had it right—our biggest enemy is likely to be the Borg. Its members could not be reasoned with, and they would have no interest in sharing or ­co-operating, because the collective would have no “members” in the way we understand them. It would be like a colony of fire ants wandering through the galaxy, destroying everything in its path.

But does any of this really matter? After all, when it comes to intelligent civilizations, we have a sample size of exactly one, and we have no reason to think that’s going to change any time soon.

But Stephen Hawking raised the Malthusian stakes last week, when he posted a comment on the website Blaming our genetically based “selfish and aggressive instincts,” he said we’re probably going to wreck this planet, and need to find another one within a couple of hundred years.

Hawking’s come full circle then. First he said our biggest fear is a rapacious alien species, and now he seems to be suggesting that that species is us. This is nonsense. Apparently, the great physicist of our times can’t help making the same silly assumption filmmakers always make, that aliens will, like us, have thinking, individual (and in the case of Hollywood, rather cute, humanoid) selves. The real concern is that they would be nothing like us at all.