Seems we're hearing a lot about aliens these days. They're humans supposedly swarming over the border, snails invading our lakes, plants taking over the dunes. Even cancer cells, those utterly homegrown products of their hosts, are deemed "alien." The word itself derives from its Proto-Indo-European root *al- (1) = "beyond" via Latin alienus "of or belonging to another, not one's own, foreign, strange."
Ask a biologist what alien means, though, and you'll be directed to a "tree of life" like the one shown here, on which every living entity on Earth, past or present, can be found, all sharing the same DNA, RNA and 20 amino acids. (Viruses, which cannot replicate or evolve, are usually disqualified from the definition of "life.") Take a look at the accompanying diagram — accepted by most biologists — to see how we're all related. You and I and jellyfish and algae are all one big happy family, cousins with a common ancestor.
But that's just here on Earth. What about other planets? What would we expect to find there? Until recently, scientists thought that finding life on Mars, living or dead, would be good evidence that life is common in the universe, invoking the late Isaac Asimov's "zero-one-infinity" rule. He was talking about gods in his 1972 science fiction novel The Gods Themselves — there might be no gods, there might be one God or there might be an infinity of gods (every tree and mountain was a deity, according to ancient Greeks). But the idea of just two gods or half a god was absurd. By this reasoning, finding life on Mars that doesn't fit on our tree of life would suggest, not just two life forms, but of an infinity of them.
To be clear, no one expects that we'll find anything living on the surface of Mars, thanks to the sun's molecule-bashing ultraviolet rays. But maybe some 3.5 billion years ago, while Mars still had a thick protective atmosphere, a magnetic field and liquid water, bacterial life thrived there, as it did then on Earth. Maybe — a big maybe — bacteria are still around just under the surface, eking out an existence on the warm flanks of Olympus Mons, perhaps. And even if microbial life died out a million years ago, we might still be able to tease out its biochemistry from fossils.
Unfortunately for those of us hoping that we find evidence for a "second Genesis" on Mars, it's starting to look more and more probable that any life there, past or present, would fit on the Earthly tree of life. That's because we're finding how comparatively easily life can migrate across space from one body to another inside meteorites. (Remember the excitement in 1996 when scientists claimed to have found evidence — since discredited — for microscopic fossils of bacteria in a 4-billion-year-old meteorite that came from Mars?) Chances are, if there is or was any life on Mars, it originated on Earth. Or, more likely, the other way around because meteorites normally spiral in toward the sun, so it would be easier to get from Mars to Earth than from Earth to Mars.
To sum up: Really pessimistically, we won't find any evidence for life elsewhere in our solar system. Somewhat pessimistically (but still awesome), we'll find life but it will be "our" life: same DNA, etc. The optimistic view, though, is that we'll find evidence of some completely unrelated form of biochemistry on Mars (or Europa or Titan or Enceladus). Optimistic because the existence of two examples of life right here in our solar system almost certainly means it can arise easily. By the zero-one-infinity rule, life will be all over the universe.
Barry Evans (firstname.lastname@example.org) can't believe that here in the colonies Brits are classified as "aliens."