By Jeremy Manier
If life has emerged more than once in the universe, where would we look for it?
That was the subject of “Weird Life,” a symposium held this morning at the American Association for the Advancement of Science meeting in Chicago.
A natural step is to use instruments like the Kepler telescope to look for Earth-like planets that could support life. But even if likely suspects emerge, it’s much more difficult to find clear signs that life has actually started and evolved on such distant planets.
An alternative tack is to search for signs that life arose more than once right here on Earth. If it did, that might be a sign that life is a cosmic imperative and that it probably is popping up on all sorts of worlds. Paul Davies, a professor of theoretical physics at Arizona State University, believes we may still be able to find traces of such “shadow life” within fossils or in the microorganisms around us. “If life did happen many times, there could be something like a shadow biosphere that either was or is all around us,” Davies said. “How do we know there isn’t an alternative form of life on Earth?”
This would be great fodder for an “X-Files” episode, but how could you prove it? One route might be to search for microbes with biochemistry based on a different “chirality” – or orientation of chemical components – than our own. For example, all DNA that we know of is “right-handed,” but some biologists think there’s no good reason why left-handed versions couldn’t have emerged. We see only right-handed DNA because all of the known life on Earth is part of a single ancestral tree. If we could find microbes that rely on a different system of chirality, they might represent a separate tree of life.
But that’s not a sure bet. Maybe life emerged twice, but the other lineage got the same coin-flip that our remote ancestors did, and wound up with identical chirality to ours. It would be a nasty little trick on nature’s part.
One way of finding out would be to craft experiments that disable life that relies on our chirality, and see if anything is left alive. Another is to develop a nutrient soup consisting entirely of compounds with the wrong chirality, and see if anything grows. But again, that wouldn’t prove that the microbes had a different origin – maybe they just evolved ways of grooving on exotic food.
Still, the possibility of finding homegrown aliens thriving under our noses – or even inside them – is so tantalizing that it seems worth figuring out ways of detecting them. “We’re open to suggestions,” Davies said.