Last week, NASA scientists gathered to reveal a discovery they promised would “impact the search for evidence of extraterrestrial life.” The public’s expectations couldn’t have been higher: widely read blogger Jason Kottke mused the space agency might even have found signs of life on another planet. But when NASA finally lifted the curtain on its finding—which turned out to be not a real-life alien, but a lowly microbe here on Earth—there was a collective yawn.
Even so, it’s not every day NASA says our biology textbooks should be rewritten. Life is thought to require six basic building blocks to exist: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. This bacteria, dug up from the salty, arsenic-rich mud of Mono Lake in eastern California, defies that expectation. As astrobiologist Felisa Wolfe-Simon and her team found, the microbe, called GFAJ-1, seems to swap arsenic for some of its phosphorus, incorporating the toxin into its DNA, proteins and cell membranes. (University of British Columbia professor Rosie Redfield slammed the research, which was published in the peer-reviewed journal Science, calling it “flim-flam” on her blog.)
Arsenic and phosphorus are actually chemical cousins, with arsenic just below phosphorus on the periodic table. “Arsenic is toxic because it looks like phosphorus,” Wolfe-Simon says. “Cells can’t tell the difference,” so when it enters the body, it’s mistaken for a nutrient. In the lab, Wolfe-Simon fed the bacteria sugar and vitamins, but no phosphorus, and a very high diet of arsenic. “Nothing should have grown,” she says, but this bacteria did. It’s a lesson on how little we still know about life on our own planet, let alone elsewhere in the solar system.
Only one NASA mission—the two Viking spacecraft, which went to Mars in 1976—has explicitly looked for signs of life on another planet. “They were focused on the idea of life like you and me,” says Ariel Anbar, director of Arizona State University (ASU)’s astrobiology program and a co-author on Wolfe-Simon’s paper. (Results from Viking, which didn’t detect even the simplest organic molecule, are still debated.) “If you were to design a Viking mission now, you’d do it differently,” since we know a lot more about the different forms life can take. McGill University’s Lyle Whyte has found methane-eating bacteria living in a very salty, sub-zero spring in Canada’s Far North. Ronald Oremland of the U.S. Geological Survey, who also co-authored this latest paper, has found bacteria that use arsenic instead of water in photosynthesis. And now we know of bacteria that appear to actually build themselves out of the toxin.
Next year, NASA will launch its Mars Science Laboratory, a rover that will try to determine whether the planet could harbour microbial life, or ever did. NASA’s loose philosophy has been to follow the water (there’s water on Mars, but it’s locked up in ice), but maybe life elsewhere doesn’t have to be water-based. On Titan, a moon of Jupiter with lakes of liquid methane, “we might look for methane-based life,” says Christopher McKay, a planetary scientist at NASA Ames Research Center. Wolfe-Simon’s work “doesn’t say that’s possible,” he adds, “but it says maybe that’s not as crazy as we thought it was.”
ASU cosmologist Paul Davies, another co-author, thinks Earth might be the best place to look. “The most convincing way we can be sure there’s life elsewhere in the universe is to find more of it here,” he says. All known life on Earth is related—branches of one single tree—but if we could find proof that life sprang up twice on our own planet, there’s a good chance the universe is full of it, he says. He calls this hypothetical second tree of life a “shadow biosphere.”
While initially there was hope Wolfe-Simon’s discovery might qualify, “this organism is not it,” Davies says. “It’s a bacteria that sits squarely on the same tree of life as you or me.” (GFAJ-1 bacteria still seem to do better with phosphorus than arsenic.) There are endless places to look for a “second genesis,” he says, like volcanic vents in the ocean, where “some people think life began.”
Wolfe-Simon’s discovery might just be a tiny bacteria, but it’s also a reminder that life, as we’ve known it, is still full of surprises. “We’ve cracked open the door to what’s possible elsewhere in the universe,” she says. “What else might we find? What else might we want to look for?”