No, they didn’t find it in space. They discovered it somewhere even stranger and more exotic, the place from which all bizarre life forms originate: California.
If, per NASA’s breathless announcement a few days ago, you were expecting something even freaky deakier, try not to be too disappointed. This is still darned deaky. In a nutshell: Every last organism on Earth is supposed to contain six essential elements — carbon, nitrogen, oxygen, hydrogen, sulphur, and phosphorus, the last of which is essential for forming the framework of the DNA double-helix. From bacteria to blue whales, if something’s alive then it’s got the big six. No exceptions to that rule.
On the tree of life, according to the results of 16S rRNA sequencing, the rod-shaped GFAJ-1 nestles in among other salt-loving bacteria in the genus Halomonas. Many of these bacteria are known to be able to tolerate high levels of arsenic.
But Wolfe-Simon found that GFAJ-1 can go a step further. When starved of phosphorus, it can instead incorporate arsenic into its DNA, and continue growing as though nothing remarkable had happened…
The DNA molecule is shaped like a spiral ladder. The “rungs” of the ladder are comprised of pairs of nucleotides, which spell out the genetic instructions of life. The sides of the DNA ladder, referred to as its backbone, are long chains of alternating sugar and phosphate molecules. A phosphate molecule contains five atoms: one of phosphorus, four of oxygen. No phosphorus, no phosphate. No phosphate, no backbone. No backbone, no DNA. No DNA, no life…
When Wolfe-Simon starved GFAJ-1 cells of phosphorus, while flooding them with arsenic, far more than enough arsenic to kill most other organisms, it grew and divided as though it had been offered its favorite snack.
The California lake where they found the GFAJ-1 has 700 times the level of arsenic that’s considered safe by the EPA, which raises a few possibilities. One: The bacteria is a true alien life form that’s capable of swapping out “essential” elements for whatever’s abundant in its environment. In that case, where’d it come from and how’d it get there? Two: The bacteria’s an Earth-based life form that somehow figured out one of the niftiest tricks in evolutionary history, altering the core structure of its own DNA to compensate for the loss of a key component. As an astrobiologist told the Times, “It’s like if you or I morphed into fully functioning cyborgs after being thrown into a room of electronic scrap with nothing to eat.” If it’s evolution at work, is it a new branch on the tree or … a whole new tree? And how many more branches are there on that tree? Read WaPo’s very user-friendly article on all this, suggesting the possibility of an entire “shadow biosphere” on Earth that doesn’t follow the usual biological rules.
Third possibility: Maybe the whole thing’s being oversold.
Steven Benner, a distinguished fellow at the Foundation for Applied Molecular Evolution in Gainesville, Fla., remains skeptical.
If you “replace all the phosphates by arsenates,” in the backbone of DNA, he said, “every bond in that chain is going to hydrolyze [react with water and fall apart] with a half-life on the order of minutes, say 10 minutes.”
So “if there is an arsenate equivalent of DNA in that bug, it has to be seriously stabilized” by some as-yet-unknown mechanism, Benner said.
Translation: Arsenic, while chemically similar to phosphorus, just ain’t rugged enough to keep the DNA matrix in place. It’s like trying to build a skyscraper out of wet sand instead of steel. Benner suspects that the bacteria isn’t using arsenic in its DNA at all but rather in other parts of the cell and that somehow there’s enough phosphorus in the lab cultures to keep it going. In fact, one of the scientists who spoke to the Times admits that there’s no hard proof yet that there’s arsenic in the DNA; the evidence is circumstantial, based on the fact that the bacteria were collected more than a year ago and starved of phosphorus ever since. The only amount available to them was the trace bits floating around in the original sample. Which means … maybe the real talent these bacteria have is subsisting, camel-like, for long periods on really small amounts of essential elements instead of swapping out those elements entirely.
Here’s Bill Nye, the science guy, to explain it all to you. My year’s quota of scientific wonkery posts is thus filled.