If you’re even remotely interested in science, you probably heard the figurative bombshell dropped by NASA yesterday: They have found life on Earth that can build its genetic structure from arsenic. If you want the whole story, please read one of the journalistic articles; they will answer all your questions. But many friends have been asking me what my take on arsenic-based life is, and why it’s important. As it turns out, Element 33 and I share some history.
Here are the basics. There are six essential elements to life: Carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. Phosphorus’ job in the life cycle is to make the backbone of the DNA double helix, and to act as a power source or battery as the “P” in “ATP,” which is the basic unit of currency for energy in all living things. It’s a pretty important job, you’d have to say. What astrobiologists working with a strain of Halomonadaceae protobacteria found in California’s Mono Lake discovered was that, if they starved the microbes of phosphate, while force-feeding them arsenic — which sits right below phosphorus on the periodic table, and shares many of its attributes — they could coerce the microbes into using arsenic in the place of phosphorus to build their DNA and energy base.
Halomonadaceae GFAJ-1, pictured above.
Disappointing, you say? Perhaps you were hoping for the discovery of poisonous Loch Ness Monsters oozing arsenic from their skin below Mono Lake? First, consider that phosphorus was once considered essential for all life. If you had a giant blender, and were to make five milkshakes out of a tree, a salamander, a mushroom, a bunch of E. coli, and a frozen mastodon penis, respectively, you’d find the same six elements in each shake. The fact that living things can replace one essential element with another means that they also probably do, which means that life is possible in far more places than we ever imagined. There could be life on a moon of Saturn that uses silicon instead of carbon, or selenium instead of sulfur. There could be a shadow ecosystem of microbes made of arsenic living unnoticed under our feet. And if so, and life evolved twice independently on Earth, it’s more than twice as likely that life has evolved on other planets, ending the supposed exceptionalism of our lonely space rock and suggesting that that we have interstellar neighbors.
This story is not about Mono Lake or arsenic, says Felisa Wolfe-Simon, the lead researcher, in the NY Times article, but about “cracking open the door and finding that what we think are fixed constants of life are not.” But for a second, let’s consider that insidious doppelganger of phosphorus, and that toxic lake in the foothills of the Sierra Nevada which is so important to so many life forms.