Arsenic and Old Lakes

NASA can’t plant expectations of a major astrobiology story without also releasing seeds of wild speculation. The actual discovery announced on December 2nd was tame compared to alien life rumors that had mutated across the internet. Researchers working in the Mono Lake area of California had found microbes that, unlike any other known organism, seem to use arsenic instead of phosphorus in DNA and other crucial molecules. This is important because astrobiologists often list the biogenic elements — carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus — as essential for life anywhere.

An epidemic of criticism soon erupted. Did the researchers actually demonstrate that arsenic is replacing phosphorus in DNA? Did these microbes really evolve to do this in the wild, or were they merely made to go along with “the plan” in an unnatural laboratory situation?

The press conference encouraged sensational news stories about “arsenic-based life,” a “fundamentally new form of life,” and even a “second genesis” or a “shadow biosphere” on Earth. But if these critters can actually use arsenic where the rest of us need phosphorus, does this really have huge implications for the search for alien life? 

Yes and no. It’s certainly an expansion of life’s known limits and chemical bag of tricks. But these microbes are still carbon based. They don’t represent “arsenic-based life,” and they don’t reveal a shadow biosphere or a distinct origin. No, these are clearly our relatives and, like the rest of us, they use giant carbon molecules to build cells and carry information. They reveal the edges of Earth’s biosphere to be a bit wider than we imagined. This shows we will have to look farther afield for any true shadow biosphere, for life that exists in conditions so different from ours that carbon or DNA cannot rule. 

The discovery may actually help astrobiologists resist an intellectual trap. At the press conference there was discussion of how to alter future missions to Mars or elsewhere to search for arsenic. But that’s the wrong lesson. We don’t need to start looking specifically for arsenic on other planets. Rather, the take-home message should be that we cannot assume we know what the biogenic elements are. Any of them. We still have no idea if life elsewhere will even be based on organic (carbon) chemistry. It’s too easy to assume that all life must be just like life here. I don’t believe we’re capable of thinking clearly about the prospects for life that is built on fundamentally different chemical or physical systems. We’re always looking for ourselves out there. We can’t think of anything better, but that may say more about us than life in the universe. 

We can follow our geocentric hunches about alien biochemistry, as long as we remain alert for life’s general signature in anomalous chemistry and disequilibrium. What we do know is that life takes advantage of excess energy and redistributes it in ways that affect the local equilibrium. Whatever it eats or breathes, it will produce waste and exhaust that will change its environment. While we search for what we think life is, we should keep in mind that all we really know is what life does.

This article originally appeared in print in the March 2011 issue of Sky & Telescope. Subscribe to Sky & Telescope.