The phrase “Life as we know it” is one of those cliches that nobody notices. It pops up in science fiction, in interviews with astrobiologists, and in serious documentaries about space. So what is life, as we know it?
We describe creatures on Earth as “Carbon based life-forms” (another sci-fi cliche), but in fact all known life is based on six elements: Hydrogen, Oxygen, Carbon, Nitrogen, Phosphorous and Sulphur.
Or at least, that’s what we thought until a team of researchers funded by NASA’s Astrobiology Institute discovered a microbe in Mono Lake, California, which breaks that rule. Strain GFAJ-1 of Gammaproteobacteria has been observed succesfully substituting arsenic for phosphorous. Phosphorus is part of the chemical backbone of DNA and RNA, the structures that carry genetic instructions for life, and is considered an essential element for all living cells. It is also a central component of the energy-carrying molecule in all cells (adenosine triphosphate) and also the phospholipids that form all cell membranes.
Arsenic is extremely poisonous to most organisms because it has very similar chemical properties to Phosphorous. When arsenic is introduced into a cell, it interferes with the chemical processes that would normally be using Phosphorous, usually with disastrous effects for the cell and whatever creature it came from. But it is this very similarity which has allowed the microbe to adapt its own biochemistry to use arsenic as a phosphorous substitute.
This is far more significant than it appears to those of us who never studied biochemistry. This discovery was not on another planet, or under the antarctic ice cap, or in a remote jungle – the microbe was found in a famous salt-water lake in California. If life can continue to surprise our top researchers like this, if it can re-write the rules right in their back yards, then imagine the possible forms it could assume elsewhere in the universe? The search for extra-terrestrial life has always assumed that life would be based on the same building blocks as on Earth, since that is all we know how to recognise. But if even earthly life doesn’t stick to the same template then perhaps we need to look wider, for more possible signs. Who knows what we might find!
To learn more about this ground-breaking discovery, visit the Astrobiology Institute‘s webpage or read their anouncement here: http://astrobiology.nasa.gov/articles/thriving-on-arsenic/