How exciting is "arsenic-based" life?

The Guardian reported commentary on the discovery of bacteria that utilise arsenic thus:

A bacterium discovered in a Californian lake appears to be able to use arsenic in its molecular make-up instead of phosphorus – even incorporating the toxic chemical into its DNA. That's significant because it goes against the general rule that all terrestrial life depends on six elements: oxygen, hydrogen, carbon, nitrogen, sulphur and phosphorus.

To be fair, many scientists seem fairly conservative in their analysis. The most bullish commentary in the article comes from Professor Paul Davies, who says that the find is

surely the tip of a big iceberg, and so has the potential to open up a whole new domain of microbiology.

Personally, I think this is an overstatement. Arsenic-based life didn't evolve separately, it seems - although the bacterium (known affectionately as GFAJ-1) can use arsenic in place of phosphorus, it actually thrives better in a phosphorus environment. Further, it didn't evolve as a separate lifeform - it has a place on the tree of life that all earthbased organisms are part of, being one of the class of gamma proteobacteria. And whilst phosphorus may be one of the "big six" elements, I suspect that arsenic is much more like phosphorus than any elements are like carbon, nitrogen, oxygen or sulphur. It is always quite remarkable to discover life at "extremes", but it doesn't really change our fundamental analysis of the requirements for life.

Life has proved itself able to adapt to hostile environments, and to "evolve" to develop the ability to metabolise unusual chemicals, even including ones that don't exist in nature. In an interesting quirk of fate, this month sees the publication of the latest paper by Michael Behe, author of "Darwin's Black Box" and "The Edge of Evolution: The Search for the Limits of Darwinism." In this paper (published in a peer-reviewed journal, please note), Behe reviews research on evolution at a molecular level, and demonstrates that most observed evolutionary changes represent loss or modification, rather than gain, of Functional Coded Elements (FCTs). The inference, which isn't drawn in the paper, is to highlight the gap between the claims that are made for evolution and what experimental work has actually been shown to be capable of.

That life is able to adapt to use arsenic in place of phosphorus demonstrates again how remarkable it is, how adaptable and well-designed (or well-designoid, if you like) it is. But the idea of "arsenic overlords" is more than a little premature.