So I would bet - and this is just pure speculation on the part of a lowly undergrad - that it probably started out like normal organisms (using phosphorus) and then it adapted the ability to use arsenic as a survival mechanism in the arsenic rich environment of Mono.

I did get to see the end of the press conference, and they addressed the age question ... mostly by saying they don't really know and further study is necessary. But she did say that while probably not all of the phosphorus in the organism was replaced by arsenic, there simply was not enough phosphorus in the samples to support life as we know it using phosphorus, so clearly arsenic is taking the place of phosphorus not only in DNA but also in membranes and ribosomes and stuff like that. They were talking about how they need to do more work to find out what the pattern is or the ratios and the mechanism and stuff like that. So many exciting future studies!!

One interesting thing they mentioned (though two of the scientists kind of argued about it a little ... lol) was that mono-Arsenic nucleotides (the analogue of the nucleotides in DNA) form spontaneously in test-tubes at room temperature while the phosphate version requires an enzyme catalyzed reaction. Interesting!