re: #55 Obdicut

This goes well beyond ordinary adaptation, though. It’s more basic.

It’s almost certainly an adaptation to low phosphate conditions. From the paper:

GFAJ-1 grew at an average μmax of 0.53/day under +As/-P [40 mM AsO4^3-, 10 mM glucose], increasing by over 20-fold in cell numbers after six days. It also grew faster and more extensively with the addition of 1.5 mM PO4^3- (-As/+P, μmax of 0.86/day, Fig. 1A, B). However, when neither AsO4^3- nor PO4^3- was added, no growth was observed

In other words, growth is slowed when only arsenate is present. It’s not an “arsenic based life form” in that sense. Now for the point about origins. The proteobacteria are clearly in the same family of life as all of us. But since prokaryotes tend to swap DNA a lot, there’s doubt that it’s even possible to trace the tree of life back to the first cell or even close. So it is conceivable that the ability to use arsenate is some kind of atavism from an “arsenic based life form” that time forgot. Although swapping arsenate-based DNA sounds a little like swapping arsenic-based brownies.