Why is it so surprising that some life form should have As substituted for P?
It has always surprised me that even scientists imagine alien life to have the same biochemistry as us... so that when they look for alien life, they look for liquid water, oxygen, nitrogen.
But why should it be so?
Alien life may be something we cannot imagine, cannot understand perhaps.
Do we really understand that our undersatnding is highly limited....just like we cannot see most frequencies of the em spectrum, or hear most frequencies, we cannot understand many things... even the most brilliant human has limited capacity to understand and imagine.
Bacteria that live in the hydrothermal vents, and now an arsenic-using life form, there may be many more such surprises even on earth and on other planets.........we may not be able to imagine what form life will take on.