Aliens?? [NASA Discovery Announcement]

WhoDat26.2 said
What's being suggested is beyond the "found a way to metabolize arsenic". Those bacteria were found and published on in 2008. What's being rumored is that what they've found something that has chemically different DNA than what we've seen before.

So this lifeform developed INDEPENDENT of the evolutionary process we're familiar with.

If that's what they announce (still rumor), then that would mean that life evolved twice on Earth, separately.
Click to expand...

Why does arsenic replacing phosphorous in a cell imply independent evolutionary processes? Chemically, they are fairly similar atoms - they ionize the same way. Why wouldn't a mutation that allowed phosphorous to replace aresenic (or vice versa) be potentially part of the evolutionary process?

(though I agree that this is a monumental discovery, if that's the case).