the reason why this theory is utter bullshit to me is the solipsistic view of life that we as humans have. we cannot fathom that there are beings that can survive and thrive in conditions that we humans can't. there might be aliens out there made of pure light who breathe ammonia and shit AIDS .
Obviously beings can survive in conditions humans can't. They are mostly single-celled organisms however. I think you need to have a much stronger understanding of chemistry, biology and physics if you want to make statements like that.
There are so many reasons "breathing ammonia" for example is just plain ridiculous.
The only thing along the lines of this theory that suggests it could be possible is that scientists have discovered alien microbes that have a ribose based backbone that opts for using Arsenate instead of Phosphate groups. They are in the same period however so it's not as wild as "breathing ammonia and shitting aids".
I get everyone trying to say "oh well there may be some unfathomable way that they can exist". Get real. The only life we have ever seen (and yes, it is all based on the same planet, however, conditions vary wildly from inside a volcano to under a sheet of ice, where the gaseous content is massively different) all runs on the same ideas - it all comes down to hydrogen diffusion being responsible for the movement of molecules
For example, let's say we are trying to replace Carbon with the next most reasonable molecule - Silicon. Both are in the same period with the same bond order. However, Silicon is simply gigantic compared to carbon. We could not pack DNA through protein bound histones due to the massive size of these molecules. So let's say these other beings are really large - they need some kind of support system. Silicon is not adept for basing proteins due to again the stereo effect of being too large, and also it's inability to both transfer (exhume) heat and create a rigid structure.
I hope you can start to get a glimpse of how it is really quite outrageous to say "there's no reason they can't be like this" when evidence so far accrued suggests life forms all run on the same basis. There's so far, no reason to suggest otherwise. The burden of proof is entirely upon the claimant in this instant.
