A principle of quantum theory is that everything that can happen, does.
Happily for the shadow biosphere, it appears that there are bacteria that can incorporate arsenic into their DNA, if required. Arsenic doesn’t appear in anyone’s genetic structure, normally. But if it can happen, it will happen.
The fact that living things can replace one essential element with another means that they also probably do, which means that life is possible in far more places than we ever imagined. There could be life on a moon of Saturn that uses silicon instead of carbon, or selenium instead of sulfur. There could be a shadow ecosystem of microbes made of arsenic living unnoticed under our feet. And if so, and life evolved twice independently on Earth, it’s more than twice as likely that life has evolved on other planets, ending the supposed exceptionalism of our lonely space rock and suggesting that that we have interstellar neighbors.
In fact it seems unlikely that there aren’t alternative ecologies on this planet. Almost all life on earth is microbial, and we know about only a minute proportion of them. Isn’t it more likely than not that they exist in ways we can’t imagine? What this shows is that “what we think are fixed constants of life are not.” Arsenic-based bacteria forces us to think about the possible and atypical not the probable and normal – and suggests the perversely uninhabitable conditions on earth that might have existed when life began. Such as naturally toxic environments like Mono Lake (pictured).
This is a long but very entertaining piece (as is the whole blog) and well worth reading in full.