Do you think there is a 'shadow biosphere'?

[ignoramus]

In one of the dna docos I watched, remember them asking this question.
Obviously to date, none have been found, but they speculated that life may not evolve any other way! And it's not that mother nature didn't try every variable.
This notion may be a universal law! So if aliens ever visit us, as long as they're not Jehova's, we can give them blood transfusions! hehe