Well I thought it was intelligent design that rules out any non sentient cause...
So what's left aliens which are NOT irreducibly complex, hence capable of evolving from scratch, or the supernatural aka GODDIDIT....
One of the key words is intelligent, either naturally occuring organism(s) or supernatural, tell me can intelligence come from anything else? Otherwise they are the only possible solutions to the question what is the designer.....
What other possible solutions can you think of?