Nobody is Earthling unless somebody is Alien
"A unless B" in predicate logic is $\sim B \rightarrow A$
A = "Nobody is Earthling" or $\sim \exists x(Earthling(x))$
B = "somebody is Alien" or $\exists x(Alien(x))$
$\sim B \rightarrow A$
$\sim \exists x(Alien(x)) \rightarrow \sim \exists x(Earthling(x))$
or $\forall x(\sim (Alien(x))) \rightarrow \forall x(\sim (Earthling(x)))$