I have just consulted my battered Chambers Dictionary which provides new evidence that the chicken came first!
More precisely "chicken" comes at the bottom of page 262 and "egg" comes much later (on page 477).
The same source indicates that "mortals" (page 969) closely followed "morons" (page 968) although it is clear that some counter-evidence of co-existence can still be found!
A chicken and an egg are lying in bed. The chicken is smoking a cigarette with a satisfied smile on its face and the egg is frowning and looking a bit ****** off. The egg mutters, to no-one in particular, "Well, I guess we answered THAT question!"