Someone once asked...
Someone answered with kindness...
Trick question. At the beginning of time last Thursday we had both eggs and chickens.
Another soul answered...
The egg cause eggs have been used by many a creature to bring their offspring into the world before chickens even came into existence
Another soul answered...
If you ask Google and AI, it will say the chicken. This is because in the timeline of evolution, the chicken would have come from a very similar ancestor who evolved during it's lifetime to become what we know as a chicken. But I think it's the egg. Because that "chicken really isn't a chicken until it has been birth as a full chicken (as an egg).