In evolutionary biology terms, neither really. Speciation is a gradual process usually taking millions of years (the shortest ever discovered are African lake cichlids- which developed into a bunch of different species in only about 15,000 years, and are studied as models of speciation, like Darwin’s finches).
Basically it was a long process of something that wasn’t a chicken laying an egg that was a fraction closer to a chicken, and so forth for millions of years until eventually they were what we call a chicken. The general definition of species is a group of animals able to have fertile offspring. There was no single generation within that chain of millions of years that couldn’t reproduce with the one before or after it. You could set an arbitrary cutoff point where you decide the egg laid was close enough to called a chicken, but it would still be able to reproduce with the generation before, so a biologist would call them the same species.