In nature, living things evolve through changes in their DNA. In an
animal like a chicken, DNA from a male sperm cell and a female ovum
meet and combine to form a zygote — the first cell of a new baby
chicken. This first cell divides innumerable times to form all of the cells of
the complete animal. In any animal, every cell contains exactly the same
DNA, and that DNA comes from the zygote.
Chickens evolved from non-chickens through small changes caused by
the mixing of male and female DNA or by mutations to the DNA that
produced the zygote. These changes and mutations only have an effect
at the point where a new zygote is created. That is, two non-chickens
mated and the DNA in their new zygote contained the mutation(s) that
produced the first true chicken. That one zygote cell divided to produce
the first true chicken.
Prior to that first true chicken zygote, there were only non-chickens. The
zygote cell is the only place where DNA mutations could produce a new
animal, and the zygote cell is housed in the chicken’s egg. So, the egg
must have come first. :wink: