You don't get evolution do you?
I'll do my best to explain :
What we define as chicken is a little bird that doesn't fly high, that has a yellow peak and that is often fatter than most birds.
There was a time when chicken did not exist, before chicken there was an unknown dinosaur bird that was different from the chicken by his proportins, size, color etc.
After time, the fattest and smallest of these birds started resisting more to the enviornment because of various factors (it was cold or something) so the other dinosaur birds had a smaller life spawn and reproduced less. In this way, the species of chicken-like dinosaur birds started geting fatter and fatter and smaller and smaller (more and more chicken-like) so after a while, the first chicken as we know them was born from an egg. Its mother was slightly taller and thinner and its mother too. Every species is defined by strict caracteristics, the first one to meet those of the present chicken was born from an egg.
Was that clear? I understand if it's confusing, it's hard to explain, I'll give you another example :
An X spicies of birds live in a Y island. They feed on Z nuts. One day, on the Y island, there is a volcano eruption and lots of trees that made Z nuts die. The new trees that grow out of the ashes have harder nuts (lets say W nuts). Only half the X birds have a peak big enough to break W nuts so half starve to death and therefore do not reproduce. The only ones to reproduce are the X birds with big peaks, they are different to precedent X birds so the scientists call them the V birds. X birds have then evolved into V birds...