Living things can only evolve through changes in their DNA; they cannot evolve once they have been born and are roaming the Earth. For example, one of us isn't going to wake up one morning with an extra toe - it's just not going to happen. In an animal, such as a chicken, DNA from a male and female chicken meet and combine, forming a zygote. A zygote is a fertilized egg and is the first cell of a new baby chicken. So, the first cell divides countless times to form all of the cells of a complete chicken. Every cell contains exactly the same DNA - the DNA from the zygote - the original cell.

Chickens evolved one of two ways: from non-chickens or by mutations. Chickens could have evolved from non-chickens through small changes. This is caused by the mixing of male and female DNA of non-chickens. Mutations to the DNA could have produced a zygote resulting in chicken evolution. However, these changes and mutations can only affect the chicken at the point where a new zygote is created. This means that two non-chickens mated and the DNA in the new zygote contained the mutations. That one, little zygote cell divided, and divided, and divided, producing the first real chicken.

Before that first real chicken zygote, only non-chickens existed. The zygote cell is located in the chicken's egg. Because the zygote cell is the only place where DNA mutations can produce a new animal, the egg came before the chicken.