Before there were chickens, there were other nonchicken species, which evolved to form the modern chicken. However, any changes to the nonchicken that evolved into a chicken had to occur in the zygote. Therefore, what probably happened is that two nonchickens mated and their zygote mutated into a chicken cell, and then multiplied. Eventually, a full chicken was ready to hatch. This means that the egg probably is the answer to the age-old question of which came first.