The question of which has formed first is being considered here, the egg or the chicken. It had always been an unsolved riddle, which has come first. If we consider a minor time scale in evolution, it will remain a riddle. But we can solve this if we go back beyond about 150 million years. If we consider that chicken as a bird, then a bird has been evolved from reptiles about 150 million years ago. Going down the lane in phylogeny from reptiles shows the following order: amphibians, fishes, invertebrates, protozoa and finally bacteria and up to invertebrate organisms have egg-laying capability. When we consider the question, which is formed first the chicken or the egg, we can change the question into this; the organism or its ‘diploid chromosome containing external viable reproductive body – the fertilized egg’ formed first.
If we follow the egg-chicken, story in the chronological order it would be like this: egg-chicken-egg-chicken-egg…reptiles-egg…amphibians-egg…fishes-egg…invertebrates-egg… protozoa-protozoa-protozoa… If we study the phylogeny of chicken down the lane, we will come to protozoa, which is a group without egg forming capability. Therefore, we can conclude that chicken, the organism not its reproductive body, the egg, formed first. The first egg-laying organism must have evolved from protozoa.
The drawback of this reasoning is that the eggs of invertebrates are different from the egg of a chicken. However, the formation of the embryo in an aqueous environment outside the animal’s body, ‘the fertilized egg’ is common to both. In other words, this capability first evolved in invertebrates and continued in all vertebrates including birds except mammals.