The eyes are formed by the fusion of different structures of embryonic tissues from different origin. The retina is a derivative of the forebrain, it therefore forms part of the central nervous system, while the cornea or lens come of the superficial ectoderm.
In the blog of Science Popularization 13 (25 of april of 2011) we showed that “Drs. Tessmar-Raible and Snyman in 2004 isolate opsins photosensitive in the ganglionar cells of a marine annelid Platyneris dumerilii (alive fossil with a trajectory of 600 millions of years), with a very similar structure with the opsins that exists in cones and rods of the eyes of the vertebrated. By this way it is established a direct biochemical relationship between the first cranial nerve, of the ganglia of the nervous system in the marine annelids, and the first cranial nerve of the ganglia of the nervous system of vertebrated (their eyes).
The retina is formed from two optical vesicles that born directly of the anterior portion of the primitive brain stem, and they are connected by the optical nerves. These two vesicles are approaching, during the embryonic development, slowly to the surface and suffer an invagination in the anterior part, and from being spherical gets to a cup form, resulting in the optical calix with double wall caused by the suffered folding. The intern wall (that overlying the interior of the optical calix), will turn into the retina and is there where are the opsins, while the external wall will form the epithelial cell sheet rich in melanin.

As with the eyes, of almost all the organs of present mammals, we can study the precedents, only in alive fossils, inasmuch as the organs hardly leave mineral fossils. For the study of the eyes of the mammals we can use alive fossils like the annelids (600 millions of years), fishes (570 millions of years) and the present birds (90 millions of years).

The chicken embryos are the most used for the dating of appearance of the organs ( at the 18 hours of hatching appear the digestive tract; at 21 hours it forms of the nervous system; at 23 hours it forms blood; at 24 hours, the eyes; at 25 hours appear the heart and the blood vessels; at 35 hours it form of auricular cavities; at 50 hours we can see the iris; at 72 hours it form the limbs and at the 120 hours the sexual organs). Thus we can design a temporal scale of the appearance of each organ during the evolution, in relation to the appearance at the hours of embryonic development. The embryonic development of the chicken lasts 500 hours and the eyes appear in the 4,8% of the process.

If we study the mammal eyes, in particular the human ones, the eyes appear in the 500 hour of fetal life in a gestation of 6720 hours, in the 7,4% of the process. The difference of percentage between birds and humans, can be explained by the neoteny process in the human beings, in other words, by the premature birth to favour the nervous system development and to correct the problems of the bipedalism (if the humans beings were born with the motor ability of the chicks, the gestation would last 408 days, and the apparition of the eyes would take the 5% of the embryonic process).

We concluded that if we consider the embrionic development equivalent to a time line of a millions of years since the apparition of the organ, in the primitive animal, up to the apparition of the specie, we can make an equational differential calculus that leads us to know why.

The organs appear in the animals in the same order and in consequence the embryos and fetus have the same morfology in the stages of equivalent development.
The organs appear in a relative similar time scale between the apparition of the organ (into evolution time) and the lasting of the embryonic development.
The apparition of the organs is related to the genetic expression of a basic pattern of codifier DNA that has remained without changes along millions of years of evolution and in consequence has genetic information basic and vital (master genes) that lead to the ordered development of each organ.
Each embryonic stage needs concrete pronutrients related with the genetic expression of the universal master genes.