Approximately 1000 millions of years ago (between 1.800 and 625 millions) appeared in the Earth the first eukaryotic and protist.

These unicellular beings lived in the sea and need to react in front of the environmental changes as a mechanism to survive.

Nowadays, there are still a lot of species from these dates, although we will focus our interests in the genres Euglena and Chlamydomona, specifically in their ocular stain. It is a small stigma formed by receptor proteins (particularly photoreceptor proteins) that allows the cell to station itself in the place were there is the optimum quantity of  light for the photosynthesis (approaching by phototaxis and moving away by photophobia when a scourge is activated generating movement). Essentially, it is the most simple eye known but it is very transcendent for the evolution about the eyesight and the apparition of new senses and organic receptor systems due to the proteins called rhodopsins.

In a nearer time, during Ediacaric, 635- 542 million of years ago, the pluricellular complex organisms appeared. These organisms were formed by lots of united cells, constituting specialised tissues. The specialised tissues joined  to form the organs, and a group of organs formed systems, and finally a group of systems became a complex organism.

In this period we can find the origin of the current jellyfish. The first eyes are still not developed (the current concept of them, can be find firstly 540 to 250 millions of years ago in trilobites) but in jellyfish, the rhodopsins have a new role in the stimulus reception through ocellar stains and ocell. They are pigmented stains appearing in the bell of some jellyfishes and are constituted in two kinds of cells: sensory cilia and cells where rhodopsins are stocked. These receptors only allow them to distinguish from light to dark, not objects.

Even more, the apparition of multicellular animal, some organs are isolated from the exterior and arises new communication requirements: information transmission between inner organs and between inner organs and the environment. So the substances produced by one part of the organism informs to other parts of situations of general interest. As the evolution proceeds, more complex is the needed extern exploration, so new senses must be developed, as taste, smell and touch such as allowing that photoperiod, through the eyes to the  pituitary, to regulate some cycles as the seasonal reproductive. Here the nature rearranges the role of the primitive receptors as rhodopsins.

Rhodopsins  can be found nowadays in eukaryotes, yeasts, plants, animals and modern animals ( birds, reptiles and mammals) where constitute class A (or 1) receptors, capable to detect extracellular signals  coming from the environment or other organs , transmit them to the cell interior and trigger the needed answers.

Rhodopsins are found nowadays in ocular stain of Euglena, Chlamydomona, in jellyfish ocell, retinal roots of reptiles, birds and mammals, in taste buds, in smell and internal cells  as adrenalin receptors,  and are a biochemical success in the performance of new functions and as the base of  operation  of new organs.

At the same way, as chlorophyll and haemoglobin are adaptations of a molecule to new evolutive requirements, rhodopsins constitute the biochemical base that , from 1000 million years ago, uses isolated cells (unicellulars) or cells from multicellular beings to detect exterior changes allowing them to adapt and survive.