Have Eyes for Vision

We have eyes for vision.

Vision exists because early organisms developed a chemical response to light. The earliest life forms, such as single-celled organisms, had chemical reactions that responded to light. These reactions involved molecules that changed their structure or function in response to light energy. It was the foundation for the development of vision. Over time, these chemical reactions evolved into more complex photoreceptors, specialized molecules that detect light and trigger a response. The earliest photoreceptors were likely simple light-sensitive proteins that helped organisms orient themselves towards or away from light sources. As organisms became more complex, their photoreceptors evolved into more sophisticated vision sensors. These sensors allow organisms to detect the presence or absence of light and its intensity, direction, and wavelength. The development of eyes for vision was a major milestone in the evolution of vision. Eyes are complex organs that use lenses, corneas, and retinas to focus light onto photoreceptors, allowing organisms to form images of their environment. Over millions of years, eyes evolved to become more sophisticated and useful for survival. 




About 3.5 billion years ago, the first life forms on Earth were single-celled organisms like bacteria and archaea. At that time, the environment was already full of electromagnetic radiation, including visible light (what we humans can see), ultraviolet (UV), infrared (IR), and lots more. Light was (and still is) everywhere, and it carried lots of information about the environment, like where the energy sources were, where shadows—potential hiding spots or predators—were, and so on.




The earliest and most basic response to light is not what we'd call vision just yet—it's called a photoreactive response or phototaxis. Even simple organisms such as certain bacteria and algae have molecules in their cell membranes called photopigments that can absorb light energy. When these photopigments absorb light, it triggers a chemical change inside the cell—a tiny biochemical domino effect.

 

In these primitive cases, the response wasn't "seeing" in the sense of forming images or perceiving shapes or colors. It was much simpler, such as moving towards or away from the light. For a photosynthetic bacterium that needs sunlight for energy like plants do today, it helped them maximize their energy production. For a microbe that gets damaged by too much UV radiation, it helped them survive. It is an extremely primitive light sensitivity. Yet it's the evolutionary precursor to vision. Organisms that moved toward sunlight as an energy source or away from UV radiation hazards were more likely to survive and reproduce.



As vertebrates evolved eyes that were more active, mobile, and agile, their visual system had to adapt to support their increasingly dynamic lifestyle. The ability to quickly and accurately track prey, navigate complex environments, and detect potential threats became crucial for survival.





Highly mobile animals have developed six extraocular muscles (EOMs) that enable wide fields of view, rapid eye movements, and independent eye movements. Eye and head movements are complexly coordinated, especially for many active vertebrates, with a large range of motion. The six EOMs help smooth conjugate eye movements—both eyes moving simultaneously—and disjugate eye movements—both eyes moving independently. This dual capability supports flexible gaze adjustments based on head position and environmental demands.





Six extraocular muscles (EOMs) allow for a wide range of eye movements. This level of control enables animals to focus their gaze on specific targets, track moving objects, and sample their visual environment efficiently. The six EOMs—four recti and two obliques—provide the fine-tuned control that wasn’t achievable with fewer muscles.






EOMs are crucial for all vertebrates with mobile eyes. Their primary functions are stabilizing gaze (keeping the visual image steady despite head movements, vestibulo-ocular reflex), tracking (following moving objects smoothly), saccades (rapidly shifting gaze from one point to another), and vergence (adjusting eyes for depth perception). These functions are vital for basic survival—finding food, avoiding predators, and navigating the environment—long before contemplating stars, which paved a way towards the patterns of existence.









 
 

Further readings

 

The evolution of eyes: major steps. The Keeler lecture 2017: centenary of Keeler Ltd
https://www.nature.com/articles/eye2017226

Human-like 'eye' in single-celled plankton: Mitochondria, plastids evolved

Scientists have peered into the eye-like structure of single-celled marine plankton called warnowiids and found it contains many of the components of a complex eye.
https://www.sciencedaily.com/releases/2015/07/150701133348.htm











Eye-like ocelloids are built from different endosymbiotically acquired components

July 2015 Nature 523(7559)
https://www.researchgate.net/publication/279732106_Eye-like_ocelloids_are_built_from_different_endosymbiotically_acquired_components




Plastids are organelles found in plant and algal cells, although they are also found in some marine animals. These organelles arose during evolution after an endosymbiosis process, when a bacterium with photosynthetic abilities, similar to current cyanobacteria, was engulfed by a eukaryote cell and became an endosymbiont instead of being digested.
https://mmegias.webs.uvigo.es/02-english/5-celulas/6-plastos.php#:~:text=Plastids%20are%20organelles%20found,found%20in%20some%20marine%20animals

The cell. 6. Non vesicular. Plastids. Atlas of plant and animal histology.

How the Eye Works

https://www.umkelloggeye.org/conditions-treatments/anatomy-eye

Anatomy of the Eye | Kellogg Eye Center | Michigan Medicine