Sanjukta Jana, B.Optom

Intern Optometrist, LVPEI, Bhuvaneswar, India


The eyes of vertebrates are complex. The ontology involved in the evolution of eyes is a debatable topic. The wide diversity in light sensory organs between different phyla makes it evident that eyes are not formed by natural selection. However, evolution shows ancient roots. Researchers have found similarities in transcription factors in the development of visual systems.

Darwin’s Theory

Darwin’s theory gives an idea of inheritance of how organisms evolve over generations and gives rise to newer species eventually. He also postulated the mechanism of natural selection – how organisms or populations learn to adapt. (1)

A Challenge for Darwin

Although it was absurd to propose that the human eye has evolved by natural selection, still Darwin strongly believed that light-sensitive cells  i.e., rods and cones have a common ancestor. (2)

The invention of the eye induced more competition. According to Darwin’s prebiotic soup theory, any structural or functional physiology was not preordained.

Light Sensory Patches

Organisms can be classified into –

  • Radially symmetric body plan
  • Bilateral body symmetry [ancestor of invertebrates and lineage of vertebrates]

Figure 1: Showing different kinds of body plans

Asymmetric sponges (The first phyla of the Animalia Kingdom) lack a nervous system or “photopsin”, yet they “can respond”  to light because of Gq-mediated signalling pathways. Hydras are the simplest organisms that are known to have photosensitive patches. They don’t have eyes but show contraction as a response to bright light.

The response is entirely dependent on two types of protein:

  • Opsins – Responsible for contraction in response to light
  • Ion channels – Generates an electrical signal. (3)

Gradual evolution – Photoreceptor cells

Planarian worms have two primitive structures called “eyespots” on their heads that can detect the direction of light rays. Light-averse planarian flatworms, known for their incredible ability to regenerate lost body parts, move away from light even after they have been decapitated. (4)

The need for cephalisation as a way of ‘struggle for existence’ was seen in bilaterally symmetric organisms leading to the development of neurology and eyes.

Initially located in the brain, light-sensitive cells as ancestors are of two types – rhabdomeric photoreceptors and ciliary photoreceptors. The basic light-processing unit of eyes is the photoreceptor cell, a specialised unit containing two types of molecules bound to each other within a membrane:

Opsin – Light-sensitive protein, Chromophore – Pigment which helps in absorption of light.

Studies concluded that vertebrates have ciliary photoreceptor cells while invertebrates have rhabdomeric photoreceptors.

Difference between ciliary photoreceptor and rhabdomeric photoreceptor

  • Morphologically different from each other.
  • Different types of opsins
  • Different transduction mechanism

Ciliary photoreceptors are advantageous over rhabdomeric in the marine environment. Nevertheless, the truth is ciliary receptors are present in many organisms to execute nonvisual purposes such as the circadian rhythm. (Example in hagfish.)

Cambrian explosion

Two different fundamental patterns of the eye developed –

  • Compound eye: Found in annelids, molluscs, and arthropods. Has a wide-angle view, and spatial resolution. (5)
  • Camera-style eyes: The second type has a lens and receptors. It acts like a camera which has a diaphragm to control the amount of light that gets through to the lens. The retina in eye is analogous to the film in a camera. (6)

Molecular Fingerprint

Discrepancies in the chronological order of the evolution of eye can be explained by Darwin’s theory of the method of Natural selection. For example, an Opsin in worm Platynereis brain resembles rods and cones in vertebrates. This phenomenon is known as a molecular fingerprint. (6)

Hence, the evolution of eyes is a topic where wide range of research scope is there.



  1. Darwin’s finches. (2016, April 25). Retrieved March 16, 2016 from Wikipedia:
  2. Darwin’s Greatest Challenge Tackled –
  3. Martin VJ. Photoreceptors of cnidarians. Canadian Journal of Zoology. 2002 Oct 1;80(10):1703-22.
  4. Woodward.A Flatworms can see even after decapitation; URL- decapitated/#:~:text=Planarian%20flatworms%2C%20which%20often%20live,detect%20the%20intensity%20of%20light.
  5. Trevor D. Lamb July 1, 2011: Evolution of the eye
  6. Schwab IR. The evolution of eyes: major steps. The Keeler lecture 2017: centenary of Keeler Ltd. Eye. 2018 Feb;32(2):302-13.


Image References

Figure 1 picture courtesy: Khushbu B. Symmetry in Animals – Definition, Types and Importance