Humans have remarkably sophisticated eyes. We view the world as a high-definition color movie in three dimensions. We also rapidly process what we see, since 40% of our brain is devoted to vision. Even as you are reading this article, your eyes and brain are effortlessly processing the words on the screen and deciphering their meaning.
Most organisms in the natural world have far less complex eyes than us. In fact, the most common eye type in the natural world, called ‘pit’ eyes, are nothing more than hollow cups that are known to sense the presence and direction of light, but little else. How did these simple eyes evolve into the complex, highly functional human visual system?
Dr. Akash Gulyani, a researcher at Institute for Stem Cell Biology and Regenerative Medicine (inStem), Bengaluru, is interested in ‘pit’ eyes, as they may hold the key to understanding how sophisticated human vision evolved. Luckily for him, Dr. Dasharadhi Palakodeti, also at inStem, studies planarians aka flatworms – simple little worms with seemingly simple eyes. The two researchers teamed up to study planarian eyes, uncovering a series of interesting and ground-breaking results that show unexpected sophistication in planarian vision. The study was published in the journal Science Advances.
Planarians are truly remarkable little critters. They have achieved a Frankenstein-ish fame in the biological world for their powers of regeneration. If you cut them up into little pieces, each piece regenerates into a small worm! If you lop off their head, they regrow that too, complete with eyes and brain.
Planarians have simple, cup-shaped eyes with a single type of photoreceptor. This means that they are colour blind, and can only view the world in grey. Dr. Gulyani and his team were curious to find what would happen if they shone light of two different colours on the worms, for example blue and green. To their intense surprise, the worms almost always chose blue over green light, and green over red light. How were the worms able to tell colours apart if they had only one photoreceptor?
The team eventually figured out that the worms’ brains were, in effect, representing different colours as different shades of grey. Even if they couldn’t actually ‘see’ the colours, their brains were using a trick to allow them to distinguish between different colours. “This result demonstrates that the planarian eye and nervous system has the ability to accomplish fairly sophisticated ‘comparative processing’ – the ability to make small comparisons. This is an exciting finding and has implications for eye-brain evolution”, says Dr. Gulyani.
Next, the team took advantage of planarians’ unique regenerative ability to study how they regained their eyesight and visual processing abilities. They chopped off the heads of the worms and watched and waited for their eyesight to recover. The decapitated worms regenerated their eyes in about 4-5 days, but at this stage, could not distinguish between different colours as before. Then as their brains strengthened, they built the right neural connections and their colour-distinguishing abilities gradually recovered. By looking at the worms’ brains during this process, the researchers were able to uncover the neural networks that regenerated to give back the worms their colour-distinguishing ability.
In a further twist, the team found that decapitated worms showed a strong aversion to small amounts of ultraviolet (UV) light, even when they had no eyes! This surprising result indicated that the worms had a mechanism to detect ultraviolet light even without their eyes, through an extraocular system.
The researchers then performed a series of experiments where they shone both UV and visible light on the worms to tease out the relationship between the eye-brain and the extraocular vision systems. In decapitated worms, the extraocular system took control and the worms avoided UV light. But when they regrew their eyes and brains, the regular eye-brain system reclaimed its dominance. “This hierarchical structure is quite striking and is reminiscent of hierarchies in neural networks in more complex animals and mammals”, says Dr. Gulyani.
What do these experiments tell us about how the human visual system evolved? The ability of planarians to perform comparative processing to distinguish between colours is likely an important landmark in the evolution of human eyes. Besides, the surprising sophistication of planarian’s vision highlights how little we know about the eyes and brains in the natural world. “Natural light sensing has many layers and textures and there are surprises galore if one is curious and willing to study this in detail”, believes Dr. Gulyani.
Further, the amazing ability of planarians to regenerate their visual systems could hold clues on how to restore human vision. The two scientists plan to study the molecules involved in eye-brain regeneration, using planarians as a model system.
Dr. Gulyani is clearly proud of his enterprising research team. “Our work illustrates how curiosity-driven, highly collaborative research not only leads to new discoveries about the natural world but also can impact problems like neural regeneration that have tremendous biomedical relevance”, he signs off.