The origin of our eyes: when our ancestors were cyclops

For decades, the eye has been one of evolution's great enigmas. Charles Darwin said that thinking about the origin of such a sophisticated organ made him shudder. How could a structure capable of detecting light have been built in such a way that the brain could form three-dimensional images with which to interpret the world, from the accumulation of small and gradual changes? A new scientific hypothesis published in the journal Current Biologyproposes a surprising answer. Our eyes could come from a single ancestral eye located in the center of the head in very simple marine animals. Perhaps it is no coincidence that so many ancient mythologies imagined cyclops, beings with a single eye in the center of the forehead. Long before vertebrates with two lateral eyes existed, our ancestors might have actually had a single primitive central eye.The research, led by neurobiologists Dan-Eric Nilsson (Lund University, Sweden) and Tom Baden (University of Sussex, England), suggests that about 560 million years ago, the ancestors of vertebrates had a single central photosensitive system, similar to a “third eye”. Over time, this organ split into two, which gradually specialized to give rise to modern retinas.

The idea is revolutionary because it overturns a very deep-rooted view: that vertebrate eyes evolved simply by adding complexity to primitive lateral eyes. According to the new model, the vertebrate retina would not be a completely new invention, as was thought, but rather the optimized reuse of an already existing ancestral system. In evolutionary biology, this pattern of evolution is very frequent: evolution does not generate anything from scratch, but rather recycles, modifies, and reconnects old structures.Detect ambient light

Researchers start from a key observation. In almost all invertebrate animals, such as insects, mollusks, or worms, their eyes use a type of photosensitive cells called rhabdomeric. In contrast, vertebrates mainly use ciliary photoreceptor cells, the so-called rods and cones of the retina. This difference led to believe that vertebrate eyes had a completely different evolutionary origin. This new hypothesis suggests that the primitive animals that would eventually give rise to vertebrates lived semi-buried on the seabed. In this lifestyle, lateral eyes were not useful. If an animal lives buried, it does not need to chase prey or orient itself visually. This could have led to the loss of ancestral lateral eyes. But it would still need a photosensitive system to detect ambient light, regulate circadian rhythms, or maintain central body posture. The most suitable place, considering their lifestyle, would be on the top of the head. Over time, some descendants of these ancestral animals abandoned their sedentary life and began to swim. At that moment, that single "central eye" would have acquired new functions. Its photosensitive cells reorganized into small cavities capable of detecting the direction of light, as current eyes do. This allowed it to know if it was tilting up or down in the water, essential information for stabilizing movement. Later, these structures moved to the sides of the head and transformed into true lateral eyes, like those we know today. Traces of the single central eye

However, is there any evidence to support this hypothesis? Yes. The most fascinating thing is that even today we retain a trace of that ancient "central eye." It is the pineal gland, a neurological structure located in the center of the brain that is responsible for regulating biological rhythms according to whether it is day or night. In fish and some reptiles, this gland is still sensitive to light. In humans, it no longer detects light directly, but it continues to function as a biological clock. The authors of this study argue that the retina of the eyes and the pineal gland of the brain, which would be a relic of the primitive cyclops eye, share a common evolutionary origin. This common evolutionary origin is supported by the presence of the same type of molecules involved in photoreception in both structures, called opsins. And also by the fact that the pineal gland has cells called pinealocytes that are very similar to the photoreceptors of the retina.This interpretation also makes it possible to explain why the retina is so complex. Far from being a simple light-sensitive layer, it would be a small extension of the brain, as it has more than a hundred different neuronal types. Naturally, this proposal still needs to be contrasted. Some paleontologists have found fossils of primitive vertebrates with four eyes, two lateral and two superior, which could indicate an intermediate stage in this evolutionary history.

In any case, regardless of the details yet to be confirmed, this research shows one of evolution's great lessons. The most spectacular innovations are often born from old structures repurposed in a new way. Our eyes, capable of reading, recognizing faces, or contemplating distant galaxies, might originate from a small light-sensitive spot on the head of an almost brainless marine animal. And perhaps this is precisely what makes evolution so extraordinary.