The Rise of Eyes

From beetles and octopuses to humans, most animals on Earth have eyes. But where did our eyes come from? The answer is complicated—especially for vertebrates, or animals with a backbone. Biologists from Sweden and England recently proposed a new hypothesis, or possible explanation, for how vertebrate eyes came to be. The story begins about 575 million years ago.

Back then, the only animals on Earth were tiny ocean-dwelling organisms. One was a wormlike creature known as urbilateria (er-bih-luh-TIR-ee-uh). It was likely no larger than a fingernail, living along the shallow seafloor. On top of the animal’s head was a patch of light-sensing photoreceptor cells that helped it tell day from night, light from shadow, and up from down. To tell left from right, it had little cup-shaped pits filled with photoreceptors on either side of its head. These were two primitive eyes!

About 98 percent of modern animal species are descended from urbilateria. This includes arthropods (insects, crustaceans, and spiders), mollusks (octopuses, squid, and snails), and vertebrates (like us!).

Because these three groups all descended from the same wormy ancestor, scientists expected their eyes to be made of the same cells. Instead, after analyzing the eyes of various animals alive today, they found that vertebrates’ eyes contain totally different photoreceptors than arthropods and mollusks. The biologists reason that vertebrates must have followed a different path as they evolved, or changed over time. Arthropods and mollusks kept the urbilateria’s original two-eyed configuration. But vertebrates followed a stranger route to end up with the vision they have today.

Most animals on Earth have eyes. Beetles do. So do octopuses and humans. But where did our eyes come from? The answer is complicated. This is especially true for vertebrates, animals with a backbone. Biologists from Sweden and England recently proposed a new hypothesis. It’s a possible explanation for how vertebrate eyes came to be. The story begins about 575 million years ago.

The only animals on Earth back then were tiny. They lived in the ocean. One was a wormlike creature. It was known as urbilateria (er-bih-luh-TIR-ee-uh). It was likely no larger than a fingernail. It lived along the shallow seafloor. On top of the animal’s head was a patch of light-sensing photoreceptor cells. They helped the animal tell day from night, light from shadow, and up from down. It also had cup-shaped pits full of photoreceptors on the sides of its head. These were two primitive eyes! They allowed it to tell left from right.

About 98 percent of today’s animal species are descended from urbilateria. That includes arthropods (insects, crustaceans, and spiders). It includes mollusks (octopuses, squid, and snails). And it also includes vertebrates (like us!).

These three groups all descended from the same wormy ancestor. So scientists expected their eyes to be made of the same cells. Scientists analyzed the eyes of animals alive today. But they found that vertebrates’ eyes contain totally different photoreceptors than arthropods and mollusks. Biologists think that vertebrates must have followed a different path as they evolved, or changed over time. Arthropods and mollusks kept urbilateria’s two-eyed setup. But vertebrates followed a stranger route. It brought them to the vision they have today.

About 560 million years ago, the descendants of urbilateria that would eventually evolve into vertebrates made a big change: They shifted from swimming freely to burrowing in one place. They became filter feeders, sticking their heads out of the seafloor to suck bits of food from the water.

“When they adopt that kind of lifestyle, they don’t really need to navigate or make left-right decisions,” says Tom Baden, a neuroscientist from the University of Sussex in England who worked on this research. They no longer needed the primitive eyes urbilateria had developed on either side of its head (see “An Early Split”). But they still benefited from knowing what time of day it was, so the light-sensing eye on top of their head remained, says Baden.

For millions of years, these one-eyed creatures stayed rooted in the sand. But there was more food out in the open water. Eventually, descendants left their burrows and started swimming. “The second you start moving again, it’s really useful to make left-right decisions,” says Baden. But by that point, “the original eyes were gone.” This animal needed new ones!

About 560 million years ago, some descendants of urbilateria made a big change. These were the animals that would eventually evolve into vertebrates. They changed from swimming around to burrowing in one place. And they became filter feeders. They stuck their heads out of the seafloor to suck bits of food from the water.

“When they adopt that kind of lifestyle, they don’t really need to navigate or make left-right decisions,” says Tom Baden. He’s a neuroscientist from the University of Sussex in England who worked on this research. They no longer needed the early eyes urbilateria had developed on the sides of its head (see “An Early Split”). But it still helped them to know what time of day it was. So the light-sensing eye on top of their head stayed there, says Baden.

For millions of years, these one-eyed creatures stayed rooted in the sand. But there was more food out in the open water. Eventually, descendants left their burrows. They started swimming. “The second you start moving again, it’s really useful to make left-right decisions,” says Baden. But by then, “the original eyes were gone.” This animal needed new ones!

Sprouting new eyes may sound like a complicated process, but eyes can actually evolve relatively quickly, says Dan-Eric Nilsson, a biologist from the University of Lund in Sweden who also worked on this research. He calculated that a patch of light-sensitive cells can transform into an image-forming eye in less than 400,000 years. In terms of evolution, that’s the blink of an eye!

For these free-swimming one-eyed creatures, being able to tell left from right was a huge advantage. Because of that, individuals with a slightly bigger and sharper top eye were more likely to survive and pass on their genes—or units of hereditary material. With each generation, the top eye grew wider. “You start to be able to look at things that are not just straight up, but sideways,” says Baden. Over time, new eyes split off from the top eye and migrated to either side of the head. These eyes were able to see blurry shapes.

By about 500 million years ago, these simple, wormy creatures had transformed into the first vertebrates—small, jawless fish. These vertebrates’ eyes were much more complex. They had a lens to focus light, creating clear images of objects and their surroundings—just like our eyes today!

Growing new eyes may sound complicated. But eyes can evolve fairly quickly, says Dan-Eric Nilsson. He’s a biologist from the University of Lund in Sweden. He also worked on this research. A patch of light-sensitive cells can transform into an eye in less than 400,000 years, Nilsson found. For evolution, that’s the blink of an eye!

Some creatures swam around with just one eye. For them, being able to tell left from right was very helpful. So individuals with a bigger and better top eye were more likely to survive. They were also more likely to pass on their genes—or units of hereditary material. With each generation, the top eye grew wider. “You start to be able to look at things that are not just straight up, but sideways,” says Baden. Over time, new eyes split off from the top eye. The new eyes moved to either side of the head. These eyes were able to see blurry shapes.

After a while, these simple, wormy creatures had transformed into the first vertebrates. They were small, jawless fish. That happened by about 500 million years ago. These vertebrates’ eyes were much more complex. They had a lens to focus light. It created clear images of objects and their surroundings. That’s just like what our eyes do today!

Over the next hundreds of millions of years, vertebrates diversified into fish, amphibians, reptiles, birds, and mammals. During that time, vertebrate eyes kept the same basic structure (see “Inside the Eye,” below).

Many vertebrate eyes did develop their own unique features though. Birds of prey, like eagles and falcons, can clearly see objects up to several miles away. Amphibians and some fish can see infrared light—energy from heat, which is invisible to humans—to help identify other animals lurking in dark water. Predators like cats and owls developed large, round pupils for hunting in low light, and prey like goats and sheep developed horizontal pupils to give them a wider field of vision for spotting predators. Some species that live in complete darkness, like Mexican cave fish, lost their eyes altogether.

Despite these differences, all modern vertebrates still carry a remnant of our cyclops ancestor. That top eye didn’t disappear—it became the pineal organ, a small structure buried in the brain. It is sometimes referred to as a vertebrate’s “third eye.” In birds and mammals, the pineal organ regulates sleep by reacting to light detected by our eyes. But in other lineages, including those of many fish, amphibians, and reptiles, the organ is still sensitive to direct light. Some reptiles have even evolved a transparent scale on top of their skull so that focused light can hit their pineal organ.

While there’s a lot of evidence to support this new idea about how vertebrates’ eyes evolved, it’s not easy to untangle events that happened half a billion years ago! With these new details, says neuroscientist Baden, “we’re putting one more puzzle piece into the story of our eye’s evolution.”

Hundreds of millions of years passed. During that time, vertebrates became more diverse. They evolved into many different kinds of animals. These included fish, amphibians, reptiles, birds, and mammals. All along, their eyes kept the same basic structure (see “Inside the Eye”).

But many vertebrate eyes developed unique features. Birds of prey, like eagles and falcons, can clearly see objects several miles away. Amphibians and some fish can see infrared light. This type of light is energy from heat. It’s invisible to humans. It allows some animals to see creatures lurking in dark water. Predators like cats and owls developed large, round pupils. They’re useful for hunting in low light. Prey like goats and sheep developed horizontal pupils. These pupils give them a wider field of vision for spotting predators. Some species that live in total darkness lost their eyes completely. One example is the Mexican cave fish.

Modern vertebrates have some differences in their eyes. But all still carry something left over from our cyclops ancestor. That top eye didn’t disappear. It became the pineal organ. That’s a small structure deep in the brain. It’s sometimes referred to as a vertebrate’s “third eye.” In birds and mammals, the pineal organ regulates sleep. It does this by reacting to light detected by our eyes. But in other kinds of animals, the organ is still sensitive to direct light. These include many fish, amphibians, and reptiles. Some reptiles have even evolved a transparent scale on top of their skull. It lets focused light hit their pineal organ.

There’s a lot of evidence to support this new idea about how vertebrates’ eyes evolved. But it’s never easy to untangle events that happened half a billion years ago! With this research, says Baden, “we’re putting one more puzzle piece into the story of our eye’s evolution.”