When we look at a painting, its colors and images enter our eyes as waves of light. Thanks to a layer of tissue at the back of our eyes known as the retina, the vibrant yellows and subtle blues of van Gogh’s Starry Night are translated into electrical signals for our brains to interpret.
This remarkable part of our eye is actually an extension of our brain tissue. And just like our brain, the retina needs a lot of oxygen to function properly.
A study published by an international collaboration of researchers recently revealed just how important a steady supply of oxygen was to the evolution of a thicker retina, and therefore better vision.
425 million years ago, the researchers found, your ancestor was a fish with mediocre eyesight. And its sight couldn’t improve until it evolved new ways for oxygen to reach the retina.
“We showed that in the ancestor of most vertebrates, the retina was likely thin and had a relatively poor oxygen supply to it,” says H. William Detrich, a professor of marine and environmental sciences at Northeastern. “As species evolved, when the retina increased in thickness, it was always accompanied by one of several mechanisms that improve retinal oxygen delivery.”
The researchers collected information about retinal thicknesses and oxygen delivery mechanisms in 87 vertebrate species around the world and examined the evolutionary links between them. They found that several unique ways had evolved to bring oxygen to the retina, and any vertebrate with good vision exhibited at least one of them.
Around 280 million years ago, when today’s continents were still squished together in a giant land mass we now call Pangea, the first of these changes showed up in fish.
Hemoglobin, the protein in red blood cells that binds with oxygen, mutated in a way that made it extremely sensitive to acid. When the blood became even slightly acidic, the mutated hemoglobin would release a large portion of the oxygen it was holding.
In the layer of the eye right behind the retina, called the choroid, a web of capillaries evolved. This network, known as the rete mirabile (latin for “miracle network,” Detrich says), maintained a slightly acidic environment. When blood passed through it, oxygen was forced out of the hemoglobin to diffuse into the retina at high concentrations.
These changes were accompanied by the evolution of thicker retinas and larger eyes in fish. The influx of oxygen allowed fish eyes to sustain more cells to help them resolve finer details in an image and see better in low light.
While the choroid rete mirabile is still prevalent in fish today, it never evolved in vertebrates on land. These animals instead evolved networks of capillaries within the retina itself, or immediately in front of it, providing oxygen more directly to retinal cells. But this solution was a tradeoff, Detrich says, because the blood vessels could potentially interfere with vision by scattering incoming light.
The researchers found that these mechanisms evolved and vanished from evolutionary history multiple times. Some animals, like the Mexican blind cave fish, adapted to environments where eyesight wasn’t that important, and lost some of the mutations that would bring oxygen to the eye. Ancient mammals evolved more capillaries in and around their retinas when they began being active in the daylight and relying more heavily on vision, about 100 million years ago.
Antarctic icefishes, which Detrich has been studying for decades, were a special case. They lost their red blood cells and hemoglobin in an evolutionary accident, and had to adapt.
“The absence of hemoglobin in the icefishes means that they cannot provide oxygen to the retina using the choroid rete mirabile,” Detrich says. “If those fish were to maintain a decent retinal sickness, another mechanism of oxygen supply had to evolve.”
Detrich was on an expedition in Antarctica when he received an email from Christian Damsgaard, the study’s lead author. Damsgaard wanted to include icefish and several other Antarctic fish species in the study, but didn’t have any high-quality specimens.
“I wrote back and said, ‘Well, I happened to be in Antarctica at the moment. And we can rectify that problem,’” Detrich says.
Detrich and his team collected fresh specimens and blood samples from five species of fish: two icefish species, and three Antarctic species that never lost their red blood.
The researchers found that the icefish species had retinas that were just as thick as those of the other Antarctic species, despite losing their oxygen-carrying hemoglobin. To keep supplying oxygen to their eyes, the icefish had evolved extensive networks of capillaries in front of their retinas.
“It was a particularly informative aspect,” Detrich says.
The odd evolutionary twist of the icefish helps to fill out a larger picture linking a steady supply of oxygen to better vision. Combined with analyses of other vertebrates around the world, it gives us a better fundamental understanding of how our eyes, and the eyes of every other vertebrate, came to be.
“This really advances our state of knowledge about eye evolution,” Detrich says. “Our study is the most comprehensive attempt to synthesize our understanding of the vertebrate eye.”