Around 425 million years ago, the researchers found, our ancestor was a fish with mediocre eyesight. Its sight couldn’t improve until it evolved new ways for oxygen to reach the retina. A study published by an international collaboration of researchers, including H. William Detrich, a professor of marine and environmental sciences at Northeastern University, recently revealed how a steady supply of oxygen helped vertebrates evolve better vision. A steady supply of oxygen helps in the evolution of a thicker retina, and therefore, better vision.
According to Detrich, they found that in the ancestor of most vertebrates, the retina was thin and had a relatively poor oxygen supply to it. As species evolved, when the retina increased in thickness, it was always accompanied by one of several mechanisms that improve its delivery. Information about retinal thicknesses and oxygen delivery mechanisms in 87 vertebrate species around the world was collected and examined. 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. The 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. However, the vertebrates evolved networks of capillaries within the retina itself or immediately in front of it, providing oxygen more directly to retinal cells. However, this solution was not helpful as the blood vessels could potentially interfere with vision by scattering incoming light.
Detrich and his team collected fresh specimens and blood samples from five species of fish: two ice fish species and three Antarctic species that never lost their red blood. They found that the ice fish species had retinas that were just as thick as those of the other Antarctic species, despite losing their oxygen-carrying haemoglobin. To keep supplying oxygen to their eyes, the ice fish had evolved extensive networks of capillaries in front of their retinas.
“This really advances our state of knowledge about eye evolution. Our study is the most comprehensive attempt to synthesise our understanding of the vertebrate eye”, Detrich says.
Shahjadi Jemim Rahman