Scientists studying microscopic organisms in a seawater sample found a creature that appeared to be a tiny floating eyeball staring back at them.
Although the creature is a single-celled organism, it possesses many of the features of the human eye, including a lens, cornea and retina. But its view of the world would be nothing like our own according to Greg Gavelis at the University of British Columbia and his colleagues, who published the work in Nature in July.
“It’s really interesting that you can get complex eye structures at a subcellular level,” says Thomas Richards, evolutionary biologist at the University of Exeter. “And what’s so striking is they figured out how the components evolved.”
The eye’s complexity has fascinated evolutionists right back to Darwin. The idea that it could have formed by natural selection seemed “absurd in the highest possible degree” he wrote in On the origin of the species, later reasoning that the eye must have evolved in little steps from a basic light-detecting organ.
The marine microorganism known as a warnowiid seems to have embarked on just such a journey of evolution. The warnowiid is found off the coast of Canada and Japan, but only in tiny numbers. Researchers first stumbled across one more than a century ago, but studying the creature proved difficult because it rapidly disintegrates after being taken out of seawater.
To catch his own warnowiid, Gavelis searched seawater samples under a microscope for a year. When he eventually found one he froze it in plastic resin, preserving it like a fly trapped in amber. He then made a 3-D model by taking snapshots of the warnowiid under an electron microscope – and was stunned by what he saw.
While other single-celled creatures can detect light using “eyespots” – simple structures that allow an organism to tell dark from light – the warnowiid seemed to have repurposed its internal organelles to form what resembled the lens, cornea, iris and retina of a complex eye.
The “cornea”, for example – the transparent outer layer at the front of the eye – was made of mitochondria, the bits of the cell normally responsible for energy production. The mitochondria in the warnowiid interlocked to form a sheet-like layer around the lens, curved to concentrate incoming light on to the “retina”.
Like our own eyes, the creature’s lens consists mainly of proteins – though the researchers haven’t yet established what proteins the warnowiid uses.
But they know much more about the light-sensing “retina” at the back of the eye, showing it was made up of plastids – structures normally involved in photosynthesis. The team extracted genetic material from inside the plastids and found it rich in algal DNA. The team suspects that at some point during evolution, a warnowiid ancestor gobbled up some algae and adopted its photosynthetic equipment. When the organism later abandoned photosynthesis for the predatory life, it repurposed its light-capturing plastids into a light-sensing organ.
It is not clear how warnowiids use their “eye”, but the authors believe they’re used to hunt. Warnowiids spiral through the water as they swim and Gavelis thinks the eye lets them see flashes of light as it bounces off the single-celled organisms that are their prey. The lens likely concentrates incoming light to increase sensitivity – rather than projecting an image as our own eye does.
Richards wants to know whether the warnowiids can recognise shapes: “That would be pretty amazing!” He thinks we’ll likely see more examples of how microbes have adapted sight in the future. “Most of the diversity of life on this planet is under-observed,” he says.
Today’s electron microscopes and DNA sequencing techniques may have answered some of Darwin’s questions about the evolution of the eye. “Critics of evolution often talk about how there are no transitional forms of eyes,” says Gavelis. “But they’re alive and well in this case.”
