A 530-million-year-old fossil may contain the oldest eye ever discovered, a new study released this week by the University of Edinburgh has suggested.
The remains of an "extinct sea creature" included an early form of the eye seen in many of today's animals, including crabs, bees and dragonflies, researchers said.
The team of scientists, made up of experts from across Europe, made the find while examining a well-preserved fossil of a hard-shelled species called a trilobite.
These were the ancestors of spiders and crabs which lived in mainly coastal waters during the Palaeozoic era, between 541 and 251 million years ago.
The scientists found that the ancient creature had a primitive form of compound eye - an optical organ that consisted of arrays of tiny visual cells, called ommatidia, which was similar to those of present-day bees.
The team, which included an expert from Edinburgh University, said its findings suggest that compound eyes have changed little over 500 million years.
The right eye of the fossil - which was unearthed in Estonia - was partly worn away, giving researchers a clear view inside the organ. This, the experts explained, revealed details of the eye's structure and function, and how it differed from modern compound eyes.
The ancient species - called Schmidtiellus reetae - had poor vision compared with many animals today but it could identify predators and obstacles in its path, researchers believe.
Its eye consisted of roughly 100 ommatidia, situated far apart compared to contemporary eyes.
Unlike modern eyes, the fossil's eye did not have a lens. This is likely to be because the primitive species lacked parts of the shell needed for lens formation, the team said.
The team revealed that only a few million years later, improved compound eyes with higher resolution developed in another trilobite species in the present-day Baltic region.
"This exceptional fossil shows us how early animals saw the world around them hundreds of millions of years ago," said Professor Euan Clarkson of the University of Edinburgh.
"Remarkably, it reveals the structure and function of compound eyes has barely changed in half a billion years."
The study is published in the journal Proceedings of the National Academy of Sciences. It was carried out with the University of Cologne, Germany, and Tallinn University of Technology, Estonia.
Professor Brigitte Schoenemann, of Cologne University, said: "This may be the earliest example of an eye that it is possible to find.
"Older specimens in sediment layers below this fossil contain only traces of the original animals, which were too soft to be fossilised and have disintegrated over time."