Fossils reveal oldest known vertebrates had four eyes

By Dean Murray

Ancient fossils reveal the oldest known vertebrates had four eyes.

Remarkably preserved fossil specimens show they could see the world using four camera-type eyes, reshaping our understanding of how vision evolved.

A new study by an international team of researchers, including a University of Leicester paleontologist, has thrown light on the anatomy of the creatures living more than 518 million years ago.

The discovery shows that ancient vertebrates had a far more complex visual system than previously thought — and that organs now linked to sleep once played a key role in seeing.

The research, published in Nature, focuses on "exquisitely preserved" fossils from the Cambrian period, a time when animals were rapidly evolving new body plans and senses in response to increasing predation.

The fossil vertebrates known as myllokunmingids were discovered in the famous Chengjiang fossil beds of southern China.

Professor Peiyun Cong of Yunnan University, who led the investigation, said: "These fossils preserve the eyes in extraordinary detail. We started by examining the obvious large eyes to understand their anatomy — and it was a complete surprise to find two smaller, fully functional eyes between them. Seeing that was incredibly exciting."

The painstaking and detailed investigations of the fossil eyes were made by first authors Dr. Xiangtong Lei and Dr. Sihang Zhang, who used high-powered microscopy to identify melanosomes — pigment-containing organelles responsible for coloration and light absorption in living eyes — in all four eyes.

Careful microscopic and chemical analysis revealed that these animals had two large lateral eyes and two smaller, centrally positioned eyes. All four eyes were similar to modern camera-type eyes.

Today, vertebrates see mainly using two eyes. A structure deep in the brain — the pineal gland — helps regulate sleep by responding to light and producing melatonin. In some fish, amphibians and reptiles, it can still detect light and is sometimes called a 'third eye'.

Professor Peiyun Cong said: "What we're seeing is that the pineal organs began as image-forming eyes. Only later in evolution did they shrink, lose visual power, and take on their modern role in regulating sleep."

Emeritus Professor Sarah Gabbott from the University of Leicester, who is one of the team, said: "Fossil eyes are incredibly rare — you wouldn't expect something as delicate as an eye to survive for hundreds of millions of years. Yet under the right conditions they can, and when they do, they open a rare window into how extinct animals saw and experienced their world."