The earliest ancestors of human beings had more than two eyes?

The two extra eyes are light-sensitive structures on the top of the head, called the pineal gland and the parapineal organs. Scientists are still trying to determine all the functions of these organs, but it is already known that they play a key role in orientation and circadian and annual rhythm cycles.

Many lower vertebrates, such as fish and frogs, have pineal organs, but parapineal organs are less common. Krister Smith of the Senckenberg Institute, however, has long suspected that ancient lizards might have had both of these light-sensitive structures in addition to their typical two eyes.

"The idea first came to me while I was in graduate school investigating the diversity of lizards in Yale's fossil collection." "Different experts have given conflicting explanations for the third eye on a particular parietal fossil," Smith said. The parietal bone clearly has two large holes, arranged anteroposterior. And I remember there were two pineal organs."

Smith admits that the search for four-eyed ancient lizards is a "bizarre" idea, but she and her team still go for it.

They analyzed museum specimens collected 150 years ago at the Grizzly Buttes site in Wyoming's Bridger Basin. CT scans have revealed that two specimens of an extinct monitor lizard (Kensesaniva) have cavities that hold two extra eyes.

The findings, published in the journal Current Biology, reveal the first known four-eyed vertebrate with jaws.

"Scientists come up with ideas all the time, but many (or even most) will not succeed. The weirder the idea, the more likely it is to fail, "says Smith. When her hypothesis about the four-eyed lizard was confirmed, she was thrilled.

This finding helps us understand the evolution of extra eyes.

Dr. Smith and colleagues Bhart-Anjan Bhullar, Gunther Khler and Jrg Habersetzer guessed that all four eyes of lampreys evolved at the same time. But they say more fossils are needed to determine when each organ was formed.

Scientists already know that the earliest mammals, our ancient ancestors, had more than two eyes.

"Dry group mammals have three eyes," Smith explains. "It's thought that the nocturnal evolutionary stage of mammalian ancestry caused the loss of the third eye in those animals. Something similar might have happened to other tetrapods."

Many tetrapods become active at night and presumably don't need extra photoreceptors, so they lose their third eye.

Humans and most other mammals retain no residual evidence of a third eye.

"In humans, in particular, the brain has grown so much that it covers the pineal organ so thickly and completely that it is almost impossible for it to reach photons," Smith said. "In addition, the paraspineal organs are thought to disappear completely."

But in mice, the pineal organ is close to the body surface, and some light-sensitive proteins are still expressed in the pineal organ of the mouse embryo. This suggests that the third eye is not completely lost in rodents.

Smith and colleagues aren't sure which organ (pineal or paraspinal) caused early mammals to form a third eye. If they can solve this problem, they should be able to better understand why the third eye gradually disappears in mammals.

We know that the third eye (pineal organ) of contemporary lizards differs from the third eye of other jawed vertebrates.

"It should be emphasized that the third eye of a lizard is different from the third eye of a frog or fish," Smith said, "but this idea is not universally accepted. If we find two species in which both pineal organs have evolved into eyes, the first one being a typical third lizard eye, then we can prove this idea."

Although the two extra eyes of extinct monitor lizards were close to each other, the researchers believe they did not function as a pair of eyes like normal eyes.

"They're really close together, but they have different central nervous connections to the rest of the brain," Smith said.

The Kansasaniwa lizard lived in Wyoming and parts of Europe during the Eocene Epoch 48 million years ago. The first fossil lizard found in North America was the Kensesaniwa lizard. Now, the animal is famous again, at least for now.

"This new finding shows that evolutionarily, under certain conditions, it's very easy to form a complex organ on your own," said Bla, the Yale paleontologist. "The eye is generally thought of as having extremely complex structures. In fact, the brain is always waiting for an opportunity to make eyes."

The eye is "an important part of the brain," he said, and its formation "is part of the process by which the brain makes connections with parts of the skin."

The central nervous system develops from the "innerfold, the outermost layer of the embryo," Smith said. This layer is called the neuroectoderm.

Previous studies have shown that octopuses can "see" the outside world with their skin. Octopus skin contains a photoreceptor found in the eyes called rhodopsin. Later studies found that pig-wrasse, which can change color, can also use their skin to "see" the outside world. Perhaps the skin of other animals is also sensitive to light, including humans.

For now, Smith and his colleagues are focusing on lizards. Smith and Burra explain that the "lizard transition" (when lizards first form a third eye) is related to the development of vision in many different vertebrates. They want to understand when this important change took place.

Smith said she and her colleagues hope to analyze the Grizzly Buttes fossil collection again, even though the fossils are "fairly fragmented and badly damaged." Such studies, she said, "highlight the important role that museums play in preserving evidence of biodiversity, past and present."