All vertebrates, including mammals and fish, possess two eyes, a trait that has persisted for millions of years. However, this was not always the case.
During the Cambrian period, a time of evolutionary experimentation, early vertebrates might have had four eyes, and these were likely high-resolution organs.
The Original Four-Eyes
A recent study published in Nature explored rare specimens of myllokunmingids, the earliest known vertebrates from the Cambrian era. These jawless prehistoric fish represent some of the initial experiments leading to the diverse vertebrate lineage we see today.
At that time, the oceans were described as a "Dark Forest," a competitive environment where the first significant evolutionary arms race took place. Fierce predators, such as radiodonts with their complex compound eyes, roamed the waters, while myllokunmingids were small, soft-bodied creatures, likely easy prey.
To survive in such a perilous environment, these early vertebrates may have relied on enhanced vision.
By reexamining fossils of Haikouichthys ercaicunensis and another unnamed myllokunmingid, researchers identified two "median" spots between the traditional lateral eyes. Initially thought to be nasal sacs, closer inspection revealed structures resembling advanced optical systems.
These structures contained melanosomes, tiny pigment packages found in the retinas of modern vertebrates. Even more astonishing was the discovery of a "distinctive, regularly ovoid structure" in both the lateral and median eyes, indicating the presence of a lens.
"We interpret this ovoid structure as an eye lens based on its shape, size, and position. This lens structure also occurs in the median paired dark spots," the authors noted in their research.
A camera-like eye, which uses a lens to focus light onto a hemispherical retina, creates sharp images rather than merely sensing light. Finding this configuration in lateral eyes is one thing, but discovering it in a second dorsal pair is remarkable. It implies that the earliest vertebrates may have possessed a 360-degree vision capability.
From Four Eyes to a Pineal Gland
If our ancestors indeed had four eyes, it raises two intriguing questions: why did evolution phase out this feature, and where did the additional pair go?
There are several theories suggesting why two eyes might be more advantageous than four. One reason is the "processing power" needed for such acute vision. Managing four high-resolution streams of visual data would demand significant neural resources and energy--a luxury not all animals could afford. Researchers also speculate that these creatures may have evolved into predators themselves, reducing the necessity for a 360-degree vision system as they spent more time hunting.
Moreover, researchers hypothesize that the second pair of eyes may have evolved into what we now recognize as the pineal gland.
The pineal gland is a small endocrine gland in the brains of most vertebrates, producing melatonin, which regulates sleep patterns in accordance with diurnal cycles. In some animals, this gland is linked to a light-sensing organ sometimes referred to as the "third eye."
This study supports the notion that this "third eye" did not originate as a simple light sensor but rather as a fully functional pair of camera eyes.
As we read this today with our two eyes, we are reminded that we are the streamlined descendants of a fascinating lineage that began in the Cambrian period, originating from a small fish that perceived the world in a way we can barely comprehend.
Journal Reference: Xiangtong Lei et al, Four camera-type eyes in the earliest vertebrates from the Cambrian Period, Nature (2026).