Humans have incredible eyes: We can sense shapes, depth, and brilliant colors—
but a hallmark of our vision is processing images from both eyes on both sides of our brain
at the same time, something known as bilateral visual projection.
All mammals are capable of this type of vision, but fish were thought to have a scaled back version:
visual information from one eye projects only to the opposite side of the brain.
That led scientists to think that bilateral vision emerged more than 375 million years ago,
when fish moved from water to land.
Such vision could have been an advantage to four-limbed tetrapods that hunted and lurked
in the night.
But is this what really happened, or were scientists just a little too eager to take the bait?
To test the old theory, researchers turned to modern techniques to study fishes’ visual systems.
First, they selected fish from two “pools”: One group, teleost, makes up 95% of all living fish.
Teleosts separated from the rest of the bony fish 300 million years ago,
after their entire genomes were duplicated.
Some think this gave rise to the astounding diversity seen in these fish
The other group—some of which are referred to as “living fossils”—are known as non-teleosts.
These ancient-looking fish are more closely related to the humans.
To find out which fish had bilateral projection, scientists injected fluorescent tracers into
their eyes: one color for the left eye, and another for the right.
They tested seven teleosts, including a mudskipper, which spends most of its time above water,
and a largescale four-eyes, which can see above and below the water at the same time.
They also tested four non-teleosts, including a sturgeon, spotted gar, and an Australian lungfish,
which can breathe air.
Two days later, researchers removed the brains of the fish and examined them with a specialized
3D fluorescence microscope.
Bilateral vision works in mammals when bundles of neurons in the retina send information
from one eye to both sides of the brain.
They found that in most teleosts, these bundles of neurons sent information to only one side
of the brain—the opposite side.
But in non-teleosts, the bundles projected that information to both sides of the brain,
like mammals.
Some of their neurons sent even more information to the same side of the brain than mice.
Surprisingly, the gene linked to same-sided projection in mammals was missing from non-teleosts,
but the existence of bilateral vision in non-teleosts suggests that it emerged early in evolution—
and didn’t coincide with the transition from water to land.
Which begs the question: How did bilateral vision give fish an advantage?
To find out, scientists plan to find the gene behind same-sided projection in non-teleost fish,
and test how removing it impacts their ability to see.
If they’re successful, our fish ancestors could soon school us with new theories on
how human vision evolved, even as old ones vanish before our eyes.