Because eyes rarely leave fossil traces,
it has been difficult to track their genesis.
But thanks to thorough research, includ-
ing the groundbreaking findings of Swiss
biologist Walter Jakob Gehring (1939-
2014), any doubts about Darwin’s state-
ments can now be laid to rest; the rela-
tionship between the individual eye types
is largely understood.
PAX6: THE CONTROL GENE
Gehring found that all eye types in the
animal kingdom arise through a master
control gene called PAX6. Whether jelly-
fish, fish, crab, human, or fruit fly, PAX6
is the main switch that triggers the devel-
opment of eyes in the organism. If this
gene is defective, the embryo remains
without eyes. PAX6 is able to set off the
entire cascade of more than a thousand
genes necessary for eye development.
In order to prove that PAX6 drives
and controls eye development, Gehring
and his co-workers also tried to grow eyes
on other body parts such as legs, wings,
or antennae in Drosophila fruit flies with
the help of PAX6. To the astonishment
of the professional world, this actually
succeeded, and the eyes were functional.
Moreover, eye formation could be trig-
gered in a fruit fly using the mouse PAX6
gene, demonstrating that the PAX6 genes
of vertebrates and invertebrates are func-
tionally identical.
EYE TYPES
The eye is a light-sensing organ, an out-
wardly positioned part of the nervous
WHAT ARE PHOTORECEPTORS?
Photoreceptors are specialized sensory cells that perceive light.
Even a single photon—the unit that forms light—can be registered
when striking a photoreceptor. The unexcited photoreceptor cell
of the vertebrate eye permanently releases a neurotransmitter that
inhibits subsequent nerve cells. When light is registered, this emis-
sion is suppressed so that a nerve pulse is transmitted and directed
to the brain.
Rod receptors contain a form of the visual pigment rhodop-
sin, which reacts to blue-green light with a wavelength of 500 nm.
Colors cannot be perceived, but the sensitivity is great, so this light
perception works even at dusk. Cone receptors use different visual
pigments and are specialized in particular wavelength ranges of the
light. They need significantly brighter light than the rods, but also
convey color information to the brain.
Color vision is the result of the fact that photoreceptors each
have only one visual color which has its absorption maximum at a
certain wavelength. Cone receptors are, for example, specialized in
red (625 nm), green-yellow (560 nm), green (535 nm), or blue-
violet light (455 nm).A B C D E F G H
Rod (1) and cone receptors (2) of the human eye.Schematic representation of
the retina in the vertebrate eye.
The yellow arrow indicates the
direction of the light.
A. Nerve fibers
B. Ganglionic layer
C. Inner plexiform layer
D. Inner nuclear layer
E. Outer plexiform layer
F. Outer nuclear layer
G. Photoreceptors
H. Retina1
2
For most coral fishes (shown is
Gramma loreto), the eye is the most
important sensory organ.