system, a branch of the brain spe-
cialized in measuring brightness.
The basis for this process is the re-
action of cells to specific radiation.
Light changes certain pigments
that are contained in cells, and
the amount of altered pigment al-
lows a perception of light and dark.
If the eye uses several pigments
specialized in different radiation
wavelengths, color vision is the
result. If the organism groups the
light-sensing cells, a simple eye is
created, and if only radiation from
certain directions can reach this
light-sensing organ, directional vi-
sion becomes possible. The evalua-
tion of light perception via a more
complex central nervous system al-
lows the perception of movement
and distance vision.
The eye types to this point in eye
development conveyed very simple
information about their environ-
ment. An important step in the evo-
lution of vision was the pinhole eye,
which we see today in the genus of
marine mollusks known as Nauti-
lus, true living fossils. A fluid-filled,
hollow eyeball has a small open-
ing to the outside world through
which a thin light beam penetrates.
There is no focusing lens, so the im-
age that reaches the retina remains
schematic and out of focus. But this
type of eye captures a directed light
phenomenon to produce a simple
image of the environment.
The next developmental step was
the emergence of a dioptric appa-
ratus which, with a refractive lens,
projected a sharp image onto the
retina. Shapes could be recognized,
which was an enormous advantage
in avoiding predators and searching
for food or prey. This could be part
of the explanation for the Cambrian
species explosion, when most major
animal groups appeared on Earth
over a period of about 25 million
years. (Oxford zoologist Andrew
Parker’s Light Switch Theory holds
that the evolution of eyes that gave
species survival advantages was a
major driver of the rapid evolution
of life 540 million years ago.) ALL: D. KNOPThe pinhole eye, which the Nautilus still possesses today,
represented substantial progress in eye development.
This eyeless tetra of the genus Astyanax is completely blind, but finds
food via olfactory sensing and can orient itself with its lateral line
system. In the embryonic stage its eyes are still present, but adaptation
to a dark environment halts their maturation.