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2 The Visual World
There is far more to seeing than having eyes. The evolution of animal vision
is one of the great epics of evolutionary development: Starting with photosensitive spots on single-celled animals a billion years ago, gradual changes have
led to the complex eyes of mammals today. But eyes do not suffice in order
for a creature to see. What the eyes capture are patches of light, dark and color
in rapid motion: This raw data must be collected and fed to a brain able to
assemble and interpret it. Finally, in the most difficult step of all, the brain
delivers to a subject’s awareness a coded image. The coded image is experienced
by the subject as a visual scene. In this final step the scene looks like something
to the animal subject—it carries meaning—and this ultimately is the purpose
of vision.
Life on our planet is mostly visual. It could have been otherwise: The earliest
animals might have developed senses responsive to other sources of information, such as electromagnetic fields or chemical messages, and any of these
senses might have evolved to be the dominant form of sensation. (In fact,
many species have secondary perceptual systems of these very kinds.) 1
What we observe, instead, is that up and down the phylogenetic scale creatures at every level and of every kind evolved with the sense of sight as the
primary source of information for navigation and action. Being able to see is
so very important to living creatures that in many phyla, more than two-thirds
of the brain is dedicated to functions involved in seeing. This is notably true
for man.
A Scene on a Very Small Stage
At the other end of the phylogenetic scale from us, there are insects so small they
cannot be seen with the naked eye—yet they have complex brains and exhibit
sophisticated visual behavior. Among them, the most intriguing are diminutive
hymenopterans related to bees and wasps and generally called fairyflies. 2 These
creatures are at the absolute limit of miniaturization for living animals. The
length of a typical fairyfly is between 0.13 and 0.25 mm—smaller than many
one-celled organisms such as amoebas. Yet they are fully-formed animals with
organs for digestion, circulation, reproduction, and most remarkably, they
have a fully developed insect brain. They have compound eyes as most insects
do, and though their visual powers are limited, they are fully visual animals.
Let’s stop momentarily to consider this: Here we have a real animal as small
as a speck of dust, whose eyes and brain can be seen only under a powerful
microscope. Yet it thrives in its habitat, which spans the Americas, Australia,
New Zealand and the South Pacific. It lives in a simple but sophisticated visual
2 The Visual World
There is far more to seeing than having eyes. The evolution of animal vision
is one of the great epics of evolutionary development: Starting with photosensitive spots on single-celled animals a billion years ago, gradual changes have
led to the complex eyes of mammals today. But eyes do not suffice in order
for a creature to see. What the eyes capture are patches of light, dark and color
in rapid motion: This raw data must be collected and fed to a brain able to
assemble and interpret it. Finally, in the most difficult step of all, the brain
delivers to a subject’s awareness a coded image. The coded image is experienced
by the subject as a visual scene. In this final step the scene looks like something
to the animal subject—it carries meaning—and this ultimately is the purpose
of vision.
Life on our planet is mostly visual. It could have been otherwise: The earliest
animals might have developed senses responsive to other sources of information, such as electromagnetic fields or chemical messages, and any of these
senses might have evolved to be the dominant form of sensation. (In fact,
many species have secondary perceptual systems of these very kinds.) 1
What we observe, instead, is that up and down the phylogenetic scale creatures at every level and of every kind evolved with the sense of sight as the
primary source of information for navigation and action. Being able to see is
so very important to living creatures that in many phyla, more than two-thirds
of the brain is dedicated to functions involved in seeing. This is notably true
for man.
A Scene on a Very Small Stage
At the other end of the phylogenetic scale from us, there are insects so small they
cannot be seen with the naked eye—yet they have complex brains and exhibit
sophisticated visual behavior. Among them, the most intriguing are diminutive
hymenopterans related to bees and wasps and generally called fairyflies. 2 These
creatures are at the absolute limit of miniaturization for living animals. The
length of a typical fairyfly is between 0.13 and 0.25 mm—smaller than many
one-celled organisms such as amoebas. Yet they are fully-formed animals with
organs for digestion, circulation, reproduction, and most remarkably, they
have a fully developed insect brain. They have compound eyes as most insects
do, and though their visual powers are limited, they are fully visual animals.
Let’s stop momentarily to consider this: Here we have a real animal as small
as a speck of dust, whose eyes and brain can be seen only under a powerful
microscope. Yet it thrives in its habitat, which spans the Americas, Australia,
New Zealand and the South Pacific. It lives in a simple but sophisticated visual