How could complex eyes have evolved?1
In this activity we will investigate several key questions:
 How could something as complex as the human eye have evolved by natural selection?
Could our eyes have evolved gradually through a series of many small improvements in the
detection and processing of visual information?
 Since very little fossil evidence is available, how can scientists learn about the evolution of
eyes?
This figure illustrates some of the complexity and similarities of human and octopus eyes.
(Figure from http://genome.cshlp.org/content/14/8/1555/F1.expansion.html)
The similarities between the human and octopus eye include:
 the overall shape and structure of the eye
 the iris and pupil, which regulate how much light enters the eye
 the lens which focuses light to form an image on the retina
 the retina which contains photoreceptor cells that respond to light and send signals
about visual information via the nerve cells
 light-sensitive molecules in the photoreceptor cells that consist of an opsin protein
combined with retinal (a form of vitamin A).
Evidence from Comparative Anatomy – Evolution of Eye Shape
One way that scientists can develop hypotheses about how a complex structure like the
octopus eye evolved is to compare the anatomy of different types of eyes in living evolutionary
relatives (other mollusks such as snails, nautilus, and limpet). None of these living mollusks was
an evolutionary ancestor of the octopus, but the different types of eye observed in these living
mollusks suggests a possible sequence of intermediate steps that may have occurred during the
evolution of the complex eye of the octopus. This evidence also indicates how each
intermediate step in the evolution of a complex eye could have been useful and contributed to
increased fitness (the ability to survive and reproduce).
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By Dr. Ingrid Waldron, Department of Biology, University of Pennsylvania, © 2013. A Word file for this Student Handout and
Teacher Notes are available at http://serendip.brynmawr.edu/exchange/bioactivities/evoleye.
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The figure below shows the structure of eyes in several types of mollusks, and the table below
describes the lifestyle of these mollusks. The light-sensitive molecules are called pigments
because they are highly colored; therefore, the photoreceptor cells are sometimes called
pigment cells and primitive types of eyes are called pigment spots or pigment cups.
Pigment spot
(in limpet)
Pigment cup
(in a snail)
Pinhole eye
(in Nautilus)
Eye with primitive
lens (in Murex)
This limpet
scrapes algae
off of rocks
and glides or
creeps along,
similar to a
snail.
This snail
feeds on
sponges and
moves by
gliding or
creeping
along.
Nautilus swims in
the open ocean
and feeds on dead
or dying prey.
This marine snail
feeds on other
snails, clams and
barnacles (bores a
hole to get at the
soft tissues inside
the shell).
Complex eye
(in octopus)
An octopus is an active
predator that relies on
vision to hunt prey. It
moves by swimming or
using its arms for
crawling.
(figure from http://www.britannica.com/EBchecked/media/74661/Steps-in-the-evolution-of-the-eye-as-reflected-in)
1a. A pigment cup, but not a pigment spot, allows an animal to detect the direction the light is
coming from. To demonstrate this, draw lines representing light coming from the left and from
the right in the figures below and show how:
 light coming from either direction can stimulate all of the photoreceptor cells (pigment
cells) in the pigment spot
 light coming from each direction can only stimulate certain specific photoreceptor cells in
the pigment cup (remember that light travels in a straight line).
Pigment spot
Pigment cup
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1b. How could a snail benefit from being able to detect which direction light is coming from? In
other words, how does a pigment cup contribute to increased fitness (the ability to survive and
reproduce)?
1c. A pigment spot can detect light versus dark, but not the direction the light is coming from.
How could a pigment spot contribute to increased fitness for a limpet?
2. Explain how a visual stimulus can produce an image on the retina of a pinhole eye. Begin by
drawing the pathway of light from the first and third dots in the visual stimulus in the diagram
below in order to indicate which photoreceptors would be stimulated by light from each of
these dots.
Visual stimulus:
•
•
•
re = retina with photoreceptor cells; ep = epithelium
(from http://commons.wikimedia.org/wiki/File:Haliotis_pinhole_eye.png)
A major disadvantage of a pinhole eye is
that very little light gets into the eye, so the
image is dim. As discussed in the next
section, an alternative evolutionary
pathway avoids this disadvantage.
Specifically, the evolution of a lens to focus
light coming through a larger opening can
result in a bright, sharp visual image. The
figure shows how a lens that bends light can
focus the light to produce a sharp image on
the retina.
(figure from
http://webschoolsolutions.com/patts/systems/focus.gif )
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Results from a Mathematical Model of the Evolution of a Complex Eye
 Researchers have developed a mathematical model of a sequence of small changes by
which natural selection could have produced an eye with the shape and lens observed in a
human or octopus eye.
 This model begins with a three layer structure with a transparent protective layer on the
outside, a layer of photoreceptor cells in the middle, and a layer of dark cells underneath.
 In this model, each small change results in slightly better spatial resolution (the ability of
light coming from different parts of the environment to stimulate different photoreceptor
cells). Improved spatial resolution results in a sharper visual image.
 The early steps in this model (shown in the left half of the figure) illustrate how selection for
improved spatial resolution results in an increasingly spherical shape, with a sequence of
steps similar to the pigment spot, pigment cup, and eyes with lenses in the comparative
anatomy evidence.
(figure from http://musingsofscience.files.wordpress.com/2011/01/nilsson-pelger_model_of_eye_evolution.jpg)


The evolution of a lens begins with increasing concentrations of special transparent proteins
in the region just behind the opening for light; these proteins increase the refractive index
(so light bends as it passes through the lens). Each tiny increase in refractive index results in
a small improvement in the ability to focus the light to produce a sharper image. Therefore,
natural selection would favor the gradual development of a lens (as shown in the right half
of the figure).
Even with very conservative estimates, this model indicates that natural selection could
produce the basic shape and lens of an octopus or human eye within less than 400,000
generations, which is a relatively short time in evolutionary terms. (Obviously, additional
generations may be required to develop the additional features of complex eyes.)
3. What does this mathematical model contribute to our understanding of how complex eyes
evolved?
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4. Explain why the ability to form images would be more useful for an octopus than for a
limpet.
Molecular Evidence – Evolution of Light-Sensitive Molecules
 Light-sensitive cells in all types of animals from jellyfish to humans use a fundamentally
similar type of light-sensitive molecule that combines an opsin protein with a molecule like
retinal (a form of vitamin A).
 The genes for the opsin proteins in a wide variety of animals have similar nucleotide
sequences. This molecular evidence indicates that all these opsin protein genes are
descended from an opsin protein gene that evolved very early in the evolution of animals
(more than half a billion years ago, long before there were any mollusks or vertebrates).
 The molecular evidence suggests that a mistake during cell division resulted in duplication
of a very ancient gene for a protein similar to opsin, and after that one of the copies of this
duplicated gene had a mutation that changed a key amino acid to produce an opsin protein
which can combine with retinal to form a light-sensitive molecule.
 This resulted in a very ancient animal that had two similar genes with different functions:
o the original gene that coded for the original protein (probably a receptor protein that
responded to a chemical signal)
o the mutated gene that coded for an opsin protein in a light-sensitive molecule (this gene
was the evolutionary ancestor for the opsin genes in all animals).
5. Explain why gene duplication can be a crucial initial step in the evolution of a new gene for a new
protein with a new function (e.g. the opsin protein in light-sensitive molecules).
6a. When were the opsin proteins in light-sensitive molecules first observed in evolution?
____ in ancient animals that were the common evolutionary ancestor of mollusks and vertebrates
____ in more recent mollusks and vertebrates
6b. What evidence supports your conclusion?
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6c. Are the similarities of the opsin-containing light-sensitive molecules in octopus and humans due
to:
___ homology (similarity which results from common descent) or
___ analogy (similarity which results from convergent evolution, i.e. similar characteristics
which evolved independently as a result of natural selection for a similar function)?
Concluding Questions
7a. Use the following evidence to decide whether the similarities between octopus and human eyes
in shape and having a lens to focus light are due to:
___ homology (similarity which results from common descent) or
___ analogy (similarity which results from convergent evolution).



Contemporary mollusks have a wide variety of types of eyes, including simple pigment spots or
pigment cups in many types of mollusks. This indicates that the common ancestor of all
mollusks almost certainly did not have spherical eyes with a lens. This in turn indicates that the
common ancestor of mollusks and vertebrates did not have spherical eyes with a lens.
Although the basic structure of human and octopus eyes is similar, there are some significant
differences in structure which result from significant differences in the way that human and
octopus eyes develop.
The results of the mathematical model indicate that natural selection could produce the basic
shape and lens of the octopus or human eye in a relatively short time (within 400,000
generations), which is much shorter than the roughly 500 million years since mollusk and
vertebrate evolution began.
7b. Explain why you chose homology or why you chose analogy.
8. Complete the following table to describe the evidence for some likely steps in the evolution of
complex eyes.
Steps in the Evolution of Complex Eyes
Evidence that Supports this Conclusion
Very early in the evolution of animals, lightsensitive molecules evolved by gene duplication
and mutation.
The shape of the eye probably evolved by
gradual changes from a flat pigment spot to an
indented pigment cup to a more spherical shape
with an opening for light.
The evolution of the lens probably began when
increased concentrations of proteins behind the
opening for light resulted in an increased
refractive index which improved the ability to
focus the light and form a sharp image.
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