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As we observe the world around us, what we see is shaped by the way our eyes process the light
that’s bouncing and absorbing and passing through our surroundings. For many of us,
what this means is that when we look at this clip of cyanobacteria, we see green strips sliding
along a blue background. For others, depending on their eyes and any other corrective measures, the
colors may be different, or the edges blurred, or the image may not exist at all. The final product
will come down to the physical and chemical reactions that are taking place in our eyes
in response to light, and how that informs the biological processing that comes next.
Microbes don’t have this sort of complex visual processing, but many still need to know where
the light around them is--whether that’s to seek out food or avoid hidden dangers.
They may not see things the way we do, but the mechanisms they have in place
allow them to respond to light in extraordinary ways to meet the most ordinary of needs.
Perhaps the most primitive of these methods are light-sensitive pigments called phytochromes.
But when we say “primitive,” we don’t mean to diminish their importance
or their complexity. If whole ecosystems are built on photosynthesis, then phytochromes
are essential to life as it currently exists. Scientists first discovered phytochromes in
plants, where they helped answer longstanding questions about how plants respond to seasonal
changes in light. But the ancestors of those plant pigments are found in cyanobacteria
like oscillatoria, whose phytochromes help regulate growth, movement, and photosynthesis.
These phytochromes respond not just to the amount of light, but also the color of it.
And remarkably, cyanobacteria phytochromes can respond to a wider range of colors
than plant phytochromes, which makes sense if we think about where
cyanobacteria live. They live in water. Because of how water absorbs light,
there’s usually more red light at the surface and then more green and blue light the deeper
you go. If a cyanobacteria were to have a pigment that detects only red light or only blue light,
it would end up restricted in where it could live. So instead, these simple little organisms perform
a complex calculation called chromatic adaptation, calibrating the amount of their
various pigments so they can maximize how much light they’re getting from their environment.
But cyanobacteria are unicellular organisms that want light. What about those that don’t?
Both blepharisma and Stentor coeruleus are well-known for their photophobic nature.
The reaction is straightforward: when confronted with light, these microbes start backpedaling.
This response is set off when light interacts with their pigments,
the pink blepharismin in the blepharisma, and the blue stentorin in Stentor coeruleus.
These pigments are used as a toxin against predators. But they can make their own organism
sensitive to light to a dangerous degree, providing excellent motivation to avoid
said light. And these photophobic responses may also help the blepharisma and stentors
avoid predators with better vision than their own.
These photosensitive pigments are like nature’s way of unlocking the foundations of vision.
They are not themselves a specialized structure. To see an example of a specialized structure,
let’s look at that bright red eyespot on the euglena. Also called a stigma, the eyespot
is not actually an eye. It’s more like a sunglass for the true photoreceptor.
That structure is located close to the euglena’s flagella, and it’s made up of around 50 layers of
stacked membranes that hold hexagonal arrangements of roughly a million photoreceptor proteins.
The stigma shades the photoreceptor, but just on one side of the euglena.
And as the microbe rotates and different wavelengths of light filter through and then
get shaded by the stigma, the euglena is actually getting information on where it can find light.
As we shift from unicellular to multicellular organisms, we start to see what microscopic eyes
look like. One of the important evolutionary developments was the formation of a cup
shape in these eye-like structures, which helps provide spatial information.
Some of these animal eyes are mysterious, like those beady little buttons atop many a
tardigrade’s heads. Sure, scientists have documented that many tardigrade species
have what they call “inverse pigmented eye-cups,” but what do they see? Well, we do not know.
Others, like the planarian, have a pigmented eye-cup connected to photoreceptor cells,
which actually provides enough similarities to our own eyes to make them useful for medical studies.
And as the animals get larger, so does their potential for complex visual systems. Similar
to many insects, Daphnia have compound eyes that are made up of units called ommatidia, which focus
light onto photoreceptor cells to form images. The larger the eye, the more information the
daphnia and other animals with these kinds of image-forming eyes can get. But size and
complexity come with trade-offs. Sure, these eyes help the animal navigate and find food. But these
are costly organs to build and maintain. One study found that the retina in a blowfly’s eye
took up 10% of its resting metabolic rate. Eyes and these analogous structures have evolved
in response to what we need to process from our surroundings. Needs inform evolution, which in
turn, informs new needs. And light can provide for those needs, but it can also pose its own dangers.
In The Origin of Species, Charles Darwin famously describes the eye as an organ
of “extreme perfection.” He introduces it as an example of biological complexity that may at first
pose some consternation to those considering natural selection.
After all, how could something so remarkable be made through iterations and iterations of tiny,
gradual changes? But in Darwin’s search through natural history, the case for stacking gradual
change upon gradual change seems less improbable, and science has continued to support his theory.
With that said, it is tempting to get caught up in this story that has been cultivated over
eons and relate it entirely to the structures that make up our own eyes. That was our entry
point into this discussion after all. But one of the most remarkable aspects of evolution is the
understanding that it is not something that was leading to us. Evolution has led to a whole world.
All of these remarkable structures and chemicals have their own unique properties that suit the
organisms they reside in, allowing creatures that, yes, may be simpler than ourselves
to use systems that are not just good enough for their survival, they are ideal for their
survival. These organisms are thus able to survive in ways that you and I could never be capable of.
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
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