Thank you to Skillshare for supporting  this episode of Journey to the Microcosmos.

If you are one of the first thousand  people to click the link in the description

you can get a two-month free trial  of Skillshare’s Premium Membership. 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. If you’d like to learn more about using  your eyes to observe the world around you

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If you want to see more from our master of

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