Every Eye In The Animal Kingdom

[Narrator] Did you know that ambush predators tend

to have vertical slit pupils to better pinpoint their prey

while grazers tend to have horizontal pupils

to better scan the horizon for incoming movement?

We created this tree of life that focuses on the evolution

of animal eyes and we're gonna walk through it

with professor Lars Schmitz.

I'm Lars Schmitz.

I study vision and the evolution of eyes

and I just love everything about eyes.

[Narrator] We'll start at the base of the tree

with an early branching fork of animals called cnidaria.

The boxed jellyfish actually have 24 different eyes

and clusters of six eyes each.

One pair of these eyes and each of these clusters

are not unlike our own eyes.

They're camera type eyes.

Our human eye works similar to a camera.

We have the aperture, the pupil that lets the light in

behind which we have the optical system.

So what we nicely see here is the curve corneal,

the optical apparatus in front of the eye,

and the second optical element,

the lens, it's a bit flatter.

Those together refract light onto the retina

and forms an image.

To stay with a camera analog,

this will be where the film will be or the receptors.

Cubozoas are really interesting in

that they don't have a centralized nervous system.

They have a nerve net which allows them

to process visual stimuli,

but they don't have a real brain,

so it's actually really puzzling

how they can actually interpret those visual signals.

[Narrator] From here, the tree of life

has a major divergent path.

We'll follow first the site of the protostomes,

which includes insects and many deep sea animals.

Scallops.

They actually have dozens of eyes around their mantle.

Every single blue dot you see in this image,

these little blueberries, those are eye structures.

This ring of blueberry eyes is actually useful for them

as a defensive mechanism

and a little bit of a navigation help.

The way the scallops form images in their little blueberry

eyes is very different from what we do.

It has a mirror that's sitting behind the retina

and that mirror is used

to focus the light back onto the retina.

So really a complicated mechanism,

but it works well enough to get you an image of the scenery.

The resolution is really not that good,

but scallops can actively move so they can visual navigate

to an extent and also when they see something approaching,

they can shut down and close down and protect themselves.

[Narrator] Let's look at a few examples

of eyes from some cephalopods.

The pinhole eye of the nautilus works just like

the pinhole camera and has a very small opening

for which light enters

and then an inverted image is formed on the retina.

So let's take a look at the human eye model here.

We take out all the optics, essentially.

Light's coming in from right here,

and then the image as crude as it is,

falls directly onto the retina.

The smaller the pupil, the better the resolution

of the resulting image.

But the issue there is if you make the pupil really small,

the image will be dimmer and dimmer

the smaller the pupil is.

They do have a central nervous system

to process visual information.

Actually a little bit of a mystery

what exactly they're using that eye for.

The resolution isn't super good.

The image will be a little bit dim, so it's kind

of limited in what it can see with that eye.

[water trickling]

Octopus have chambered eye type

and in difference to the nautilus, this one has a lens,

and what we have in common here is this deep eye cup,

a pupil that's really well defined.

That's the only part where light can enter the eye

and nowhere else.

If light from all directions could enter the eye,

we wouldn't know where it's coming from.

One really important difference between octopus

and our own eye,

we have a retina that is inverted.

The photoreceptors are actually pointing

away from the light.

And the axons that transmit the signal

approach the incoming light.

But in octopus, the retina is the other direction

where the photoreceptors pointing towards the incoming light

and the axons going away.

The side effect of the inverted retina that we have,

the so-called blind spot

in the retina where we do not have any photoreceptors.

So in this model we can see the optic nerve

really clearly right here.

So this is where all the axons from the photoreceptors

are bundled together and exit the eye

and transmit the signal off to the brain.

But in this part right here where the axons exit,

that's where we cannot have any photoreceptors,

so that's our blind spot.

From my understanding, it's not really well known

how exactly and what may explain that pattern.

There may be a small advantage

of having the photoreceptors a little bit further back.

The resolution of the image may be a tiny little bit better.

Personally, I'm not sure if that's a huge reason

that would provide additional evolutionary fitness.

The bottom line is our retina, even though it's inverted,

it works well enough.

[water trickling]

One thing that's really interesting about squid

is their cool pupil shapes.

If we just look at squid, here are two examples.

On your right, you see this U shape,

on the left, we have this W shape in here,

so quite a bit of variety.

The function of these pupil shapes is a little bit

of a mystery, and a really interesting hypothesis

in this context is color vision.

Color vision in squid is not well supported behaviorally,

but it would make sense given

that they use colors for signaling, for example.

In humans in our own color vision,

we use three different opsin types.

So three different photo pigments

to distinguish different colors.

So we have red, green, and blue.

Depending on the strength of the relative excitation

of these pigments, we can infer what color wavelength

we're actually looking at.

In squid, we don't have different opsin types.

We have one opsin, the image will be essentially

a grayscale image.

How can they see color if they only have one opsin type?

Many animals correct an optical problem,

chromatic apparition.

Chromatic apparition results from the fact

that shorter wavelength light is refracted more strongly

than longer wavelength light.

If you have blue light versus red light,

the blue light will be focused a little bit

before the retina,

and the red light a little bit behind the retina,

so you get a little bit of an offset.

Now this is something that most animals

actually try to get rid of.

There's the hypothesis that these pupils

actually accentuate the chromatic apparition.

They make it worse,

and they can use that to tell

what color light they're actually looking at.

This blur hypothesis is a bit

controversial in our community.

Some people say while theoretical possibly,

the practical benefit isn't all that big.

The key will be to test how these animals perceive colors

and if there's good behavioral evidence to find out.

[Narrator] As we head back through the tree of life,

let's stop to talk about the flatworm.

This little fellow has two very conspicuous eyes

right here.

It's a very simple eyes, like little eye spots.

What we see here is eyes

that are really not good from our perspective,

but are beneficial to this aquatic flatworm.

Essentially, it would help them to avoid getting eaten.

These eyes are consisting of just two cell types.

They have photoreceptors in a shallow cup.

That cup is shielded by pigmented cells,

so light can only reach these eye cups from above

and a little bit from the side,

but it cannot reach the eye cup from below,

because that's where the pigmented cells are.

This flatworm would be able to tell

if it's in bright versus dark environments

and it would know if the light is coming from directly above

or perhaps a little bit from the side.

Now that seems like utterly useless to us,

but for this flatworm, it's good to know like it's bright,

it's exposed, it's in the dark,

it's perhaps a bit protected.

[Narrator] Now let's cover the other fork

of the protostomes, which include insects,

spiders, and some other arthropods.

Most spiders have eight eyes.

In the jumping spider,

the two front facing eyes are clearly enlarged.

That tells us that that pair of eyes is most important

for that particular spider.

They're actively hunting during the day.

It would make sense for them

to have like forward-facing eyes that have high acuity

so they can see their prey better.

They have eyes that more or less look like a camera-type eye

that we have seen before.

One lens, one aperture in here,

we see this quite nicely.

It's a different principle, different structural principle

than pretty much all the other arthropods.

A little bit of a mystery how they evolved.

It may have been derived from a compound eye,

but how exactly, we don't know yet.

Having four pairs of eyes

that are surrounding your head gives you essentially

perfect parameter vision,

splitting the job into different parts.

The accessory pairs could be used to scan the environment

for any possible movement,

whereas the forward facing eyes in this case have really

good acuity and give you lots of detailed information.

Mantis shrimp.

One of the most bizarre animals.

They're really cool, they can throw really hard punches

and they're super fast.

These compound eyes are situated in stalks

that kind of reach up a little bit.

They truly have panoramic vision, 360 degrees,

no problem whatsoever.

Mantis shrimp are also known to have

not just three opsins that we have,

but they actually have 12 opsins.

For many years it was thought

that mantis shrimp had superb color vision.

It turns out that color vision

of mantis shrimp isn't actually all that good.

They process that information very differently from us.

Our opsins have very broad range over which

they actually respond to light.

So we have three of these different peaks

and they all overlap, and they're excited

to different degrees from different wavelength's light.

In mantis shrimp, having 12 opsins,

each of them probably captures a smaller range

of wavelengths so they have more like discreet channels

instead of like largely overlapping absorption ranges.

So and that's probably explaining why they actually don't

have superb color vision, still good, but could be better.

[Narrator] Now let's look at some examples

of arthropod eyes.

Maybe let's start with the fly eye right in here.

This style of eye is called a compound eye,

because it consists of many different individual lenses.

The resulting image as a whole is a mosaic

of all the different images put together.

Probably don't think of our eyes as simple quote unquote,

but compared to these compound eyes,

they actually are pretty simple because we have one lens

and these have thousands of lenses.

We have this really like, these bulging eyes,

so they're spherical,

but they're coming towards us,

so they're kind of inside out from our own eyes.

The part of the eye that's here will not receive light

that's coming from behind.

The part of the eye that's here will get light from the side

a little bit from the front, but not from the left side.

It gives you really good idea where light's coming from.

The flies cannot move the eye structures around,

because these are all little crystalline lenses.

No eye movements in here or lens shape changes.

This is what you got.

Depending on the position of the eyes

and the orientation on the head,

many insects, including these flies,

can essentially have full panoramic vision

all the way around, 360 degree vision.

[insect chirping]

So the elephant hawk moth has eyes

that are very light sensitive

and it can see colors at night, which is really hard to do.

We only see this in these hawk moths and also in geckos.

So different type of compound eye.

We call this the superposition eye.

In the apposition eye, what we talked about before,

we had each ommatidium producing an average image

and those images were put together in the mosaic.

Here what we see in this compound eye,

there's a separation between the lenses

and the retina where the image is received.

So we have a projection of an image onto the retina,

so we have a single image

that's being formed instead of a mosaic.

It allows for much brighter images to be formed.

So this eye type is often found in insects

that are active at night.

Many of the superposition eyes have a little

layer that's reflective.

It's called the tapetum.

The tapetum lucidum to be specific.

You may have seen this when you're driving at night

and you see a deer in the headlight,

the eyes are like popping up really bright,

this eyes shine.

So imagine you have light arriving at the retina.

Many of the photons are being absorbed

by the photoreceptors, but not all.

Those that travel through get to the tapetum,

which is reflective.

They're bouncing back

and going through the photoreceptors again

for a second chance to be absorbed.

We see this tapetum evolving several times independently

in all groups that are nocturnally active.

[insects chirping]

One thing we can see in this dragonfly eye,

the lenses here are relatively small,

so the smaller the ommatidia in this case,

in this particular eye type, the better the acuity

or the better the resolution of that image.

Now one thing that we see often is that the ommatidia

change their shape depending on where you are,

horizontal, or on top, or more towards the bottom.

So they have zones of high acuity

that tells us something about how they live

and what they're going after,

what they're trying to monitor.

One thing that is a limitation

to compound eyes is this design structure.

If these animals want to make a really high acuity eye,

we need very long focal length

and with this kind of setup, that's not gonna work.

They would need to have eyes

that are a hundred times bigger

than what they can really fit on the head.

And I mean their visual ability is pretty good.

I mean, they're tiny animals but can still see

quite a bit, good enough so we cannot catch them, often.

It tells us that their visual processing

must be reasonably fast.

So this is quite incredible task actually,

and they do it well.

[Narrator] Let's go back to the trilobite for a moment.

Existing in the Cambrian era, trilobites provide some

of the oldest eyes in the fossil record.

So here is the pair of eyes that this trilobite has.

You can imagine it's really hard

to find squishy eyes in the fossil record, right?

These compound eyes with their crystal lenses have a bit

of an advantage to become fossils,

and this is really important for us

to study vision in deep time.

For example, we can look at the size

and the shape of the lenses

and that tells us a bit about how the acuity

and sensitivity must have been like.

And this particular type of trilobite,

which is a Phacops, the lenses are quite large

and this is a really interesting cue for us

to study the behavior in ecology of these fossils,

which is otherwise really hard to do

with just the fossil record.

Their large size would suggest to us

that this animal is probably able

to see in very dim conditions,

so it was either in deeper water

or primarily active at night.

[Narrator] We'll now double back

and explore the other side of the tree, the deuterostomes.

And we'll start here with our underwater friends,

the echinoderms.

Sea stars have light sensitive cells

at the top of their arms.

Mm, it's not really image forming,

it's really low resolution vision,

but it's good enough for them

to tell shade from bright parts of the sea floor

so they can use this to stay hidden

for protection from predators.

Again, adding to the fact that even like very rudimentary

visual structures are useful

for the organism that way natural selection easily explains

the evolution of sophisticated eyes.

Nilsson and Pelger determined in the '90s,

with a very conservative computational model

that it would take only about a million years,

256,000 generations to evolve from patch of light sensitive

cells to a complex image forming eye.

And if you think about like how much time was available

to these organisms to evolve eyes,

that's just like nothing, like literally like a blink.

[water trickling]

Similar to the sea star,

sea urchin have light sensitive cells

that are distributed across their little feet,

and this is another example of diffuse visual system

that kind of gives the information about the presence

or absence of light.

Not well studied how widespread photosensitive cells are

across the kind of echinoderms.

None of them are known, at least to the best

of my knowledge, to have full image forming eyes.

But photosensitivity is probably more

widespread than we know so far.

Not many people are looking at sea urchins

in the visual structures, people should, though. [laughs]

[Narrator] We move on now to chordates,

which are all vertebrates,

including fish, amphibians, reptiles, and mammals.

First up are the fish.

You're looking at the four-eyed fish.

Actually it doesn't have four eyes but it has four pupils.

One set of pupils that see through air,

and the other set of pupils they see through water.

Upper pupil, lower pupil.

And the reason for that is

that once the eyes pop outta water,

the cornea of these eyes becomes functional

and helps with the refraction and bending of light.

So it helps with the formation of images.

Underwater, the cornea does not work at all

and the lens is the only optical element

that fish have at their disposal to form an image.

When seeing it through air, the lens can be flatter,

meaning it's not as strong

because you have the cornea too.

In the four-eyed fish, Anableps, has it both.

So the lens is completely irregularly shaped,

functionally spherical for underwater vision,

but flattened functionally for seeing through air.

Vision through water and vision through air

is also different from another perspective.

We take one step back and think about like how much light

is actually available and how far you can see underwater

versus seeing through air.

It's a huge difference.

Vision is metabolically actually pretty expensive,

similar to brain tissue.

It has to be maintained, it's very costly.

From an evolutionary perspective, big eyes are expensive.

We looked at exactly this step in the history of vertebrates

where tetrapods came on land.

We had the evolution of vision from seeing through water

to seeing through air,

and that transition was coupled with a significant increase

of eye size at that time.

Having big eyes really only pays off,

evolutionarily speaking, if you can see a long distance.

[water trickling]

In blennies, a group of really colorful

and diverse reef fishes, we have independent eye movements.

One eye is stationary, the other one is moving around,

and one thing that's interesting about this pupil shape

is that if you look really closely,

we can see that there's a little gap in this teardrop shape.

What we see here is an example for a different way

to focus the image.

Humans and many other vertebrates that live on land,

the lens shape is actually changed when we focus on

different objects in the visual field.

Now, fish don't do that.

Fish have a spherical lens that are actually really hard,

almost like a marble and it cannot change its shape.

What these fish do is move the lens around

to focus on different parts in the visual scene.

This little blenny wants to focus on something

that's directly in front of it.

What it will do is it will move the lens from here into this

little gap up there and then focus on it.

That's called the lens-less space or the aphakic gap,

and that's something I've studied in quite a bit

of detail looking across like many different reef fishes

and understanding how eye shapes and sizes change

with the ecology and lifestyle of these animals.

So if you look for example at this soldierfish,

in here we see that we have a very large eye

and a very large rounded pupil

and that helps to collect more light.

This is a nice contrast to the blenny, which is diurnal,

where we have this pointed pupil shape

and the eye is actually quite a bit smaller.

This other soldierfish here,

there's hardly any lens-less space

that's like fully expanded.

While we can constrict and dilate the pupil,

these fish can't.

If it's bright or dark,

that pupil shape will always be the same.

Not ideal, because if they're exposed

to very rapidly changing light levels,

they're kind of screwed,

may be completely overexposed to light

and actually not see that much during that time.

Now fish have an interesting mechanism to deal

with changing light levels.

If you stay with a camera analog,

they can switch out the film

and change it for different light sensitivity.

Vertebrates have rod and cone photoreceptors.

Rods are really light sensitive

but don't give you any color vision.

The cones are less light sensitive

and they provide color vision.

When fish approaching dusk, there is migration

of the photoreceptors within the retina.

So what they're doing is they're switching out the cones

for rods, the cones are withdrawn and shielded by pigment,

and the rods come out and become functional.

The problem with that mechanism,

even though it seems brilliant, it's not fast,

it takes about 20 to 30 minutes,

which is a time for predators to strike.

[dramatic music]

This is a very hectic and chaotic time on the reef.

[frog croaking]

[Narrator] We'll explore now the amphibian branch

where animals need the ability to see both

above and below water.

Frogs are known to have charismatically big eyes

and what's really cool is that they have a ton

of variety in terms of the pupil shapes.

So starting with the vertical slit pupil in here

where we have the pupil extending,

so from the bottom towards the top,

and we can contrast that perhaps

with a horizontally aligned pupil that we have here.

Our simple circular pupil that we have here.

So many aquatic frogs tend to have that,

but this is much different.

For example, from this inverted triangle shape in here

where we have like this pointed tip pointing downwards,

we also have fan shapes that can be inverted

or regular, so variations to the same theme.

We also have pupil shapes that are shaped like this,

this diamond here where we have like six major corners,

which is really unique.

The diversity is just stunning.

What are these different pupil shapes good for?

With this particular study,

they didn't really find a strong ecological

signal in that data.

None of these pupil shapes which really tightly connected

to a particular type of ecology except for those

of aquatic frogs, which is perhaps the, hate to say it,

most boring pupil shape, it's just circular like ours.

In a way, not finding a clear ecological signal

makes it really hard to find a clear function

for these different pupil shapes.

Traditionally, we have vertical slit pupils associated

with predators and horizontal pupils in prey species.

This model, we would assign this pupil shape to a predator,

and this pupil shape rather to a prey species.

In frogs, this is not supported by the data,

so evolution is never selecting for the optimal

or the single best structure.

There's many different ways to do something.

They all get the job done, it's works well enough

and hence we see this diversity.

[birds chirping]

So geckos are really light sensitive.

They're primarily nocturnal, probably say,

well, they probably have really good rods,

cones for day vision, rods for night vision,

but geckos do everything with just cones.

They have evolved from lizards that have lost their rods

during their evolutionary history.

The ancestors were largely diurnal and day active

and they secondarily return to life at night.

Geckos have modified the cone for the receptors to be able

to work in the dark, so they do all that night vision

with the machinery that had evolved originally

for day vision.

We know that gecko eye is a very, very,

very light sensitive.

They have to make sure that they're not overexposing

their retina to too much light, so they really have

to constrict their pupils a lot to avoid that problem.

What we see here is that we have a series

of four little pinholes that are being formed in here.

They minimize the amount of light that gets into the eye,

but that gives you a really large depth of focus,

so it will be really hard to get any distance estimation.

One hypothesis is related to Scheiner's discs,

which is used by ophthalmologists.

Essentially, for Scheiner's disc you use two

tiny little pupils and then you determine if that's resolved

or if it's not resolved.

And then you know if there's any vision deficiencies.

Adding the string of four pupils on top of each other,

a decreased depth of focus, you introduce some blur

and that blur is useful to judge distances.

[rattlesnake rattling]

So snakes are a bit odd.

They can see heat, for example, pit vipers,

the name pit comes from a pit

that would sit pretty much right here in front

of the eye lined with thermoreceptors

and the information that's coming from that pit goes

to the same brain region where the visual information goes,

and that's actually combined with visual information.

They're a bit odd among the vertebrates.

For example, they don't have any bone structures

within their eyes that many other reptiles do.

They don't have the so-called scleral ring.

The shape of the snake eye,

traditionally there have been two hypotheses.

One says that the shape of snake eyes evolved

because snakes went through a phase in the evolution.

they were mostly burrowing underground,

which came with a lot of modifications to the eyes,

which they didn't really use.

Alternate hypothesis was that snakes were aquatic

early on the history

and that could explain many of their somewhat odd structures

and similarities with other organisms in this.

[insects chirping]

Chameleons can move the eyes independently,

they can fixate on one object while moving the other eye

and scanning its environment for potential predators.

So for all the animals that we have talked about so far

that have a lens, those are converging lenses

where the image is focused on one part.

Now in chameleons we do find a negatively powered lens,

meaning it's actually diverging, it's enlarging,

its magnifying the image on the retina,

but it's looking at a very small part

of the visual scene surrounding it.

Essentially it's using its eye

as somewhat more like a telescope.

It also comes with like a depth of focus

and by focusing through that visual field like close

and further away, it can judge distance.

They need that visual acuity and the distance perception

to actually successfully capture the prey

with a tongue flick.

[Narrator] As we make our way through the chordates,

we're now looking at reptiles, which include squamates,

archosaurs, and also surprisingly, birds.

[reptile growling]

Among alligators and crocodiles,

we find really pronounced vertical slit pupils.

Really good examples for ambush predators.

One idea is vertical slit pupils enhance the way predators

can see prey species standing on the ground,

and they really pop out nicely from everything else

that's kind of more towards the horizon.

That's a little bit blurred.

Highlights vertical contours,

kind of blurs horizontal contours,

objects that are closer maybe in focus,

objects that are further away are not in focus,

and that is useful to gauge the distance.

So one other really interesting thing about alligator

and crocodile eyes is their position as well.

They live at the water air interface.

Their eyes are just poking through the water surface

and they kind of scan everything that's surrounding them

to find possible prey items and that is enabled

by having the eyes in the kind of upward facing position

above the water.

[birds chirp]

Birds.

Okay, our living dinosaurs,

let's start with the owl, perhaps.

I mean, amazing birds, forward facing eyes.

They have really good night vision,

but they also have really quite good acuity,

so they actually have eyes that kind of do it all.

Owls have really found a way to maximize the size

of their eyes for their given head

and that gives them a somewhat odd eye shape,

which is called tubular.

And normally an eyeball is shaped kind elliptical,

and in owls, the sides are kind of cut off.

It's more like tubular.

For a wide eyeball,

there was just not enough space.

The eyes are so big

that the eyes cannot move within the eye sockets,

they have scleral rings that support the eye structures.

The eyes are wider inside the eye socket

than the eye socket diameter is.

They're filling out all that space.

You see that kind of eye shape also in in deep sea fish,

totally different group independently evolved

for likely similar reasons, trying to make the eyes as big

as possible for the space they've got.

They can't move the eyes within the eye sockets,

but owls can move their head exceptionally well,

almost doing a full turn,

and that gives them that peripheral vision

that they would need to detect something behind them.

[birds chirp]

We often think of ourselves as having the best eyes,

but that's actually not true.

Whatever we do, the eagle does it about twice as good.

So they can see things that are half the size

from the same distance and they do it with eyes

that are actually somewhat smaller than our own eyes.

That's remarkable.

In their zone of most acute vision in their retina,

they're packing the photoreceptors as densely as possible

and you can somewhat compare that to the number

of pixels you have on a display, for example.

The more of these pixels you pack in there,

the better the resolution,

and that's what these eagles have pushed to an extreme.

There's a physical limit to how small

a photoreceptor can be.

It cannot be narrower than the wavelength of light.

These eagles push at the physical limits of visual acuity.

[Narrator] And now towards the extent

of the deuterostome branch of the tree,

we find the mammals.

[horse whinnies]

Horses have among the largest eyes

that we find on land.

Five to perhaps six centimeters in diameter,

a pretty sizable eyeball

and horses have their eyes placed laterally

pointing to left and right.

So you get much more of a peripheral vision surrounding you,

enabling you to see things

and movement all the way around your head

and behind your head a little bit too.

Good for screening the horizon in open grassland habitats.

And they also have a very pronounced horizontal slit pupil

that may enhance the ability to see

and scan objects moving along that horizon.

Other horizontal pupils will be found.

For example, in this goat,

it's almost like a little angular in here,

like a horizontal stripe.

So we also see this horizontal pupil in this alpaca,

a blue iris and there's the horizontal pupil in here.

So very similar structure

if you compare the alpaca and the goat.

And zebras, in here with the iris.

And again, a very pronounced horizontal slit pupil.

So vertical slit pupils would allow predators

to detect vertical contours rising up from the ground

with a horizontal slit pupil, story's different.

This would give them the ability

to detect movement surrounding them,

especially in combination with the laterally facing eyeballs

that are pointing to the sides and not forward.

This is considered to be an adaptation

to those kind of environments.

[bear grunting]

Bears as omnivores, they have nicely forward facing eyes

in here, nicely contrast the laterally facing eyes

of the grazing species.

We also see forward facing eyes in dogs.

Dogs' vision, well, it's not super good, no color vision,

Acuity is okay.

I mean, they're largely driven through their nose.

Their olfactory system is phenomenal,

so they rely mostly on that.

But the vision is good enough.

Cats also have really strongly forward facing eyes.

Here we see again a nice vertical slit pupil.

It's a predator, that should give them pretty much

a stereoscopic depth perception.

Also have of a large cat in here

where we see a rounded pupil.

So we see some differences among the cats.

So it's maybe interesting, that may have to do something

with the absolute size of the animal, the body size.

Maybe the vertical slit pupil is really effective

at lower heights and perhaps not as important

at larger body sizes.

Probably worth investigating a little more actually.

[water trickling]

So whales are mammals that have returned

to life in the ocean.

What we see in whales is several features

that make it possible for them

to see fairly well through water.

The lens returns to very spherical rounded shape,

because the cornea is no longer any good underwater.

And we also see difference in terms of

what wavelength of light, what color they can see,

because light is very limited, especially at the depth

where these whales are actively foraging.

Also, we've seen eyes are facing pretty much laterally.

They can't see much forward at all.

For many whales vision is important,

perhaps not the primary sense that they're using.

It depends a little bit on what they do

with their diet, I mean you have filter feeding whales.

We have toothed whales that are predatory,

but still, in water, whales reach the largest eye size

of vertebrates that live today, so they get

to about 12 centimeters approximately in diameter,

which is about three times the size of a horse eye

that we see as the largest vertebrate eye on land.

Giant squid get twice the size of eye diameters

and the largest vertebrate eye that ever was

around on Earth was that of ichthyosaurs,

about the same size as a giant squid, about 30 centimeters

or good soccer ball size, really.

[Narrator] And the last branch we'll look at

of the tree is the primates, of which we are apart.

So tarsiers are really cool.

Tarsiers are nocturnal

and their eyes are clearly adapted to being able

to see in the dark, which is very different from our eyes.

We are day active.

Our night vision really kind of sucks,

but I think the eyes of tarsiers are actually bigger

in volume than their brains.

So really, really big eyes, proportionally speaking.

You know, we see a tarsier here in bright daylight,

so it's pupils are all the way constricted even at this tiny

little opening right there.

But in the dark, these pupils would pretty much cover

all of the visible surface of the eye.

The tarsier eyes functionally pretty similar to that

of an owl eye, meaning it has really good ability

to collect photons.

What's different about the tarsier is that it doesn't have

that tubular eye shape that we see in owls.

So it does have enough space to fit its eyeballs,

but functionally they're pretty similar.

[macaque chirping]

So moving closer to humans, the macaque,

an old world monkey, can't deny the similarity.

Primates are kind of the odd one out among the mammals

in that they are primarily active during the day hours

and so their eyes are more equipped

to work in bright light conditions.

We see a circular pupil, see the iris color in here,

and pretty much forward facing eyes.

That gives them a stereopsis

and really good 3D vision ability.

Again, chimpanzees similar to us are day active,

again, forward facing.

We have a red iris in here in dark, we see the pupil,

so these are actually very similar to our own eyes.

Full circle, all the way back to humans.

The pupil right here, the dark area, this is the iris,

the sclera that kind of holds it all together.

The cornea will be like sitting right on top.

We have eyelids in here that help clean our cornea

to make sure there is no dust on there

and dust particles and so forth.

Well equipped for day vision, not good at night,

pretty good at color vision, pretty good acuity.

Not the best, the eagle has us beat,

but yeah, that's our own eye.

[insects chirping]

One thing that will be really worth investigating

and what people are doing is trying to find structures

of eyes that could help develop new technologies.

If eyes work well in extreme environments,

be it in dark environments,

being really good at color vision,

there's something we can learn from nature.

Our color vision is pretty good.

I mean, we can distinguish lots of different hues,

but many birds and also lizards go into the UV range,

something we cannot pick up at all.

It would be really fascinating to find out,

like what is it like to see all that,

a whole world not visible to us?

And I think that would be really, really cool to look at,

as well.

[gentle music]