Professor Lars Schmitz joins WIRED to guide us through a giant tree of life mapping the evolution of eyes in the animal kingdom: how they work, why they've taken the form they have, and the evolutionary advantages they've unlocked across species.
Director: Joe Pickard
Director of Photography: Olivia Kuan
Editor: Matthew Colby
Expert: Lars Schmitz
Creative Producer: Christie Garcia
Line Producer: Joe Buscemi
Associate Producer: Amy Haskour
Production Manager: Peter Brunette
Casting Producer: Nicole Ford
Gaffer: Nick Massey
Sound Mixer: Kari Barber
Production Assistant: Fernando Barajas
Researcher: Paul Gulyas
Post Production Supervisor: Christian Olguin
Post Production Coordinator: Ian Bryant
Supervising Editor: Doug Larsen
Assistant Editor: Justin Symonds
Designer: Violet Reed
Director: Joe Pickard
Director of Photography: Olivia Kuan
Editor: Matthew Colby
Expert: Lars Schmitz
Creative Producer: Christie Garcia
Line Producer: Joe Buscemi
Associate Producer: Amy Haskour
Production Manager: Peter Brunette
Casting Producer: Nicole Ford
Gaffer: Nick Massey
Sound Mixer: Kari Barber
Production Assistant: Fernando Barajas
Researcher: Paul Gulyas
Post Production Supervisor: Christian Olguin
Post Production Coordinator: Ian Bryant
Supervising Editor: Doug Larsen
Assistant Editor: Justin Symonds
Designer: Violet Reed
Category
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TechTranscript
00:00Did you know that ambush predators tend to have vertical-slip pupils to better pinpoint
00:04their prey, while grazers tend to have horizontal pupils to better scan the horizon for incoming
00:09movement?
00:10We created this tree of life that focuses on the evolution of animal eyes, and we're
00:14going to walk through it with Professor Lars Schmitz.
00:17I'm Lars Schmitz.
00:18I study vision and the evolution of eyes, and I just love everything about eyes.
00:22We'll start at the base of the tree with an early branching fork of animals called
00:26cnidaria.
00:27The box jellyfish actually have 24 different eyes in clusters of six eyes each.
00:33One pair of these eyes in each of these clusters are not unlike our own eyes.
00:38They are camera-type eyes.
00:40Our human eye works similar to a camera.
00:42We have the aperture, the pupil, that lets the light in, behind which we have the optical
00:48system.
00:49So, what we nicely see here is the curved corneal, the optical apparatus in front of the eye,
00:54and the second optical element, the lens.
00:57It's a bit flatter.
00:58Those together refract light onto the retina and forms an image.
01:02To stay with a camera analog, this would be where the film would be, or the receptors.
01:07Cubozoans are really interesting in that they don't have a centralized nervous system.
01:12They have a nerve net, which allows them to process visual stimuli, but they don't have
01:17a real brain.
01:18It's actually really puzzling how they can actually interpret those visual signals.
01:22From here, the tree of life has a major divergent path.
01:25We'll follow first the site of the protostomes, which includes insects and many deep-sea animals.
01:30Scallops.
01:31They actually have dozens of eyes around their mantle.
01:36Every single blue dot you see in this image, these little blueberries, those are eye structures.
01:42This ring of blueberry eyes is actually useful for them as a defensive mechanism and a little
01:46bit of a navigation help.
01:48The way the scallops form images in their little blueberry eyes is very different from what
01:52we do.
01:53It has a mirror that's sitting behind the retina and that mirror is used to focus the
01:59light back onto the retina.
02:01So a really complicated mechanism, but it works well enough to get you an image of the scenery.
02:06The resolution is really not that good, but scallops can actively move so they can visually
02:12navigate to an extent.
02:14And also when they see something approaching, they can shut down and close down and protect
02:18themselves.
02:19Let's look at a few examples of eyes from some cephalopods.
02:22The pinhole eye of the Nautilus works just like the pinhole camera and has a very small
02:28opening through which light enters and then an inverted image is formed on the retina.
02:32So let's take a look at the human eye model here.
02:35We take out all the optics, essentially.
02:37Light's coming in from right here and then the image, as crude as it is, falls directly
02:42onto the retina.
02:44The smaller the pupil, the better the resolution of the resulting image.
02:49But the issue there is, if you make the pupil really small, the image will be dimmer and
02:54dimmer the smaller the pupil is.
02:55They do have a central nervous system to process visual information.
03:00Actually a little bit of a mystery what exactly they're using that eye for.
03:04The resolution isn't super good.
03:06The image will be a little bit dim.
03:07So it's kind of limited on what it can see with that eye.
03:15Octopus have chambered eye type and in difference to the Nautilus, this one has a lens.
03:21And what we have in common here is this deep eye cup, a pupil that's really well defined.
03:26That's the only part where light can enter the eye and nowhere else.
03:29If light from all directions could enter the eye, we wouldn't know where it's coming from.
03:33There's one really important difference between octopus and our own eye.
03:37We have a retina that is inverted.
03:39The photoreceptors are actually pointing away from the light.
03:43And the axons that transmit the signal approach the incoming light.
03:48But in octopus, the retina is the other direction.
03:52With the photoreceptors pointing towards the incoming light and the axons going away.
03:57A side effect of the inverted retina that we have, the so-called blind spot in the retina
04:03where we do not have any photoreceptors.
04:06So in this model, we can see the optic nerve really clearly right here.
04:09So this is where all the axons from the photoreceptors are bundled together and exit the eye and
04:15transmit the signal off to the brain.
04:17But in this part right here where the axons exit, that's where we cannot have any photoreceptors.
04:23So that's our blind spot.
04:25From my understanding, it's not really well known how exactly and what may explain that
04:29pattern.
04:30There may be a small advantage of having the photoreceptors a little bit further back.
04:34The resolution of the image may be a tiny little bit better.
04:38Personally I'm not sure if that's a huge reason that would provide additional evolutionary
04:42fitness.
04:43The bottom line is, our retina, even though it's inverted, it works well enough.
04:52One thing that's really interesting about squid is their cool pupil shapes.
04:55If we just look at squid, here are two examples.
04:57On your right, you see this U-shape.
04:59On the left, we have this W-shape in here.
05:02So quite a bit of variety.
05:04The function of these pupil shapes is a little bit of a mystery.
05:07And a really interesting hypothesis in this context is color vision.
05:11Color vision in squid is not well supported behaviorally, but it wouldn't make sense given
05:18that they use colors for signaling, for example.
05:20In humans, in our own color vision, we use three different Opsin types.
05:24So three different photopicments to distinguish different colors.
05:28So we have red, green, and blue.
05:30Depending on the strength of the relative excitation of these pigments, we can infer what color wavelength
05:36we're actually looking at.
05:37In squid, we don't have different Opsin types.
05:40We have one Opsin.
05:41The image would be essentially a grayscale image.
05:44How can they see color if they only have one Opsin type?
05:47Many animals correct an optical problem, chromatic aberration.
05:52Chromatic aberration results from the fact that shorter wavelength light is refracted more
05:57strongly than longer wavelength light.
06:00If you have blue light versus red light, the blue light will be focused a little bit before
06:04the retina and the red light a little bit behind the retina.
06:07So you get a little bit of an offset.
06:09Now this is something that most animals actually try to get rid of.
06:13There's the hypothesis that these pupils actually accentuate the chromatic aberration.
06:19They make it worse.
06:21And they can use that to tell what color light they're actually looking at.
06:25This blur hypothesis is a bit controversial in our community.
06:30Some people say, while theoretically possibly, the practical benefit isn't all that big.
06:34The key would be to test how these animals perceive colors and if there's good behavioral
06:39evidence to find out.
06:41As we head back through the tree of life, let's stop to talk about the flatworm.
06:46This little fellow has two very conspicuous eyes right here.
06:50So very simple eyes, like little eye spots.
06:54What we see here is eyes that are really not good from our perspective, but are beneficial
07:00to this aquatic flatworm.
07:01Essentially, it would help them to avoid getting eaten.
07:05These eyes are consisting of just two cell types.
07:09They have photoreceptors in a shallow cup.
07:11That cup is shielded by pigmented cells.
07:14So light can only reach these eye cups from above and a little bit from the side.
07:20But it cannot reach the eye cup from below because that's where the pigmented cells are.
07:24This flatworm would be able to tell if it's in bright versus dark environments.
07:30And it would know if the light is coming from directly above or perhaps a little bit from
07:34the side.
07:35Now that seems like utterly useless to us.
07:38But for this flatworm, it's good to know like it's bright, it's exposed, it's in the
07:42dark, it's perhaps a bit protected.
07:44Now let's cover the other fork of the protostomes, which include insects, spiders, and some other
07:50arthropods.
07:51Most spiders have eight eyes.
07:53In the jumping spider, the two front-facing eyes are clearly enlarged.
07:57That tells us that that pair of eyes is most important for that particular spider.
08:01They're actively hunting during the day, and it would make sense for them to have forward-facing
08:06eyes that have high acuity so they can see their prey better.
08:09They have eyes that more or less look like a camera type eye that we have seen before.
08:15One lens, one aperture in here, we see this quite nicely.
08:18It's a different principle, different structural principle than pretty much all the other arthropods.
08:23A little bit of a mystery how they evolved.
08:25It may have been derived from a compound eye, but how exactly we don't know yet.
08:30Having four pairs of eyes that are surrounding your head gives you essentially perfect parameter
08:36vision, splitting the job into different parts, the accessory pairs could be used to scan the
08:41environment for any possible movement, whereas the forward-facing eyes, in this case, have
08:45really good acuity and give you lots of detailed information.
08:50Mantis shrimp.
08:52One of the most bizarre animals.
08:54They're really cool.
08:55They can throw really hard punches, and they're super fast.
08:57These compound eyes are situated in stalks that kind of reach up a little bit.
09:01They truly have panoramic vision, 360 degrees, no problem whatsoever.
09:06Mantis shrimp are also known to have not just three opsins that we have, but they actually
09:12have 12 opsins.
09:13For many years, it was thought that mantis shrimp had superb color vision.
09:18It turns out that color vision of mantis shrimp isn't actually all that good.
09:23They process that information very differently from us.
09:26Our opsins have a very broad range over which they actually respond to light.
09:30So we have three of these different peaks, and they all overlap.
09:33They're excited to different degrees from different wavelengths light.
09:37In mantis shrimp, having 12 opsins, each of them probably captures a smaller range of
09:42wavelengths.
09:43So they have more like discrete channels instead of like largely overlapping absorption ranges.
09:49So that's probably explaining why they actually don't have superb color vision.
09:53Still good, but could be better.
09:55Now let's look at some examples of arthropod eyes.
09:59Maybe let's start with the fly eye right in here.
10:02This style of eye is called a compound eye because it consists of many different individual
10:07lenses.
10:08The resulting image as a whole is a mosaic of all the different images put together.
10:13Probably don't think of our eyes as simple quote unquote, but compared to these compound
10:18eyes, they actually are pretty simple because we have one lens and these have thousands of
10:22lenses.
10:23And there's really like these bulging eyes.
10:26So they're spherical, but they're coming towards us.
10:29So they're kind of inside out from our own eyes.
10:32The part of the eye that's here will not receive light that's coming from behind.
10:36The part of the eye that's here will get light from the side, a little bit from the front,
10:41but not from the left side.
10:42It gives you a really good idea where light's coming from.
10:45The flies cannot move the eye structures around because these are all little crystalline lenses.
10:51No eye movements in here or lens shape changes.
10:54This is what you got.
10:55Depending on the position of the eyes and the orientation on the head, many insects, including
11:00these flies can essentially have full panoramic vision all the way around 360 degree vision.
11:10So the elephant hawk moth has eyes that are very light sensitive and it can see colors
11:16at night, which is really hard to do.
11:18We only see this in these hawk moths and also in geckos.
11:22It's a different type of compound eye.
11:23We call this the superposition eye.
11:26In the apposition eye, what we talked about before, we had each ometidium producing an average
11:31image and those images were put together in the mosaic.
11:35Here what we see in this compound eye, there's a separation between the lenses and the retina
11:40where the image is received.
11:42So we have a projection of an image onto the retina.
11:46So we have a single image that's being formed instead of a mosaic.
11:50It allows for much brighter images to be formed.
11:53So this eye type is often found in insects that are active at night.
11:58Many of the superposition eyes have a little layer that's reflective.
12:02It's called the tapetum, the tapetum lucidum to be specific.
12:07You may have seen this when you're driving at night and you see a deer in a headlight.
12:10The eyes are like popping up really bright.
12:13This eye shine.
12:14So imagine you have light arriving at the retina.
12:17Many of the photons are being absorbed by the photoreceptors, but not all.
12:21Those that travel through get to the tapetum, which is reflective.
12:25They're bouncing back and going through the photoreceptors again for a second chance to be absorbed.
12:31We see this tapetum evolving several times independently in all groups that are nocturnally active.
12:43One thing we can see in this dragonfly eye, the lenses here are relatively small.
12:47So the smaller the omatidia in this case, in this particular eye type, the better the acuity
12:52or the better the resolution of that image.
12:54Now, one thing that we see often is that the omatidia change their shape depending on where
12:59you are, horizontal or on top or more towards the bottom.
13:03So they have zones of high acuity that tells us something about how they live and what they're
13:08going after, what they're trying to monitor.
13:10One thing that is a limitation to compound eyes is this design structure.
13:15If these animals want to make a really high acuity eye, we need very long focal length.
13:21And with this kind of setup, that's not going to work.
13:24They would need to have eyes that are 100 times bigger than what they can really fit on their head.
13:29And I mean, their visual abilities is pretty good.
13:32I mean, they're tiny animals, but I can still see quite a bit.
13:35Good enough so we cannot catch them often.
13:38It tells us that their visual processing must be reasonably fast.
13:41So this is quite an incredible task, actually, and they do it well.
13:44Let's go back to the trilobite for a moment.
13:46Existing in the Cambrian era, trilobites provide some of the oldest eyes in the fossil record.
13:52So here is the pair of eyes that this trilobite has.
13:57You can imagine it's really hard to find squishy eyes in the fossil record, right?
14:02These compound eyes with their crystalline lenses have a bit of an advantage to become fossils.
14:07And this is really important for us to study our vision in deep time.
14:11For example, we can look at the size and the shape of the lenses.
14:14And that tells us a bit about how the acuity and sensitivity must have been like.
14:18In this particular type of trilobite, which is a fake copse, the lenses are quite large.
14:24And this is a really interesting cue for us to study the behavior and ecology of these fossils.
14:29Which is otherwise really hard to do with just the fossil record.
14:33Their large size would suggest to us that this animal was probably able to see in very dim conditions.
14:39So it was either in deeper water or primarily active at night.
14:43We'll now double back and explore the other side of the tree, the deuterostomes.
14:48And we'll start here with our underwater friends, the echinoderms.
14:52Sea stars have light sensitive cells at the top of their arms.
14:57It's not really image forming, it's really low resolution vision.
15:00But it's good enough for them to tell shade from bright parts of the seafloor.
15:05So they can use this to stay hidden for protection from predators.
15:09Again, adding to the fact that even like very rudimentary visual structures are useful for the organism.
15:15That way natural selection easily explains the evolution of sophisticated eyes.
15:19Nielsen and Pelger determined in the 90s with a very conservative computational model
15:25that 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.
15:35And if you think about like how much time was available to these organisms to evolve eyes, that's just like nothing.
15:42Like literally like a blink.
15:48Similar to the sea star, sea urchin have light sensitive cells that are distributed across their little feet.
15:53And this is another example of diffuse visual system that kind of gives you information about the presence or absence of light.
16:00Not well studied how widespread photosensitive cells are across echinoderms.
16:05None of them are known, at least to the best of my knowledge, to have full image forming eyes.
16:10But photosensitivity is probably more widespread than we know so far.
16:14Not many people are looking at sea urchins in their visual structures.
16:17People should though.
16:19We move on now to chordates, which are all vertebrates, including fish, amphibians, reptiles and mammals.
16:26First up are the fish.
16:28You're looking at the four eyed fish.
16:31Actually, it doesn't have four eyes, but it has four pupils.
16:34One set of pupils that see through air.
16:37And the other set of pupils, they see through water.
16:40Upper pupil, lower pupil.
16:42And the reason for that is that once the eyes pop out of water, the cornea of these eyes becomes functional and helps with the refraction and bending of light.
16:52So it helps with the formation of images.
16:55Underwater, the cornea does not work at all.
16:58And the lens is the only optical element that fish have at their disposal to form an image.
17:04When seeing through air, the lens can be flatter, meaning it's not as strong because you have the cornea too.
17:09And the four eyed fish, anablips, has it both.
17:12So the lens is completely irregularly shaped, functionally spherical for underwater vision, but flattened functionally for seeing through air.
17:21Vision through water and vision through air is also different from another perspective.
17:25We take one step back and think about like how much light is actually variable and how far you can see underwater versus seeing through air.
17:33It's a huge difference.
17:35Vision is metabolically actually pretty expensive.
17:38Similar to brain tissue, it has been maintained.
17:40It's very costly.
17:41From an evolutionary perspective, big eyes are expensive.
17:45We looked at exactly this step in the history of vertebrates where tetrapods came on land.
17:51We had the evolution of vision from seeing through water to seeing through air.
17:56And that transition was coupled with a significant increase of eye size at that time.
18:01Having big eyes really only pays off, evolutionarily speaking, if you can see a long distance.
18:11In Blenys, a group of really colorful and diverse reef fishes, we have independent eye movements.
18:17One eye is stationary, the other one is moving around.
18:19One 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.
18:27What we see here is an example for a different way to focus the image.
18:32In 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.
18:40Now, fish don't do that.
18:42Fish have a spherical lens that is actually really hard, almost like a marble, and it cannot change its shape.
18:48What these fish do is move the lens around to focus on different parts in the visual scene.
18:53This little blenny wants to focus on something that's directly in front of it.
18:57What it will do is it will move the lens from here into this little gap up there and then focus on it.
19:04That's called the lensless space or the aphagic gap.
19:08And 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.
19:20So, if you look, for example, at this soldier fish in here, we see that we have a very large eye and a very large rounded pupil.
19:29And that helps to collect more light.
19:31This 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.
19:39This other soldier fish here, there's hardly any lensless space. It's like fully expanded.
19:44While 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.
19:52Not ideal, because if they are exposed to very rapidly changing light levels, they're kind of screwed, maybe completely overexposed to light and actually not see that much during that time.
20:02Now, fish have an interesting mechanism to deal with changing light levels.
20:07If you stay with a camera analog, they can switch out the film and change it for a different light sensitivity.
20:13Vertebrates have rod and cone photoreceptors. Rods are really light sensitive, but don't give you any color vision.
20:20The cones are less light sensitive and they provide color vision.
20:24When fish approaching dusk, there is migration of the photoreceptors within the retina.
20:30So, what they're doing is they're switching out the cones for rods.
20:34The cones are withdrawn and shielded by pigment and the rods come out and become functional.
20:40The problem with that mechanism, even though it seems brilliant, it's not fast.
20:45It takes about 20 to 30 minutes, which is the time for predators to strike.
20:50This is a very hectic and chaotic time on the reef.
20:53We'll explore now the amphibian branch, where animals need the ability to see both above and below water.
20:59Frogs are known to have charismatically big eyes.
21:03And what's really cool is that they have a ton of variety in terms of their pupil shapes.
21:07So, starting with the vertical slip pupil in here, where we have the pupil extending from the bottom towards the top.
21:13And we can contrast that perhaps with a horizontally aligned pupil that we have here.
21:18Our simple circular pupil that we have here, so many aquatic frogs tend to have that.
21:24But this is much different, for example, from this inverted triangle shape in here, where we have like this pointed tip pointing downwards.
21:32We also have fan shapes that can be inverted or regular, so variations to the same theme.
21:38We also have pupil shapes that are shaped like this diamond here, where we have like six major corners, which is really unique.
21:47The diversity is just stunning.
21:48What are these different pupil shapes good for?
21:51With this particular study, they didn't really find a strong ecological signal in that data.
21:57None of these pupil shapes were really tightly connected to a particular type of ecology, except for those of aquatic frogs,
22:05which is perhaps the, hate to say it, most boring pupil shape, which is just circular, like ours.
22:10In a way, not finding a clear ecological signal makes it really hard to find a clear function for these different pupil shapes.
22:18Traditionally, we have vertical slit pupils associated with predators and horizontal pupils in prey species.
22:26This model, we would assign this pupil shape to a predator and this pupil shape rather to a prey species.
22:34In frogs, this is not supported by the data.
22:38So, evolution is never selecting for the optimal or the single best structure.
22:43There's many different ways to do something.
22:45They all get the job done.
22:46It works well enough and hence we see this diversity.
22:49So, geckos are really light sensitive.
22:58They are primarily nocturnal.
23:00Probably say, well, they probably have really good rods, cones for day vision, rods for night vision.
23:05But geckos do everything with just cones.
23:09They have evolved from lizards that have lost their rods during their evolutionary history.
23:14Their ancestors were largely diurnal and day active and they secondarily returned to life at night.
23:21Geckos have modified their cone for the receptors to be able to work in the dark.
23:26So, they do all that night vision with the machinery that had evolved originally for day vision.
23:31We know that gecko eyes are very, very, very light sensitive.
23:35They have to make sure that they are not overexposing their retina to too much light.
23:41So, they really have to constrict their pupils a lot to avoid that problem.
23:45What we see here is that we have a series of four little pinholes that are being formed in here.
23:52They minimize the amount of light that gets into the eye.
23:55But that gives you a really large depth of focus.
23:58So, it would be really hard to get any distance estimation.
24:02One hypothesis is related to Shiner's discs, which is used by ophthalmologists.
24:07Essentially, for Shiner's discs, it was two tiny little pupils.
24:10And then you determine if that's resolved or if it's not resolved.
24:14And then you know if there's any vision deficiencies.
24:16Adding the string of four pupils on top of each other,
24:20the decreased depth of focus, you introduce some blur.
24:23And that blur is useful to judge distances.
24:26So, snakes are a bit odd. They can see heat.
24:33For example, pit vipers.
24:35The name pit comes from a pit that would sit pretty much right here in front of the eye,
24:39lined with thermoreceptors.
24:41And the information that's coming from that pit goes to the same brain region where the visual information goes.
24:46And that's actually combined with visual information.
24:49They're a bit odd among the vertebrates.
24:51For example, they don't have any bone structures within their eyes that many other reptiles do.
24:57They don't have the so-called scleral ring.
24:59The shape of the snake eye.
25:01Traditionally, there have been two hypotheses.
25:03One says that the shape of snake eyes evolved because snakes went through a phase in their evolution.
25:10They were mostly burrowing underground, which came with a lot of modifications to their eyes,
25:14which they didn't really use.
25:15Alternate hypothesis was that snakes were aquatic early on in the history.
25:20And that could explain many of their somewhat odd structures and similarities with other organisms in this.
25:29Chameleons can move their eyes independently.
25:31They can fixate on one object while moving the other eye and scanning its environment for potential predators.
25:37So, for all the animals that we have talked about so far that have a lens,
25:41those are converging lenses where the image is focused on one part.
25:46Now, in chameleons, we do find a negatively powered lens, meaning it's actually diverging.
25:51It's enlarging. It's magnifying the image on the retina.
25:55But it's looking at a very small part of the visual scene surrounding it.
25:59Essentially, it's using its eye as somewhat more like a telescope.
26:03It also comes with, like, a depth of focus.
26:06And by focusing through that visual field, like close and further away,
26:12it can judge distance.
26:14They need that visual acuity and the distance perception to actually successfully capture the prey with a tongue flick.
26:21As we make our way through the chordates, we are now looking at reptiles,
26:25which include squamates, archosaurs, and also, surprisingly, birds.
26:30Among alligators and crocodiles, we find really pronounced vertical slit pupils.
26:37Really good examples for ambush predators.
26:40One idea is, vertical slit pupils enhance the way predators can see prey species standing on the ground.
26:48And they really pop out nicely from everything else that's kind of more towards the horizon.
26:53That's a little bit blurred.
26:54Highlights vertical contours.
26:57It kind of blurs horizontal contours.
26:59Objects that are closer may be in focus.
27:01Objects that are further away are not in focus.
27:03And that is useful to gauge the distance.
27:05So, one other really interesting thing about alligator and crocodile eyes is their position as well.
27:11They live at the water-air interface.
27:13Their eyes are just poking through the water surface and they kind of scan everything that's surrounding them
27:19to find possible prey items.
27:21And that is enabled by having the eyes in a kind of upward-facing position above the water.
27:29Birds.
27:30Okay.
27:31Our living dinosaurs.
27:32Let's start with the owl, perhaps.
27:34I mean, amazing birds.
27:36Forward-facing eyes.
27:37They have really good night vision.
27:40But they also have really quite good acuity.
27:42So, they actually have eyes that kind of do it all.
27:44Owls have really found a way to maximize the size of their eyes for their given head.
27:50And that gives them a somewhat odd eye shape, which is called tubular.
27:54And normally, an eyeball is shaped of elliptical.
27:58And in owls, the sides are kind of cut off.
28:01It's more like tubular.
28:02For a wide eyeball, there was just not enough space.
28:05The eyes are so big that the eyes cannot move within the eye sockets.
28:09They have scleral rings that support the eye structures.
28:12The eyes are wider inside the eye socket than the eye socket diameter is.
28:16They're filling out all that space.
28:18You see that kind of eye shape also in deep sea fish.
28:22Totally different group, independently evolved, for likely similar reasons.
28:27Trying to make the eyes as big as possible for the space they've got.
28:30They can't move the eyes within the eye sockets.
28:33But owls can move their head exceptionally well, almost doing a full turn.
28:38And that gives them that peripheral vision that they would need to detect something behind them.
28:43We often think of ourselves as having the best eyes.
28:50But that's actually not true.
28:51Whatever we do, the eagle does it about twice as good.
28:54So they can see things that are half the size from the same distance.
28:58And they do it with eyes that are actually somewhat smaller than our own eyes.
29:02That's remarkable.
29:03In their zone of most acute vision in their retina,
29:07they're packing the photoreceptors as densely as possible.
29:10And you can somewhat compare that to the number of pixels you have on a display, for example.
29:17The more of these pixels you pack in there, the better the resolution.
29:21And that's what these eagles have pushed to an extreme.
29:24There's a physical limit to how small a photoreceptor can be.
29:27It cannot be narrower than the wavelength of light.
29:30These eagles push at the physical limits of visual acuity.
29:34And now, towards the extent of the deuterostome branch of the tree, we find the mammals.
29:41Horses have among the largest eyes that we find on land.
29:45Five to perhaps six centimeters in diameter.
29:48It's a pretty sizable eyeball.
29:50And horses have their eyes placed laterally, pointing to left and right.
29:54So 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.
30:04Good for screening the horizon in open grassland habitats.
30:08And they also have a very pronounced horizontal slip pupil that may enhance the ability to see and scan objects moving along that horizon.
30:18Other horizontal pupils would be found, for example, in this goat.
30:22It's almost like a little angular in here, like a horizontal stripe.
30:26So we also see this horizontal pupil in this alpaca.
30:30A blue iris.
30:31And there's the horizontal pupil in here.
30:34It's a very similar structure if you compare the alpaca and the goat.
30:37And zebras in here with the iris.
30:41And again, a very pronounced horizontal slit pupil.
30:45So vertical slip pupils would allow predators to detect vertical contours rising up from the ground.
30:51With a horizontal slip pupil, the story is different.
30:53This would give them the ability to detect movement surrounding them,
30:57especially in combination with the laterally facing eyeballs that are pointing to the sides and not forward.
31:03This is considered to be an adaptation to those kind of environments.
31:08Bears as omnivores, they have nicely forward-facing eyes in here.
31:17And I see contrast the laterally facing eyes of the grazing species.
31:21We also see forward-facing eyes in dogs.
31:24Dogs' vision, well, it's not super good.
31:28No color vision.
31:29Coyote is okay.
31:30I mean, they're largely driven through their nose.
31:32Their olfactory system is phenomenal.
31:34So they rely mostly on that.
31:36But the vision is good enough.
31:38Cats also have really strongly forward-facing eyes.
31:41Here we see again a nice vertical slit pupil.
31:44It's a predator.
31:45That should give them pretty much stereoscopic depth perception.
31:48We also have a large cat in here where we see a rounded pupil.
31:53So we see some differences among the cats.
31:55So this may be interesting that they may have to do something with the absolute size of the animal, the body size.
32:00Maybe the vertical slit pupil is really effective at lower heights.
32:05And perhaps not as important at larger body sizes.
32:09Probably worth investigating a little more, actually.
32:12So whales are mammals that have returned to life in the ocean.
32:20What we see in whales is several features that make it possible for them to see fairly well through water.
32:28The lens returns to very spherical rounded shape because the cornea is no longer any good underwater.
32:35And 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.
32:45Also, we've seen eyes are facing pretty much laterally.
32:48They can't see much forward at all.
32:50For many whales, vision is important.
32:53Perhaps not the primary sense that they're using.
32:55It depends a little bit on what they do with their diet.
32:58I mean, you have filter feeding whales.
33:00We have toothed whales that are predatory.
33:02But still, in water, whales reach the largest eye size of vertebrates that live today.
33:09So 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.
33:20Giant squid get twice the size of eye diameters.
33:24And 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.
33:34And the last branch we'll look at of the tree is the primates, of which we are a part.
33:39So tarsiers are really cool.
33:42Tarsiers are nocturnal, and their eyes are clearly adapted to being able to see in the dark, which is very different from our eyes.
33:51We are day active. Our night vision really kind of sucks.
33:55But I think the eyes of tarsiers are actually bigger in volume than their brains.
33:59So really, really big eyes, proportionally speaking.
34:03You know, we see a tarsier here in bright daylight.
34:05So its pupils are all the way constricted, even at this tiny little opening right there.
34:10But in the dark, these pupils would pretty much cover all of the visible surface of the eye.
34:16The tarsier eyes functionally pretty similar to that of an owl eye,
34:20meaning it has really good ability to collect photons.
34:23What's different about the tarsier is that it doesn't have that tubular eye shape that we see in owls.
34:29So it does have enough space to fit its eyeballs.
34:31But functionally, they're pretty similar.
34:36So moving closer to humans, the macaque, an old world monkey, can't deny the similarity.
34:42Primates are kind of the odd one out among the mammals in that they are primarily active during the day hours.
34:49And so their eyes are more equipped to work in bright light conditions.
34:53We see a circular pupil, see the iris color in here.
34:56And pretty much forward facing eyes that gives them a stereopsis and really good 3D vision ability.
35:02Again, chimpanzees, similar to us, are day active.
35:05Again, forward facing.
35:06We have a red iris in here.
35:08In dark, we see the pupil.
35:10So these are actually very similar to our own eyes.
35:12Full circle, all the way back to humans.
35:15The pupil, right here in the dark area.
35:17This is the iris.
35:19The sclera, that kind of holds it all together.
35:21The cornea would be like sitting right on top.
35:24We have eyelids in here that help clean our cornea to make sure there is no dust on there and dust particles and so forth.
35:31Well equipped for day vision.
35:33Not good at night.
35:34Pretty good at color vision.
35:36Pretty good acuity.
35:37Not the best the eagle has us beat.
35:38But yeah, that's our own eye.
35:45One thing that will be really worth investigating and what people are doing is trying to find structures of eyes
35:52that could help develop new technologies.
35:55If eyes work well in extreme environments, be it in dark environments, being really good at color vision.
36:01Is there something we can learn from nature?
36:02Our color vision is pretty good.
36:04I mean, we can distinguish lots of different hues.
36:07But many birds and also lizards go into the UV range.
36:10Something we cannot pick up at all.
36:12It would be really fascinating to find out like what is it like to see all that.
36:16A whole world not visible to us.
36:18And I think that would be really, really cool to look at as well.
36:22You think about it as images.
36:23Remember to get rid of blood and flu stamping.
36:25We've chosen the room.
36:26The和ars and life tool.
36:27Our bodies in theli IP group is especially this great system that is designed for,
36:28you know, really stance such as this system that offers come into mind.
36:29That's why I love this, if I am always building a room for those buildings,
36:31the mortgage anneary or the delegate stack of water.
36:32Very nice to it than I want.
36:33It's got nothingся like holding a suspect.
36:34Thank you and Arianna pense.
36:35We are now building a drunk kanssa.
36:37It's better to get into meditation by the Thanunks and Felix once.
36:38What do you spend your time doing with the m quarters?!
36:40What do you want to do with the next burning place?
36:41Do you think?
36:42The total OreBA
36:44andرك are trying to ignore how to touch violence.