How Animals See: Crash Course Zoology #6 - Free Educational videos for Students in K-12

How Animals See: Crash Course Zoology #6 - By CrashCourse

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00:0-1	If we collected the whole evolutionary history of animals in
00:03	a book , we'd start with the original common ancestor
00:07	and before we knew it , even if we manage
00:09	to fit a million years on each page , we'd
00:12	still end up with over 600 pages . There's a
00:16	lot of important stuff spread throughout the grand evolutionary history
00:21	of animals . But one of the most foundational chapters
00:24	would come fairly early on For the animals we know
00:28	today with very few exceptions . Seeing means being an
00:32	animal . In fact , we wouldn't even make it
00:35	10% of the way through the book before seeing well
00:39	, seeing the earliest known fossil with eyes is for
00:43	Gina ability , arian from over 550 million years ago
00:47	, seeing or vision is the ability to interpret your
00:50	environment based on how light interacts with it . And
00:53	about 96% of all animal species have eyes , the
00:57	organ or collection of tissue and cells that make vision
01:00	happen . Even many animals that have evolved to be
01:03	eyeless or have eyes that don't function to their full
01:06	capacity still have some ability to sense light . In
01:10	fact , vision is so evolutionarily advantageous . It's evolved
01:14	multiple times and in multiple ways across all metas Owens
01:18	. And it took eyes a surprisingly short time .
01:21	Evolutionarily speaking to be seen all over the animal tree
01:25	of life . I'm Ray Wynne Grant and this is
01:28	crash course zoology eyes can be intricate structures or simple
01:42	collections of cells but basically eyes detect light and process
01:46	that information into signals that the nervous system understands .
01:50	And there are two minimum requirements for an eye to
01:53	work photo receptors which are cells that react to light
01:57	by sending electrical signals and a nervous system made of
02:00	special cells that can process the signals into colors ,
02:04	brightness , darkness and other visual information . Everything else
02:08	you've heard eyes have like lenses , pupils , corneas
02:12	is extra to make the signal from the photo receptors
02:15	and its interpretation by the nervous system more effective .
02:19	So there are lots of different types of eyes .
02:21	But we can split animal eyes up into two big
02:25	categories eye patches and image forming eyes . Animals with
02:30	eye patches can have something as basic as a flat
02:33	cluster of a few photo receptors . They can perceive
02:36	light and dark and if that patches in a small
02:39	cup tell vaguely where the light is coming from but
02:43	they can't see shapes , details or patterns at all
02:46	. To really take advantage of the sense of vision
02:49	. Animals need an image forming I which not only
02:53	detects light but thanks to some extra I machinery uses
02:57	it to make an image in the back of the
02:59	eye . The most basic image forming eyes form dim
03:03	, blurry images to see fine and far away details
03:07	. Animals need a lens or a transparent crystal like
03:10	structure that focuses incoming light onto the retina . The
03:14	specialist layer of photo receptors located in the back of
03:17	the image forming eyes . But while it's convenient to
03:20	divide animal eyes into eye patches and image forming ,
03:24	it doesn't truly reflect the diversity of animal vision .
03:27	Like box jellyfish , octopus is fire worms , horseshoe
03:32	crabs and parrots all have image forming eyes . But
03:36	even if they were all looking at the same object
03:38	, they all see very different things because some have
03:42	stronger lenses or more sensitive retinas than others . And
03:45	some build a picture of the world in a fundamentally
03:48	different way . So it's not easy to compare eyes
03:51	and it's hard to not be biased by how we
03:54	humans see . Animal eyes are specialist to see in
03:57	a way that's evolved to match their lifestyle and environment
04:01	not necessarily ours . So we have to be more
04:05	specific , like by looking at how animals perceive fine
04:08	details called acuity or resolution . Visual acuity depends on
04:13	lots of factors like how many photo receptors are packed
04:17	into the retina . I size and how the structure
04:20	of the eye focuses light into a picture . The
04:23	compound eyes of crustaceans , insects and some mollusks are
04:27	made of multiple , sometimes even thousands of light sensitive
04:31	I units called um Atiya that combine their information to
04:34	form an image . These eyes generally have poorer visual
04:39	acuity than camera type eyes , which focus light onto
04:42	a single much larger retina . But because they require
04:47	less space , Compound eyes can wrap around an animal's
04:50	head giving them a wider field of view or how
04:54	much of their environment and animal can see without moving
04:57	. Their eyes or head . And compound eyes are
05:01	great at detecting movement . So if resolution is the
05:04	most important to us , we want camera type eyes
05:08	, but if we wanted to see a larger area
05:11	maybe to keep a lookout for predators , we could
05:13	go with compound eyes , especially if our animal is
05:17	tiny . This trade off between seeing a small area
05:20	and lots of detail and seeing a larger area in
05:23	less detail is really common in animal eyes . Some
05:27	animals take it to the extreme , like jumping spiders
05:31	have evolved telescope eyes . Two of their eight camera
05:35	type eyes have two lenses instead of one which magnifies
05:39	the image and really bumps up the resolution , but
05:42	their field of view is tiny so the spiders six
05:45	other eyes see a wide area to compensate . We
05:48	can also compare eyes based on how much light they
05:51	need to work called sensitivity . Most animals see in
05:55	different lighting by having two types of photoreceptors , rods
06:00	, which are so sensitive to light that they get
06:02	overwhelmed in daylight and cones , which pick up finer
06:06	details and color . Animals can also change how much
06:09	light gets into their eyes with other adaptations like pupils
06:13	, the hole in the center of some eyes that
06:15	grows and shrinks to let more or less light in
06:18	. Or with tap it um lucid um a layer
06:21	of reflective tissue at the very back of the eye
06:24	that bounces light so it passes the retina twice ,
06:27	giving the photo receptors a second chance to catch the
06:30	light . So for working in a low light environment
06:33	, we'd want wide open eyes with a to PDM
06:35	lucid . Um to maximize light sensitivity . The last
06:39	property of vision will talk about to distinguish animal eyes
06:42	because there are others is what types of light they
06:46	see first there is the wavelength which is based on
06:49	how much energy the light has We . Humans can
06:53	only see light with wavelengths from about 380 to about
06:56	750 nm . So we named that the visible spectrum
07:01	, but most animals can see light outside that range
07:04	like infrared or ultraviolet light , or even polarization ,
07:08	which is the direction that light waves are vibrating in
07:11	. By evolving to be more sensitive to specific wavelengths
07:15	. Animals can more easily notice things that are important
07:18	to them , like camouflage predators or the colours of
07:21	tasty food . So the best set of eyes are
07:24	, well , that depends on what you need them
07:27	for . But the wildest eyes and one of the
07:30	most complex visual systems discovered yet can be found in
07:35	the ocean punching things . Let's live a day in
07:39	the life of a mantis shrimp . Allow me to
07:44	introduce you to the peacock mantis shrimp . This technicolor
07:48	boxer has a pair of compound eyes that are set
07:52	on stocks and can look two places at once as
07:56	he swims through shallow coasts of the indo pacific .
07:59	Not only does he see both ultraviolet and polarized light
08:03	, but while we have just three types of cones
08:06	are Mantis Shrimp has at least 12 . But even
08:10	with all that visual firepower , he doesn't distinguish colors
08:14	nearly as well as we'd expect , which is weird
08:17	because he definitely lives a colorful life . He scares
08:21	off other males of his species by flashing a bright
08:24	patch of color on his clubs called a Merrill patch
08:28	. Though 12 cones still seems like overkill . Lots
08:32	of other animals put on colorful displays with only three
08:36	or four cones , It might come down to what
08:39	goes on in our crustaceans brain when he sees color
08:42	Instead of comparing the signals from a few codes to
08:45	perceive different colours . Our Shrimp uses a limited 12
08:51	piece palette of finely tuned photo receptors that only respond
08:55	to very specific colors . One cone reacts to UV
09:01	. One reacts to violet , one reacts to blue
09:05	, another for turquoise and so on , spanning a
09:09	rainbow and more of wavelengths with our three cones .
09:14	There's a slight delay as our brains do the math
09:16	to figure out the color we're looking at . But
09:19	instead , a mantis shrimp knows pretty quickly how much
09:23	UV or violet or whatever light he's seeing . He
09:27	isn't very good at telling apart small color differences ,
09:31	but he doesn't need to wait long for his brain
09:34	to process so he can grab a snack or chase
09:37	off an intruder at lightning speed . Have a great
09:41	day . Mantis shrimp eyes especially ones as tricked out
09:45	as what mantis shrimp have , might seem to be
09:48	an overwhelmingly complicated structure to have evolved . In fact
09:52	, many animals that live exclusively in underground habitats ,
09:55	like caves , have evolved to have little or no
09:58	sense of vision , likely because I zar too expensive
10:02	to maintain if they're not being used . But even
10:04	though evolution doesn't always lead to more complexity , eyes
10:08	are one of the best examples of how tiny changes
10:11	in structure and function can build into sophisticated traits over
10:15	time . In 1994 Swedish psychologist , Danny , nelson
10:20	and Suzanne Pell Ger proposed a sequence of events where
10:23	a camera type eye evolved from a light sensitive patch
10:27	of photoreceptors in well , the blink of an eye
10:31	evolutionary history wise , nelson and Pilger used something similar
10:35	to the maximum likelihood approach , assuming that eyes with
10:39	sharper resolution always helped animals survive and reproduce . They
10:43	calculated how many steps or mutations it would take to
10:47	go from this I to this I first the flat
10:51	patch of light sensitive cells develops into a depression than
10:55	a cup and finally a pinhole shape like in a
10:58	nautilus . Eventually the eye area evolves to be enclosed
11:03	with a membrane and filled with fluid to keep junk
11:05	out . And a more complex systems take advantage of
11:09	how fluids bend light to focus it more sharply on
11:12	the retina . Next the lens develops as another structure
11:16	to help focus light on the photo receptive patch .
11:19	Then we get an iris and the pupil for fine
11:22	control of light going into the eye , and what
11:24	they found was that it might have taken 364,000 years
11:29	or less to evolve . All of the complicated eyes
11:33	out there today , including the ones mantis shrimp path
11:37	, which sounds like a long time , but it
11:39	took tens of millions of years for feathers to go
11:42	from this to this and one size did evolve .
11:46	They were an evolutionary game changer , which might explain
11:50	why they seem to have become wildly popular almost overnight
11:54	. Instead of relying mostly on close proximity senses like
11:58	touch animals could literally see other animals coming today .
12:02	Most known animal species have eyes , but like other
12:06	senses , eyes are fine tuned to an animal's lifestyle
12:09	so they all see very different things . Next episode
12:13	will cover another sense that's evolved many times , hearing
12:18	before you go , you should feast your super awesome
12:21	image forming eyes of yours on a new PBS show
12:25	other words , other words on story , digs into
12:29	the quintessential human trait of language and finds the fascinating
12:34	thought provoking and funny stories behind the words and sounds
12:38	we all take for granted incorporating the fields of biology
12:41	, history , cultural studies , literature and more other
12:45	words has something for everyone and offers a unique perspective
12:49	into what it means to be human . Give it
12:52	a peak and let them know . Crash Course sent
12:54	you thanks for watching this episode of Crash Course ideology
12:58	, which was produced by complexity in partnership with PBS
13:00	and Nature . It's shot on the team Sandoval Pierre
13:03	stage and made with the help of all of these
13:05	nice people . If you'd like to help keep crash
13:08	course free for everyone forever , you can join our
13:10	community on Patreon .

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