The Power Of Sight Evolution Of The Eye Documentary Discovery TV

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The Power Of Sight Evolution Of The Eye Documentary Discovery TV

the animal kingdom encompasses millions of species and more than 95% of them share a common trait the power of sight eyes are everywhere but look closer no two species see the world in quite the same way animals have a tremendous variety of eyes we see different appearances of eyes we see different sizes of eyes we see different configurations of eyes and this is because animals have different lifestyles among the sharpest eyes in the animal kingdom are those belonging to birds of prey searching high in the sky for tiny prey demands remarkable vision and birds of prey have evolved eyes that are second to none this Eagles flies its prey from more than a mile away even while diving at speeds of 200 miles per hour it tracks its target with deadly precision the secret to the Eagles success the anatomy of its eye in most cases the larger the animal’s eyes the sharper its vision and the eyes of the eagle are huge Eagles are a hundred pounds lighter than we are but their eye weighs the same as ours size is just for starters the back of our eye has 200,000 light capturing cells per square millimeter the Eagles has 1 million providing 5 times the image resolution its lens is flatter and placed farther away from the retina than ours the result gaze as sharp as a telescope magnifying its helpless prey to three times what our own eyes can see what force drives the development of such a remarkable innovation evolution the sparks of evolution are tiny random genetic changes every once in a while does little mistake made when the DNA has passed on from generation to generation and those mistakes are called mutations and sometimes the mistakes cause problems other times they create new opportunities mutations that boost survival and reproduction are favored by natural selection species that can’t adapt disappear 99% of all species have gone extinct over time the survivors are equipped with mutations that have given rise to uniquely powerful traits the eagle has evolved one of the most powerful eyes in nature but it is just one of the many different types of eyes that have evolved over time animal eyes are an excellent example of convergent evolution all the eyes on different animals across the planet today did not evolve from the same ancestral I in fact eyes have evolved numerous times in different lineages remarkably evolution has used the same basic genes to build the eyes of creatures as profoundly different as flies squid and humans research points to an origin for these building blocks of all vision 600 million years ago the Earth’s oceans are calm there are no monstrous predators to contend with no creatures armed and ready for combat sea life is simple animals come in two sizes small and extra small they’re soft with little protection and they have one speed slow but it is here that one group of creatures took a dramatic step toward vision these pioneers are long extinct but one of the oldest living lineages of animals still swimming in today’s oceans provide a peek into the past jellyfish biologists Alex Goodell and Chad Widmer of the Monterey Bay Aquarium are on the hunt for a test subject go get them one that can give them insight into the roots of vision yeah

that’s a nice-looking one right there this is Polly orcas more commonly known as the belle jelly the anatomy of this delicate creature is about as simple as animals get it has no skeleton no heart and no brain just a loose network of nerves but it has a crucial innovation that its survival depends on a ring of small dark spots line its base light-sensitive organs called eye spots we can only imagine what these most basic of eyes can see vision for them is very different than for us because we are able to form images quite well and they are so it’s kind of more of a blurry world for them than for us scientists don’t know why the distant ancestors of Mon and jellies evolved light sensing cells when they did but Goodell can show how they have helped jellyfish survive for 600 million years she has designed a simple experiment showing how Bell jellies use light detection to their advantage I’m interested in seeing if different animals that don’t form images can see different colors if they respond to different wavelengths of light goodell subjects the jelly to light waves that it would find in its natural environment first she hits the jellyfish with a dose of green light on cue its body goes limp and drifts to the bottom of the tank so you can definitely see when they’re relaxing tentacles stand they pulse less frequently the green light usually makes them appear to relax they usually will drop down to the bottom and their tentacles will extend and there are male pulse rates will decrease why does green light put the jelly so at ease it’s the wavelength of its home turf the ocean floor they usually spend most of their time on the bottom where they would naturally be getting a lot of green light they find most of their food on the bottom so they’re used to being in this type of light Goodell then switches things off purple light fills the tank suddenly the jelly pulses wildly when I found that the purple light that jelly seemed to go into this escape response their tentacles shortened up and their belt full straight increases it’s a reaction akin to us fleeing from a fire to the jelly purple light means one thing injury or even death might be imminent the reason is simple the short wavelength light like purple or ultraviolet lights it’s actually higher energy and it can be very damaging for organisms that are transparent so a lot of the transparent animals rise up in the water column in the evening and hang out on top at night and then in the morning as the sense coming out they migrate back down light triggering action a simple vision system to be sure but it’s helped jellyfish survive over half a billion years as they find food and evade predators but the eyes evolution would not stop there more than 500 million years ago the calm waters are to be overrun with a new breed of creatures eyes are about to get a powerful upgrade an unleash an evolutionary arms race that will forever change life on Earth the dawn of the Cambrian period 544 million years ago the animal kingdom is undergoing a time of transition early animals were simple passive creatures drifting through the current or anchored to the seafloor but over the course of 50 million years a lot is going to change life undergoes an explosive transformation thousands of new species burst into being including the oceanic ancestors of dinosaurs elephants and humans this is evolutions big bang the Cambrian explosion for that time there was little of any animals present on planet Earth and within about a 20 to 30 million year period basically all of the major animal groups appear in the fossil record and

start diversifying so it’s a time of major evolutionary change scientists like Bruce Lieberman are discovering ancient clues about why the explosion happened this really is one of evolutions big unanswered questions Darwin talks about the Cambrian explosion in the Origin of Species back in 1859 and he was puzzled by how quickly organisms appear we’d like to know what were the triggers that caused this real explosion of life the creatures that emerge from the Cambrian were larger and more mobile than any of their predecessors and what’s more they evolved weapons for battle an important aspect of the Cambrian explosion is that it is an evolutionary arms race for the first time creatures evolved the natural weapons they still use on today’s battlefields jaws claws body armor and most critical of all eyes appear they’re obvious they’re there and there’s no evidence for complex eyes before this Cambrian explosion these eyes are the first type to appear in the fossil record compound eyes the invention of a group of arthropods called trilobites arthropods are joint limb – animals that today include things like crustaceans insects spiders and so on the compound eye is a type of eye that’s possessed by these arthropods on closer look it’s clear that these eyes are nothing like our own so you can see these two round structures on the head and when you look at them in detail you can see small circular bodies in a series of rows each row has several of these circles and when we look at them it can recognize that these are eyes with many lenses trilobite eyes are one of the earliest results of the animal kingdom’s most feverish arms race coincidence or was the evolution of more powerful eyes the spark that triggered the Cambrian explosion eyesight is part and parcel with predation and things avoiding predators so if there were not the development eyes you might have had some of the sparks of the Cambrian explosion but I don’t think you would have had the full bang just how did the trilobite manufacture an eye of such complexity they took advantage of the materials at hand their eyes are made of rock this rock is calcite and in fact it’s the same rock or mineral that makes up the trilobite i trilobite secreted this mineral from their skin to form both their rigid shells and equally rigid eyes that’s what their skeleton was made of they had to evolve an eye out of this substance so it means we’ve got a beautiful record of what their eyes look like without calcite trilobite s’ would be not only naked but blind these crystal eyes represented a major advance from the simpler light sensing cells that came before and likely gave trilobite to supreme survival advantage the eye is present before the Cambrian explosion would have been nowhere near as complex as the type of eye that you’d see in the trilobite the compound eye would have allowed them to better track down prey or find dead carcasses of things to eat true visionaries trilobite to diversify and endure for nearly 300 million years before a mass extinction event wipes out the last of their species but other arthropods survived and one group takes the compound eye to new heights in sex if we look at the most abundant animal life form on land it’s the insects there are literally more than a million species of insects that are present they all have a complex compound on the first insects evolved around 400 million years ago while they share a common ancestor with trilobites they are not their direct descendants studies show they evolved their compound eyes independently but from the same genetic blueprint an array of microscopic lenses that work together to form an image the greater the number the finer the detail our fruit flies have hundreds of lenses per eye bees boast an astonishing 7,000 lenses but evolution drove one insect to develop a compound eye of staggering complexity the dragonfly each eye has a mind-boggling 29,000 lenses making for the ultimate motion detector

in pursuit of fast-moving prey dragonflies give chase at a staggering 38 miles per hour their eyes focusing power is poor but their processing speed is spectacular resolving images five times faster than our own the result lightning reflexes ideal for timing strikes in avoiding high-speed collisions the success of insects has made the compound eye the most prevalent in all of nature but it would not be the only vision system to emerge from the Cambrian explosion another group of animals evolved one that would launch the prototype for an advanced eye of their own the vertebrates the eyes of insects and the eyes of vertebrates represent different instances of the origin of eyes they’ve evolved from different ancestors to achieve the same end the vertebrate bye gets its start as a simple light detector over time it will evolve and become the targeting system for history’s largest predators when we look at Earth’s creatures we find that many of them look back with an eye akin to our own in fact one type of eye powers the vision of all vertebrates animals with a backbone a single lens camera made of soft tissue known as the vertebrate eye the vertebrate line includes reptiles mammals and birds and they all have a common ancestor a primitive worm like animal that lived during the time of the Cambrian explosion how this creatures crude eyes developed into the intricate organs we see with today has fascinated scientists since Darwin the vertebrate eye is really an icon of evolution early detractors of Darwin used it as an example of a feature that is so unbelievably complicated and it’s so obviously a machine and they’re so obviously the mark of an intelligent designer they’re Darwin’s famous response was to go through a series of intermediate evolutionary steps and show how you could start with a cluster of photosensitive cells on the surface of the skin from there you could create a pit eye which essentially putting those photosensitive cells at the bottom of a pit creates the first directional sensitivity and from there you could evolve other sorts of focusing or accommodating parts of the eye and essentially show that it was plausible the you can have a whole series of steps that lead up to a complex camera I this knew I would sweep through the ocean providing an advanced weapon in the escalating battle between predators and their prey the genes for building the eye were passed down to all the branches of the vertebrate tree from the first fish to sharks and on to shallow water swimmers who would boldly set their sights on an unexploited world land a new arena that one group of predators would come to dominate these creatures carnivorous dinosaurs first evolved 230 million years ago for the next 160 million years they would rule the animal kingdom littering the fossil record with the bones of their vanquished prey what was it that made carnivorous dinosaurs such successful predators was it their size their speed their array of vicious weapons such traits are the stuff of any predator but they’re useless without another key skill the ability to target prey carnivorous dinosaurs surely possess some of the largest eyes in history but how might their vision have helped them become such extraordinary predators paleontologist Kent Stephens made it his mission to find out very interesting to know what it was like to be a dinosaur what kind of vision did it have how did it use its eyes Stephens faced a major obstacle unlike their bones the eyes of

dinosaurs have not stood the test of time it’s essential to fill in the missing pieces because the soft tissue of the eyeball of course never is preserved what you mostly have is just the boat Stephens use the bones to make scale models of dinosaur heads these models would serve as a window into dinosaur vision when I came up with is a way to estimate how well the dinosaur could look out into the world by my looking at the animal Stephens devised a novel way to make his dinosaurs see lasers the beam lit up the i allowing stevens to plot its line of sight when the illuminated eye became obscured by the creatures brow or snout stevens had reached the limit of what that eye could see I decided I will trace those lines of sight on an intervening glass between me and the animal where I couldn’t see it it couldn’t see me by mapping the lines of sight of both eyes Stephens could calculate their degree of overlap when I had these dots now if I connect them this actually represents the region which both eyes could see the area were Stevens’s markings overlap is known as the binocular field of vision what advantage does binocular vision provide binocular vision seeing something with two eyes is very good for making fine depth judgments if you have two eyes and you can simultaneously focus on an object in space in front of you your brain can do a little bit of trig and decide how exactly how far away that object is how would this aid a predator imagine a large dinosaur whose two eyes face to the side resulting in two fields of view that don’t overlap this makes it difficult for the creature to see an object is distinct from its background and to gauge the distance to it but if its eyes face more forward their fields overlap providing a degree of binocular vision suddenly it’s a 3d world engaging distance to a target is no longer a problem Stevens knew if he could work out the binocular vision of carnivorous dinosaurs he could determine how they hunted their prey he trained his sights on the most iconic member of their group tyrannosaurus rex with 13 inch canines this 6 ton beast was certainly armed to the teeth but did t-rex have the targeting system to back up its bite t-rex had 55 degrees which is very substantial binocular overlap as much as hawks Stevens concluded that this vision system allowed t-rex to target its prey at a distance and pursue it on the run that certainly would have selective advantage toward being a better predator you can just stay locked on to your target and the amount of overlap could allow you to see how the world is moving in as visual report 3d vision was a vital part of t-rexes Arsenal and looking at skulls from different points in the animals history Stevens found it evolved even better 3d vision over time its snout narrowed and its cheeks hollowed out to allow for increasingly forward-facing eyes these animals reshaped their heads evolutionarily for a purpose see the steady progression toward more and more binocular vision so it must have been of Vantage to be edible with forward-facing four inch diameter eyes t-rex’s vision likely ranked among the most detailed in the animal kingdom but as Stevens discovered not all carnivorous dinosaurs could hope to follow t-rex’s lead this is Allosaurus he was the top predator in the Late Jurassic this animal has very lateral facing eyes and not only that but there’s a whole structure here along the top of the snout of this animal which completely obliterates the view from the left eye to the animals right and vice versa this animal has essentially blinders on it that keep it from being able to have a very wide field of view Allosaurus turns out to have about

twenty degrees of binocular overlap how could a dinosaur saddle with blinders be a top dog of the Jurassic it turns out less binocular vision led Allosaurus to assume a craftier mode of predation modder creditors that have so little overlap between the two eyes you tend to find that they are ambush predators like a modern crocodile one of the important aspects of an ambush predator is to be able to have surveillance and awareness of what’s going on around it so the idea of lateral facing eyes is completely consistent with a an ambush predator they basically wait until the meal comes to them they have a task of judging whether the time is right to lunge and try to take out the prey dinosaurs show how the same winning strategy evolves again and again in predator vision the opposite strategy holds true for prey animals their vision has evolved to help them escape their killers as predators evolved eyes closer and closer together for targeting the eyes of their prey moved further and further apart today rabbits take this strategy to the extreme 360 degree vision rabbit eyes evolved to sit high into the sides of its skull while its vision is not three-dimensional it allows the rabbit to see any threat at any angle rabbits like humans are mammals and back when dinosaurs still ruled the earth our mammal ancestors had to do much more than keep an eye out to survive they had to find a new niche in vision one that would allow them to coexist with creatures threatening their existence 100 million years ago dinosaurs rule the earth the odds are stacked against any animal competing with these mega predators especially for the evolutionary newcomers the world’s first mammals pint-sized animals not much larger than mice when the dinosaurs were running around there were also mammals but the only mammals that were around were very small they’d fit in the palm of your hand and they were just trying to keep out of the way of the dinosaurs for mammals the era’s underdogs it was either scurry or be squashed these tiny creatures adapted to an unexploited niche the nighttime and evolved a vision system that could thrive in the dark night vision a triumph of AI evolution and a trait essential for survival in many mammal species today many varieties of modern mammals also see well in the dark and many of them are also nocturnal and in some cases at least this is probably leftover from their evolutionary heritage as small mammals hiding in the shadows of dinosaurs emerging at night and feeding what changes did evolution make to the eyes of nocturnal animals that’s a question Chris Kirk is trying to answer I tell my friends it’s one of those things that you would have expected that somebody back in the 18th century went around collecting eyeballs but to my surprise when I started looking for data on just the gross anatomy of the pie there was very little that had been published and so spent you know about a year and a half collecting eyes Kirk uses his extensive collection of eyeballs to study the mechanics of night vision by putting his subjects eyeball to eyeball he’s able to compare them and see how their Anatomy has evolved to deal with the dark what’s the difference between the eye of a day active monkey called a marmoset and a night active fat-tailed dwarf lemur it’s the size of their corneas the cornea is the window to your eye it sets the ultimate limit on how much light your eye can gather the cornea is a clear curved shell that provides focusing power and ushers light into the pupil the light hits the retina where it is converted into electrical impulses that are funneled through the optic nerve to the brain a complex process in

the blink of an eye the diurnal monkey has got a very small cornea because it doesn’t need to let in as much light but you can see here as the nocturnal lemur which has a very large cornea that takes up nearly the entire anterior surface of the eyeball for night hunters a bigger cornea is key to catching fast-moving critters in the dark and one night hunter has pushed this Anatomy to the max tarsiers are something completely unique there’s nothing else like this in the world of vertebrate eyes the diameter of tarsier cornea is almost equal to diameter of the eyeball there are 54 hundred species of mammals relative to size none have eyes as large as the tarsier at 6 inches tall it’s one of the world’s smallest predators but its surveillance system out sizes the competition’s so tarsiers need both highly sensitive vision and acute vision one of the only ways you can get this is to build a bigger and bigger eye and that’s exactly what tarsiers have done how big try bigger than its own brain so this is the eye of a philippine tarsier and you can see that it’s cornea is actually a little bit larger than my own corneas but what’s really amazing about this is the fact that tarsiers head is only this big the tarsiers eyes are so large they need extra support to keep from tumbling out the eyeball extends past the margins of the orbit and they have to have strong connective tissue in their eyelid to hold the eyeball in place with immobile eyes the creature compensates with a neck that rotates more than 180 degrees tarsiers visual adaptations show the extremes to which evolution will go many mammals such as big cats evolved another way to boost their nocturnal vision a staple in fables and fairy tales glowing eyes have haunted children for centuries a nightmare to some is evolutions gift to others and they can see us just fine they can see us a lot better if we can see them for sure if one of them was over there stalking us we wouldn’t even know that they were there even you know without this kind of light they know and we’re coming and they know who’s coming I shine functions is more than a mere scare tactic it’s an adaptation that has made cats spectacularly successful night hunters these guys they’re rather very active at night and in fact you can hear them making their noises particularly the lion when he starts to roar even at this hour hiking also the eye shine not only can they see their prey better but also I think it makes it more intimidating to any predator other predators that are out the source of the eye-shine is buried deep in the anatomy of the cat’s eye Kirk’s careful dissection reveals its inner workings first the retina then the light-sensitive tissue lining the back of the eye this area is known as the tepee dome lucidum the peetam lucidum is flattened for bright carpet so it’s the bright carpet in the back of your eye behind your retina so how does the tepee dome work in your eye or my eye there’s a black pigment behind the retina that absorbs the light so that it’s not scattered but in species that Sepideh any light that’s not absorbed by the retina bounces off the tepee dome the mirror behind the retina and it essentially has a second chance to be absorbed so you’ve got twice the chance to absorb every incoming photon of light that your eye captures on the rebound some of the light shoots back out of the eye resulting in a glowing effect for cats this mechanism yields extraordinary benefits they require only 1/6 of the light that humans need to be active in the dark I think of the tip edom is just this marvelous adaptation you know it’s such an elegant solution to a pretty straightforward biological problem and you know putting a mirror behind your retina to increase sensitivity what could be simpler cats were far from the only mammal group to evolve such an ingenious solution to night-vision other species would reach the same result entirely on their own but I think really the most interesting thing about the evolution of the tip edom lucidum is that it’s such a good solution to this problem of enhancing sensitivity that many many different groups of mammals have arrived at this same solution independently what about us even though we are mammals

we struggle to see clearly at night that’s because 30 million years ago our ancestors shifted from the night to the day when these monkey like creatures moved to a new niche they would evolve eyes that could see the world like never before the human eye able to decipher 2.3 million colors with speed and precision that makes a computer look slow our color vision is superior to that of many other mammals who must get by with far less to a dog the world is virtually devoid of color only a small number of species including our primate relatives can see a full range of yellows blues greens and most significantly reds but such a depth color vision does not exist in the earliest primates its evolution has its roots in the aftermath of a mass extinction about 66 million years ago with gigantic asteroid caused the extinction of all non-avian dinosaurs and paved the way for the rise of mammals after the dinosaurs disappear mammals radiate all of a sudden they start diversifying and one group of mammals is the primates and they go straight for the trees once primates settled into the forest canopy they evolved into new species one of those lineages becomes day active the ancestors of today’s monkeys apes and humans they evolved a new adaptation in their eyes not seen in earlier species an expansion of their color vision a standard range of colors consisting of blues and greens now include red why would primates need to see red in their new world for clues biologists look to primates today howling monkeys are this wonderful unique animal that may give us some insights into our own evolutionary past biologist nate domine has spent his career researching primates as a means of understanding our own evolution they’re almost like a time machine for looking at what the ancestral monkey in Africa and Asia might have been like we might get clues to what its diet was like its social behavior its anatomy and that ancestral animal was the one that gave rise to all subsequent monkeys and apes and ultimately humans domini wanted to know why natural selection favored improved color vision in our ancestors what did they gain by evolving to see the color red domine went on a year-long journey to study primates in their natural habitats part of my PhD research was to go to Africa to study monkeys with different diets and to study chimpanzees and to see if there was some sort of common or unifying food item that they might have all turned to that might explain why these monkeys all routinely share a very similar timely color Mission Domini scanned the rainforest canopy for test subjects hoping to capture leaf samples from where the monkeys had stopped to eat but he had a problem what a monkey eats was high above his reach it’s challenging in the rainforest in particular when an animal is at the top of that tree eating something and you want the sample not many ways to get it left with no other choice he had to take up arms we used this very stout handmade slingshot from Panama that fire small pebbles up into the canopy and leaves will come down and it’s a great way for sampling the types of foods that primates might eat after collecting and packing up the key samples Domon e’s real work began using an instrument called a spectrometer he recorded the color of each leaf he had gathered in the field you see that peak in this green spectrum so that’s telling you that this is a green leaf now you see how compared to that last one this is picking much more here in this orange region into the red region Nate data yielded a remarkable discovery not only were the monkeys eating red leaves red leaves composed the majority of their diet it turns out the red of the leaf the younger and more nutritious it is old mature leaves the green leaves they

are tough they’re holding toxins they’re generally leaves you want to avoid so primates in general tend to go for the youngest leaves possible and if those young leaves also have a color cue that distinguishes them from mature leaves then it may be advantageous to evolve a mechanism for detecting those on the leaves – primate red wasn’t simply another color it was a beacon a homing device allowing them to target their key food source from a distance if it sees red leaves in the canopy then it knows automatically that those particular leaves are young and so that can save energy save time it can travel directly to those young red leaves rather than randomly searching throughout its environment increased color vision a strategic evolutionary leave that scientists discovered was one of many primates would soon reap the rewards of another crucial adaptation binocular vision it’s a trait that predators have repeatedly evolved but for primates it served a new function primates have eyes at the front of their head but they’re not predators so why would they do that great thing about having two eyes pointing the same direction as you get depth perception if you’re gonna jump from one tree and try to land on another tree depth perception is a really good trick to have over time primates evolved a binocular field of vision of 60 degrees on par with birds of prey it’s an evolutionary step that allows primates to capitalize on their hand eye coordination but this increased binocular vision comes at a price unlike the rabbit primates now had a limited field of view most animals have eyes that are laterally placed on their heads was to say the eyes are on the side of the head and this provides these animals with a panoramic view and that’s really really useful for detecting predators that may be coming from behind you so there’s a significant cost to putting your eyes in the front of your head but the advantages are that you have better depth perception you have better acuity once primates abandon a vision system built for protection birds of prey were ready to benefit swooping in from the sky and plucking primates from the treetops these aerial terrors were threats to be reckoned with based upon the evidence that we have from the fossils birds of prey have been exerting some type of selective pressure on primates for as long as there have been primates and Raptors primates were easy targets for these winged predators under siege primates evolved a new kind of behavior they had to depend on each other you can imagine then that once these animals of all de pies in front of their heads they needed to live with other animals so that they could improve the probability of detecting a potential predator group living was the primates answer to predation it allowed them to maintain their high degree of binocular field of vision without sacrificing safety they’re keeping watch with a communal eye resulted in an evolutionary side effect larger more powerful brains so you can see the evolution of group living and once group living evolves you have a strong selective pressure or you might say that evolution favored larger brains because that’s more individuals that you have to remember fossils show how the visual system of primates improved over time both the eye sockets and the optic nerve canal grew larger is brain size increased primates put huge demands on their site from color vision to increased binocular vision they became extremely sensitive to the slightest change of expression on the faces of other primates this increased need for visual processing fueled the development of intelligence and you can see how this sort of runaway process for group living larger brains avoiding predation could give you the the suite of characteristics that define the primate ordered to which we belong a complex and mysterious organ one that has fuelled evolution for more than 500 million years driving an eternal arms race between predator and prey and becoming an indispensable tool for millions of species the eye an engine of evolution and one of its ultimate masterpieces