My Science School

Having no thumbs would make you all thumbs, fumbling to tie your shoes or assemble a hamburger. (Don’t believe us? Tape one your thumbs against the side of your hand and see how hard life becomes.) We inherited a fully ‘’opposable’’ thumb – named for its ability to close tip-to-tip against our other fingers – from our primate ancestors around two million years ago. These ancient relatives needed handier hands to help get a grip on simple tools. So give a thumbs-up to your thumbs. They’re the mains reasons you can text with one hand and build a burger without fumbling the bun.

It might not sound very fancy, but lots of animals don’t have this opposable finger and cannot grasp things the way we do. The most important thing we get from grasp is probably the ability to use tools. The use of tools has made it possible for humans to do everything from creating fire for cooking food and making warmth, to building homes and shelters, to learning how to write!

 

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Scientists have several ideas why humans can high-five each other instead of, say, high – four or high – six. One theory suggests four fingers and a thumb on each hand are the perfect number and length to grip objects firmly. (Another study suggests we can grasp most things with just our thumb and index finger if necessary; the other four fingers are spares.)

The process of evolution determined the most beneficial number of fingers and toes for our survival. Pandas, after all, have thumb like digits to help them grasp bamboo shoots, while some birds have quadruple digits for perching and tucking away during flight. Occasionally, babies are born with extra fingers and toes (a condition known as polydactyly), but those additional digits have never offered enough of an edge to survive to later generations. In other words, evolution determined that five fingers per hand are just right for humans.

 

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Your eyes are amazing machines, but it takes only one small imperfection in the shape of your cornea or the lens of cause fuzzy focusing. Doctors call these imperfections astigmatisms, and they’re often inherited from parents. Glasses and contact lenses (or corrective surgery) can fix the problem.

Glasses or contact lenses correct vision because they allow the eye to focus light in the right spot on the retina — the spot that produces the clearest image. Because everyone's eyes are different, a pair of glasses that makes one person see wonderfully may look terribly blurry to someone else. You know this if you've ever tried on somebody else's glasses!

If you need glasses or contact lenses, your doctor will write you a prescription. In this case, a prescription doesn't mean medicine you'll pick up at the drugstore. A vision prescription is a piece of paper with numbers on it. The people who will make your glasses for you need these numbers to create lenses that will correct the way your eye bends light. Remember, the target is right in the center of the retina.

 

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Humans blink automatically to flush away the stream of cleansing tears produced by ducts in the corners of our eyes. Adults blink about 15 times per minute, but our rate of blinking slows when we read (which is why our eyes tire after tackling a long book) or focus on a distant object. No matter how hard we try not to blink, the need to flush the eyes eventually trumps our willpower – as anyone who’s lost a staring contest will tell you.

Every time you blink, your eyelids spread a cocktail of oils and mucous secretions across the surface of the eye to keep your globes from drying out. Blinking also keeps eyes safe from potentially damaging stimuli, such as bright lights and foreign bodies like dust.

Scientists have found that the human brain has a talent for ignoring the momentary blackout. The very act of blinking suppresses activity in several areas of the brain responsible for detecting environmental changes, so that you experience the world as continuous.

 

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That depends on the kind of crying you’re talking about. Our eyes produce tears of three types...

Basal tears flow constantly to keep our eyes from drying out. Our bodies produce about five to ten ounces (148 to 295 ml) of basal tears each day.

Reflex tears protect our peepers from irritants in the air, such as smoke and dust.

Emotional tears flow when our brain registers sadness or stress, which triggers the release of body chemicals called hormones that turn on the waterworks. Some scientists believe that emotional tears help rid our body of bad chemicals that build up during stress – which is why you feel better after ‘’having a good cry’’. With the possible exceptions of elephants and gorillas, humans are the only animals that shed tears of this type.

 

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Because sea water contains about three times more salt than your tears. The higher concentration of salt can cause a mild stinging in your eyes even if you wear a mask.

There is a tiny amount of salt in your tears, but its minor compared to the amount of salt present in salt water. Your tears contain about 0.9% sodium chloride, while salt water in the ocean contains 3.5%! That means the oceans is almost four times saltier than tears.

Opening your eyes in a swimming pool exposes them to dirt and other debris that might be floating in the water that could cause an irritation of the eye. It also exposes them to germs and to other bacteria in the water that could cause an eye infection.  A brief exposure to say, swim down and pick something up off the bottom of the pool is probably not going to be harmful, if the water is clear and reasonably well-maintained.

 

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Sodium chloride – the most common type of salt – is in all your body’s fluids: blood, sweat, and (yep) tears. Your tears contain a little less than one percent salt.

Tears contain greater quantities of water along with other organic and inorganic chemical components like mucin, lipids, lysozyme, lactoferrin, lipocalin, lacritin, sodium and potassium. The salinity of tears is attributed to the presence of salts of sodium and potassium.

This salinity of tears along with the presence of enzymes like lysozyme is responsible for their antimicrobial activity. Basal tears have a salt content similar to blood plasma. The salinity of basal tears disturbs the osmotic balance of bacteria and keeps the cornea in a healthy microbial-free environment. The mineral content of tears also nourishes the tissues associated with eyes. Saline nature of tears also indicates our evolutionary descent from marine organisms.

 

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Chickens have terrible night vision, but they’re capable of seeing colors – including vibrantly purple ultraviolet colors – that humans cannot. Researchers think chickens and other birds inherited their visual capabilities from their dinosaur ancestors. Because most dinosaurs weren’t nocturnal (active at night), they developed exceptional color perception and motion – detection vision for hunting a broad daylight.

 Chickens have very few cones, and they are not especially sensitive.  This difference between rod to cone ratio and the light sensitivities of cones in birds vs. mammals is explained because mammals all but disappeared from evolution long ago, and the only types of mammals that survived were nocturnal and insect eaters.  Mammals that survived this evolutionary bottleneck re-developed colour vision after millions of years, but since we evolved our cones from a different starting point than birds (they evolved from dinosaurs, and never spent millennia as nocturnal creatures), we developed our colour vision a little differently.  It’s another case of convergent evolution like whales and dolphins evolving to look like fish, because that’s the body type that works best in the water.

 

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Brown Eyes: Brown eyes are the most common eye color in the world with over 55% of the world's population having brown eyes. Brown eye color is a dominant genetic trait, and is created by the presence of melanin in the eye. Brown eyes are typically darker than other eye colors, and they may even appear black in certain individuals.

Hazel Eyes: Hazel eyes are similar to brown eyes, although they are typically lighter in color, and have more of a green-yellow tint. Hazel eyes have a higher concentration of melanin (pigment) around the eye's border, which can result in a multi-colored appearance that varies between copper and green depending on the lighting. Most people estimate that around 5-8% of the world's population has hazel colored eyes.

Blue Eyes: Blue eyes are genetically recessive and therefore much less common worldwide. Blue eyes are formed by the absence of pigments in the eye, where the blue color is formed by the scattering of light as it's reflected off the iris. It's estimated that approximately 8% of the world's population has blue eyes.

Green Eyes: Green eye color is often confused with hazel eye color, yet is entirely separate and distinct. Green eye color is the rarest color found around the world, and it is estimated that only around 2% of the world's population has green colored eyes. Green eye color is a result of a mild amount of pigmentation in the eye with a golden tint. When combined with the natural blue scattering of the eye, the colors mix to give a green appearance.

Silver Eyes: Silver eye color is also quite rare, although many consider silver eyes to be a variation of blue eye color. Like blue eyes, silver eyes are the result of a very low amount of pigmentation in the eye, which reflects a gray-silver appearance.

Amber Eyes: Amber eyes show off a yellow-copper tone, which results from the yellow colored pigment lipochrome. Amber eye color can range from golden yellow to a more copper tone.

 

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Sometimes, a person’s melanin pigment doesn’t spread evenly to each iris. Which can lead to one eye being darker than the other or even splashes of color in each iris? This extremely rare condition – known as heterochromia – doesn’t affect a person’s vision.

Heterochromia can be either present from birth (congenital) or acquired. Most people will have someone else in their family with the same condition, although if it develops at an early age it’s still considered to be congenital.

Acquired heterochromia can occur either from injury or illness, but won’t be the cause of any difficulty with vision. If any parts of your iris does change colour make sure you visit an ophthalmologist, just to be on the safe side.

Your eye colour is set by a variety of genes, but heterochromia occurs due to the concentration and distribution of a pigment called melanin. Melanin is also important in determining what colour skin you have and is found in your hair too.

It’s known that blue eyes contain the lowest amount of melanin, whereas brown eyes have the most, and so heterochromia is caused from one eye having either considerably more or considerably less melanin than the other.

 

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Eyelashes have one main purpose -- protecting your eyes. They help to keep dirt, germs, and other stuff out of your eyes, which are very delicate and need to be kept clean! Our eyelashes also help us know when a foreign object is headed towards our eye – when they sense the touch of some dirt or dust, they quickly send us the message to shut our eye and keep out the invader! 

Like the hair on your head, eyelashes sometimes naturally fall out and naturally grow back, too. It takes approximately four to six weeks for an eyelash that has fallen out to grow back, although because you have so many, it’s unlikely you’ll ever even notice it was gone! And don’t forget – when an eyelash lands on your cheek, pick it up, blow it away, and make a special wish!

 

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Your retina is covered with millions of special cells called rods and cones that process light from the lens. Cones detect colors (people who are colour blind are missing cone cells for a particular color), while the rods process light information. Scientists can guess at how animals perceive vision by counting the rods and cones in their eyes. Cats, for instance, have eight times as many rods as humans but far fewer cones, which explain their excellent night vision and their relative color blindness.

Photoreceptors cells take light focused by the cornea and lens and convert it into chemical and nervous signals which are transported to visual centers in the brain by way of the optic nerve.

In the visual cortex of the brain (which, ironically, is located in the back of the brain), these signals are converted into images and visual perceptions.

 

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CORNEA: The eye’s protective, transparent cover, the cornea is similar to the protective glass on a camera lens. It bends the light entering your eye to pre – focus the image before it reaches your lens.

PUPIL: Muscles in the iris control this hole in the center, which, like a camera’s shutter, allows light to enter the eye and strike the lens. In bright sunlight, the pupil contracts to let in less light. In darkness, it opens wide to let in as much light as possible.

SCLERA:  The whites of your eyes, sclera from a protective cover about the size of a ping – pong ball.

LENS: Like a projector in a moving theater, the lens focuses light onto the retina. It’s suspended in a muscle that changes the shape of the lens to focus on objects near and far faster than any computerized camera.

OPTIC NERVE: This cable carries visual information from your retina to the brain. Your brain processes the information and translates it into what you’re actually seeing.

 

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Like all human beings, you have ‘’binocular vision’’, meaning both your eyes face toward the front and provide your brain with two slightly offset images. Your brain processes the differences in these two images to create a perception of depth, or a three – dimensional view.

In order to see 3D and with stereo depth perception your brain has to use the visual information from both eyes. If the two eye views are too different and cannot be matched up, the brain will be forced to make a choice. It will reject all or part of the information from one eye. The brain can ignore, suppress or turn off visual information it cannot use.

 

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Just like the shape of your nose and the color of your hair, your eye color is determined by the genes you inherit from your parents. Those genes determine how much melanin – a colored chemical matter – you have in each iris, which is the colorful part of our eye. The more melanin you have, the darker your eyes. Less melanin makes for lighter eyes, which is why fair – skinned people often have light blue or gray eyes.

Brown is the most common eye color. Individuals with brown eyes have more melanin present, and over half of the people in the world have brown eyes. People who don this hue are said to be very independent, self-confident and determined. You are known to be trustworthy, and when people look at you they get a sense of security and stability.

Green is the least common eye color, but it is found most frequently in northern and central Europe. People with green eyes are curious about nature, very passionate in their relationships with other people and have an overall positive and creative outlook on life. These people tend to get jealous easily, but possess large amounts of love.

 

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