Why do we have two eyes?

Why do we have two eyes?

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Humans, animals and birds all have two eyes. Why? In addition to why two instead of one, why aren't there more than two?

First, it's not true that all animals have two eyes. For example many spiders have four, six or eight eyes whilst worms have none, but all are animals.

In simple terms, the reason typical mammals have two eyes is probably similar to the reason that cars usually have four wheels, the reason houses keep out the rain and why coffee cups have handles. It's because that's what works.

Each time an organism develops, there is a high chance that the genetic information that built it will include something new. When that happens, most often the phenotype (essentially, the organism's appearance, such as how many eyes it has) does not change. The organism looks the same as its parent(s) did. If something significant does change, it is most likely to be bad news for the new organism. The chances are the organism will not survive and the particular genetic information that built it will not used again. Hence, organisms don't tend to be that different to their parent(s). Dogs don't give birth to cats.

But just rarely, this new combination produces something different and useful - useful enough that the organism is particularly successful and a good fit for its environment. In that case, simply because it can, the organism has more offspring and more of those offspring themselves survive to have offspring of their own. The genetic information that built the organism is thus used to create more, similar organisms.

So, the first eyes were produced by accident. When eyes turned out to be useful, subsequent generations from the first organism with eyes were more likely to have eyes themselves. As it turns out, analysis of genomes shows that eyes have evolved like this multiple times.

We can only assume that since mammals typically have two eyes, either the random genetic accident that would produce different numbers of eyes in mammals has never happened, or if it did happen, the result was not a good fit to the environment.

Why not no eyes? Human beings are occasionally born without eyes and the cause may be genetically inherited. One might reasonably imagine that for much of human history such a child would be unlikely to survive into adulthood and thus pass on this mutation.

Why not one eye? One reason in the case of primates is that two eyes are useful for estimating distance, which is difficult with one eye. A two-eyed animal has an advantage against predators, making it more likely the next generation is going to have two eyes.

Why not more than two eyes? Perhaps a larger number of eyes requires a larger head making childbirth impossible, or perhaps it requires too much energy to sustain. Who knows? What we observe today - humans typically have two eyes - means that, if that mutation for more than two eyes did happen at some stage, then more than two eyes wasn't optimal for the environment in which the resulting organism found itself, so it didn't get passed on to the next generation.

Eagle eye

The eagle eye is among the strongest [ ambiguous ] in the animal kingdom, with an eyesight estimated at 4 to 8 times stronger than that of the average human. [1] Although an eagle may only weigh 10 pounds (4.5 kg), its eyes are roughly the same size as those of a human. [1] Eagle weight varies: a small eagle could weigh 700 grams (1.5 lb), while a larger one weighs 6.5 kilograms (14 lb) an eagle of about 10 kilograms (22 lb) weight could have eyes as big as that of a human being who weighs 200 pounds (91 kg). [1] Although the size of the eagle eye is about the same as of a human being, the back side shape of the eagle eye is flatter. Their eyes are stated to be larger in size than their brain, by weight. [2] Color vision with resolution and clarity are the most prominent features of eagles' eyes, hence sharp-sighted people are sometimes referred to as "eagle-eyed". Eagles can identify five distinctly colored squirrels and locate their prey even if hidden. [3]

In addition to eagles, birds such as hawks, falcons, and owls also known as raptors have extraordinary vision which enable them to gather their prey easily. Raptors are also known as "birds of prey" and are categorized by their predator hunting style. This means that they use their sharp senses to locate and capture prey. An eagle is said to be able to spot a rabbit 3.2 km (

2 miles) away. [1] As the eagle descends from the sky to attack its prey, the muscles in the eyes continuously adjust the curvature of the eyeballs to maintain sharp focus and accurate perception throughout the approach and attack. [1]

Why do you have two lungs but only one heart?

Your body is pretty amazing. At any given point you have a great many biological processes going on, such as digestion, respiration, metabolism, and fighting off invading bacteria. Different regions and systems within your body work together to create a state of balance -- just the right amount of blood sugar here, just enough electrolytes there -- to keep you working at peak performance.

But have you ever asked yourself how your body got to be the way it is? Why do you have two of some organs and just one of others? Take the heart and lungs, for instance -- why do you have two lungs but just one heart? Wouldn't it be better to have two hearts?

Your vital organs -- like your lungs, your heart, your pancreas, brain and liver -- are just that, vital. Not only are they vital to life, they are also vital to one another. Your lungs, for example, breathe in oxygen and exhale carbon dioxide (one of your body's waste products). The lungs transfer oxygen to the blood, which is carried to the heart for distribution throughout the rest of the body. The blood carries waste carbon dioxide back to the lungs, where it is absorbed and exhaled. It's a beautiful system. But how did it come about?

It's a very ancient system, says Rutgers University anthropologist Susan Cachel -- and it's not unique to humans. The organ systems we find in most animals contain one heart and two lungs. That is, with the exception of earthworms and cephalopods -- the invertebrate class which includes octopi and squid. Earthworms have five heart-like structures. Cephalopods have three hearts (two to send blood to the gills, and one to send blood to the rest of the body) and no lungs.

Cachel says that the one heart/two lungs system began to emerge about 300 million years ago, when animals first moved from sea to land to escape predators and find new sources of food. From that point on, it's been the norm. But why didn't it continue to change?

In this case, Occam's razor provides the key -- the simplest explanation is usually the right one. Ultimately most animals developed a system of two lungs and one heart (along with the rest of their organs) because that's what was needed to survive and thrive on Earth. People didn't develop two hearts or eight legs or wings because we didn't need them for survival. And we developed two lungs because we need them.

Phylogeny is the study of how the first ribonucleic acid (RNA) strands in Earth's primordial soup developed into humans and other animals. As these animals evolved into such divergent species as birds, insects and humans, the organ systems in those animals remained similar to one another. We still have stomachs to digest food, lungs to breathe air, and kidneys to filter waste. All of this indicates that species -- including humans -- have been shaped and molded specifically to live on Earth.

So does this mean that our system of internal organs is perfect? We know through our study of disease that going from two lungs to one is detrimental to our health, but what about adding an extra heart? Wouldn't that make us better able to survive? Read the next page to find out about what it would be like to have two hearts.

You might imagine that having two of some organs is redundant. We have two lungs, two kidneys, two eyes -- each doing the same job at the same time. But Dr. Tony Neff, a professor of anatomy and cell biology at Indiana University - Bloomington, warns against downplaying the role of duplicate organs. It takes both organs in those sets to carry out their job fully Although one can function alone, the process it carries out will not be done at full capacity, and the rest of the body suffers. For example, you can see with only one eye, but the eyes' function of providing depth perception will suffer and you'll bump into things much more frequently seeing with one eye than you would with two.

So if you need both lungs to function at full capacity, what would happen if you had an extra heart? Would the performance of the processes it carries out double?

Not at first, says physiologist Bruce Martin, a colleague of Dr. Neff's at Indiana University. Your body is a system, and it's built so that the system is always functioning at its full capacity. When the system is attacked -- for example, through starvation -- all parts of the system suffer at the same rate. Conversely, when one part breaks down, the whole system suffers. If your lungs become irreparably damaged -- say, through emphysema -- the rest of the system will slow down to accommodate the broken part.

So since your system is already functioning at full bore, the addition of an extra heart wouldn't do much. But your system also possesses potential function, as seen in the muscles, when they're called upon to act beyond their normal capacity, like in the case of hysterical strength. We can train our bodies to function at higher levels, like athletes do. Since the heart pumps blood to the muscles, with a second heart your muscles would eventually grow stronger with time. Once the rest of the system is used to having a second heart, a person could grow stronger and have more endurance [source: Martin].

But the same can't be said for your brain. The brain is already getting more than enough blood to it, so it wouldn't function at a higher level, theorizes Dr. Martin.

Interestingly, when we are in the embryonic stage of development, we actually do have two hearts. The heart primordia (which describes the stage of the heart's development) in the embryonic stage is actually two hearts, which eventually fuse together into one heart with four chambers. Embryologists in the 1920s and '30s kept the heart primordia from fusing in embryonic frogs, and the frogs that grew up developed two hearts. The same also goes for our eyes. We begin with one primordia of the eye, which eventually separates to form two. If the primordia is kept from splitting, one central eye develops, like a cyclops, says Dr. Neff.

So it's theoretically possible for us to develop two hearts. And if we could determine how to use both fully, we could also advance ourselves into a species of super-strong, intellectually average beings. But wouldn't tampering with our own evolution as a species be dangerous?

"We've already taken ourselves out of evolution," says Rutgers' Susan Cachel. "[Humans are] all effectively tropical animals, and through our use of technology, like winter clothes, we've shielded ourselves from the effects of cold weather."

So we've beaten natural selection by the elements. We'll see what we can achieve with two hearts.

For more information on physiology, evolution and related topics read the next page.

The Science of Eye Contact Attraction

Eye contact is one of the easiest and most powerful ways to make a person feel recognized, understood and validated. The simple act of holding someone’s gaze — whether it’s a new girl, a prospective employer or an old friend — has the power to ignite or deepen a relationship. That’s why it’s so important, and that’s why it’s one of the fundamental skills we emphasize so strongly at The Art of Charm.

To understand why eye contact is so important, we need to appreciate how central it is to the human experience. As it happens, humans — the only primates with white eyes — are drawn to eye contact from an early age. A 2002 study from MIT found that infants were far more likely to try and follow an adult’s eyes rather than just their head movements. And beyond the science, think about your personal experience: We study people’s eyes to judge their character, we notice when someone meets our gaze, and we are highly conscious of where our eyes wander. Eye contact is deeply rooted in our DNA. In fact, you’re reading this article in large part because your caveman ancestors had an intuitive mastery of eye contact. Back then, eye contact meant the difference between life and death, attraction and indifference.

And yet most people have never given any thought to how good or bad their eye contact is. If you want to become a master of eye contact, it’s going to take some practice, along with a good grasp of the underlying theory. And that’s the focus of this piece: five things every person should know about the science of eye contact, the benefits of mastering it, and how to make the most of your gaze.

Eye contact can make people more resistant to persuasion.

Eye contact makes you more persuasive, right? Yes. But it can also have the opposite effect.

In many cases, people can be more resistant to persuasion when making eye contact, as researchers at the University of Freigburg, Germany found. Using the latest in eye-tracking technology, Frances Chen (now an assistant professor at the University of British Columbia) and her team of researchers found that the more eye contact subjects made with a video while they listened, the less likely they were to believe what was being said.

There are some caveats to this general rule. First, the study involved people watching videos about controversial subjects. Participants who disagreed with the viewpoint they were listening to were less likely to be persuaded by someone the more eye contact they made with the speaker. This might be because liars are scientifically proven to hold more and deliberate eye contact than people telling the truth.

On the other hand, if someone already agreed with what was being said, they were likely to agree even more if they made steady, prolonged eye contact with the speaker.

In either scenario, we can appreciate how powerful eye contact really is. Someone’s gaze can be a channel of truth or a barometer of lies, depending on the speaker’s intent and the listener’s sensitivity. Both are communicated through the eyes.

The implications? For one thing, steady eye contact will help you motivate people to complete actions they’ve already agreed to undertake. It can also convince people to become more zealous about your mutual position. If you’re seeking allies around the office, eye contact can be a powerful tool. If you’re a parent looking to instill discipline and connection in your children, it can be equally effective.

At the same time, if you feel like someone is trying to “sell” you on something you’re not all that interested in, then focus on making eye contact. This will make you less susceptible to a deceptive sales pitch. On the other hand, if you’re even a little bit interested, but still don’t want to buy, looking away might be to your advantage — more eye contact will just make you agree more.

Eye contact makes your words more memorable.

If you want people to remember what you said long after you’re done talking, maintain good eye contact. This was the finding of a joint study between the University of Wolverhampton and the University of Stirling.

In this study, participants were put on a video call with another person. Researchers found that eye contact increased retention of what was said on the call. What’s more, this didn’t even require all that much eye contact: A mere 30 percent of time spent making eye contact added up to a significant increase in what participants remembered.

Which means that a little eye-gazing goes a long way. Making eye contact 30 percent of the time isn’t hard — that’s less than 20 seconds out of every minute. If you are working on making a lasting impression, or want your colleagues to remember your words long after a meeting, then find their gaze and hold it, because memory, impression and eye contact are deeply connected.

Eye contact and movement helps people notice and remember you.

Eye contact alone doesn’t explain what makes your words memorable. Researchers have also found that movement, when coupled with eye contact, has a profound effect.

Two researchers working out of Radboud University and Rutgers University did the first research on eye contact and movement. What they found is that eye contact, coupled with a sudden movement (such as an out-of-nowhere hand motion or a turn of the head while you make eye contact) makes people both more memorable and more noticeable.

Changing your direction and making eye contact will help you make an even stronger impression. All you have to do is turn your head, move your hand onto the bar, or focus on strong body language as you make eye contact. That in turn will make you more noticeable and memorable.

Eye contact makes people more honest.

Paradoxically, liars make more eye contact than truth tellers, but eye contact tends to make people more honest when confronted. That’s what researchers at Tufts University found when they left a dime in a phone booth and had strangers approach claiming the dime as their own. When eye contact was made, those who had just found a dime in the phone booth were far more likely to return it.

This phenomenon reflects the age-old “eyes as windows of the soul” concept. You know logically that someone can’t read your mind by looking into your eyes, and yet you intuitively know that eye contact makes it less likely that you will get away with lying. Our eyes give away way more about our internal processes than we might like.

If you’re dealing with someone you think is shady, holding eye contact can be a simple way to keep them honest. Even if you’re dealing with someone who isn’t deliberately trying to deceive you, eye contact can still be a power tool. Think, for example, about trying to buy a car. Your eye contact keeping the seller honest can help you to get vital information about the history of the car, or get them to offer you a more realistic starting figure.

Whatever the scenario, remember this: eye contact is fundamentally connected to our perceptions of truth and honesty.

Eye contact makes you more self-aware.

French researchers at the University of Paris found that people are far more self-aware (defined as being more or less aware of what is going on with one’s physical body) when someone else makes eye contact than when they are not. The researchers believe that we become more focused on ourselves and aware of our behavior when others are looking at us.

There’s a thin line between self-awareness and self-consciousness, but being aware of this phenomenon can help you make the most of it. In the company of other people — in a meeting, at a party, on a date — you might find that someone’s eye contact makes you more sensitive to your own thoughts, feelings and behaviors. Once you notice that happening, you can check in with yourself and give a bit of thought to the impression you’re making. That is where self-consciousness heightened by eye contact can actually work for you.

You can also use your eye contact to heighten self-awareness in others. I certainly wouldn’t advise anyone to bully someone with eye contact, but what about maintaining eye contact with a girl you’re really getting along with? That cool, nervous butterflies feeling can be a lot of fun. When you maintain eye contact with her, heightening her self-awareness in the process, you’re giving her a little gift. You’re allowing her to enjoy that feeling even more. And you’re communicating a number of important things: that you’re listening, that you’re paying attention, and that you want her to enjoy the present moment with you.

You’re also doing something essential, because…

Hold onto your hats, gents. What I’m about to tell you will make you love eye contact: eye contact will make people like you. Because according to research, holding someone’s gaze has been proven to create attraction.

And while you might know that a winning smile is a great way to appear attractive, putting direct eye contact behind that winning smile is your best bet, says one study conducted by Aberdeen University in Scotland. It seems that looking directly at someone does most of the work in creating attraction, and dramatically enhances other behaviors like smiling, touching and listening.

What’s more, smiling while making eye contact has the most power when talking to women, according to the researchers. “What we’ve shown is that people seem to like someone who likes them – based on the direction of their gaze – and it’s particularly true of the opposite sex.” (My emphasis.) However, while it might be more powerful with women, it will also have an impact when you’re meeting new people at work, networking or making friends in a new city. Eye contact is universal.

So whether you’re hustling to meet new people or trying to deepen a connection you already have, smile big, make eye contact and go straight after what you want. It won’t always be easy, but it’s the best strategy you have.

Eye contact can even make you fall in love.

“Love at first sight” is apparently a real thing. But here’s the deal: it only works on your end. Researchers using hidden cameras found that men who stared at a woman for 8.2 seconds or more were far more likely to feel like they had fallen in love at first sight. On the other hand, if a man looked at woman for 4.5 seconds or less, he was likely to not have any interest in her at all. There’s just one problem: The women did not reciprocate the men’s amorous feeling. It was all in the man’s head.

The lesson here is that eye contact can be powerful, but not always mutual — and, in the case of this study, not always reliable. Because our gaze can create an incredibly powerful experience (sometimes only for ourselves), it’s easy to project those feelings onto its objects. Be aware of that, and resist the feeling that you’ve “fallen in love” before she even opens her mouth. Eye contact is a powerful attractor and opening fundamental, but there’s a human to get to know on the other side.
The bottom line? Eye contact is an immensely powerful capability that creates better connections, keeps people honest and generally enriches relationships. With a bit of practice, you can become a master of this essential skill.

Push and pull get eyes to work together

Researchers appear to have found a better way to correct sensory eye dominance, a condition in which an imbalance between the eyes compromises fine depth perception. The key is a push-pull training method in which the weak eye is made to work while vision in the strong eye is actively suppressed, according to a report published online on Oct. 14 in Current Biology, a Cell Press publication.

"After a 10-day training period, we found our participants' sensory eye dominance is significantly reduced as the two eyes become more balanced," said Teng Leng Ooi of Pennsylvania College of Optometry at Salus University. "As a consequence, their depth perception also improves significantly."

Most people have excellent three-dimensional depth perception because their two eyes work together as an even team, explained Ooi and her colleagues Zijiang He and graduate student Jingping Xu of the University of Louisville. That's why it is easier to thread a needle with two eyes opened than with one eye closed.

"By using the visual images from both eyes, the brain can construct a 3D visual world that enables us to precisely judge the depth of objects," He said. For that to work optimally, the two eyes have to contribute equally. If one eye becomes stronger, depth perception degrades. In the extreme, that imbalance is similar to amblyopia, more commonly known as lazy eye, a condition that affects two to three percent of children in the United States.

If the imbalance of amblyopia goes uncorrected early, the weak eye can become severely suppressed. Treatments today aim to correct the problem by covering up the stronger eye part of the time, making the weaker one do all the work, a treatment that follows the logic of "use it or lose it."

But, the researchers report, that "push-only" training strategy doesn't work very well in adults with sensory eye dominance, whose neural wiring is less flexible. They now show in a study supported by the National Eye Institute that what does work is an alternative method in which the two eyes are exposed to different patterns in a way that ensures that only the images presented to the weak eye are perceived.

The method is based on Ooi and He's earlier work in which they studied how the brain determines which eye's image is perceived when the two eyes receive very different images, for example, horizontal grating in one eye and vertical grating in the other eye.

"Typically, such a stimulation results in one alternately perceiving the image in each eye," He said. "At one moment, the left eye's image is seen, and the next moment, the right eye's image is seen, and so on. It is as if the two eyes compete for perception."

Ooi and He found that when the two eyes are forced to compete in that way, they could tilt the competition for perception in favor of one eye by attracting a form of visual attention to it. This is done by cueing one eye before the competition begins. In the new method, this simply means that a square frame is presented to the weaker eye before the competitive images appear.

The researchers say that they don't yet understand exactly how this push-pull training method works to readjust the balance between the eyes. "Possibly, by causing the strong eye to be suppressed at all times during the training, we reduce the inhibitory hold of the strong eye on the weak eye," Ooi said. Further behavioral and neurophysiological studies are needed to explore the mechanism.

The new push-pull strategy could be used to reduce sensory eye dominance, which could be especially important in those for whom fine depth perception is critical for their vocations, including dentists, surgeons, machinists, and athletes. The researchers also expect that it can be adapted for treating children with amblyopia.

The researchers include Jingping P. Xu, University of Louisville, Louisville, KY Zijiang J. He, University of Louisville, Louisville, KY and Teng Leng Ooi, Pennsylvania College of Optometry at Salus University, Elkins Park, PA.

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Why do my brother and I look so different?

-An elementary school student from California

This is a great question! There are two main reasons you two do not look alike. The first is that the two of you didn't grow up the same way. And the second is that even though you have the same parents, you don't have the same genes.

Different Environment

The way you grew up is what geneticists call your environment. It includes where you grow up, what you eat, and what you do. It also includes what your mom did and ate while she was pregnant with you.

All of these things play a role in the way you look. People are different in part because no two people grow up in the same environment. Not even twins are together all the time!

One example of how your environment can affect the way you look is in your height. There are certain periods in your life when what you do plays a big part in your height. Two of these times are when you are first born and when you hit puberty.

If you do not eat well during these two times then you will be shorter. But if you eat well then you will be taller. Extreme examples of this include neglected kids and kids in war zones. They tend to be much shorter than average.

There are lots of other traits affected by the environment. Things like personality, weight, and intelligence just to name a few. While the environment is important, getting different gene versions from your parents is probably an even bigger reason for why you and your brother ended up so different.

Different Gene Versions

Your genes play a big role in making you who you are. The color of your hair, the color of your eyes, and the dimples on your cheeks are all controlled in part by your genes.

At first it might seem like kids from the same parents should look alike. After all, kids get their genes from the same parents.

But brothers and sisters don't look exactly alike because everyone (including parents) actually has two copies of most of their genes. And these copies can be different.

Parents pass one of their two copies of each of their genes to their kids. Which copy a child gets is totally random. And this is a big reason why you don't look like your brother.

This is all a bit abstract so let's use a specific example to hopefully make it all clearer. Let's look at the dimples some people have when they smile.

The gene that makes dimples comes in two forms (or alleles), D and d. D gives you dimples and d means no dimples.

Like almost all genes, you have two copies of the gene that can give you dimples. It is obvious what happens if you have two D copies -- you have dimples. And two d copies clearly means no dimples.

But what about a D and a d copy? Then you have dimples. In genetics speak, we say that D is dominant over d (or that d is recessive to D).

OK so what does that have to do with you and your brother? Let's do an example with some parents to show why this matters.

Let's say your dad has a D and a d copy. He has dimples but might pass the d (no dimple) copy to you. He is a carrier of no dimples.

Which copy he passes to you is random. It's like flipping a quarter. Half of the time you get heads and half of the time you get tails. So there is a 50% chance you get D and a 50% chance you get d from your dad.

Let's say your mom doesn't have dimples. This means that both of her copies are d. Which means she can only pass a d to you.

So combining one copy from your dad and one copy from your mom means you either have D and d (Dd) or d and d (dd).

What this means is that each of your parents' children has a 50-50 shot at dimples (Dd) and a 50-50 shot at no dimples (dd). If you got d from dad and your brother got D, then your brother would have dimples and you wouldn't.

Having dimples or not is just one example of many ways in which you and your brother may be different. Every person has about 20,000 genes. And many of these genes come in different versions.

So for every gene where your dad has two different copies, then you and your brother have a 50-50 shot of getting a different version. Same thing with your mom.

Let's say that your parents each had 10,000 genes with different versions. The odds that you and your brother would get the same versions of each gene is really, really small. It's the same as flipping a quarter and getting heads 10,000 times in a row!

The odds aren't actually that low

A big assumption we made in coming up with these odds is that genes are independent of each other. In other words, each gene is passed on without any other gene affecting it.

This would be true if each of our genes is physically separate from the others. But they're not. Your genes are strung together on chromosomes. We have 20,000 genes on just 23 pairs of chromosomes. That means each of the chromosomes has lots of genes.

Parents do not pass genes to their kids -- they pass chromosomes. So if genes are next to each other on chromosomes, then they often get passed down together.

Let's say that the gene for dimples is next to the gene for wet earwax. Wet earwax comes in two versions W and w. Imagine your dad has D and W on one chromosome and d and w on the other chromosome.

So if you have dimples, then most likely you'll have wet earwax too. And if you don't have dimples, you'll have dry ear wax (at least in this example).

In other words, if you get D, you are almost certain to get W. And if you get d, you are almost certain to get w.

Brothers Can Be Really Different

Just like how you don't think you look like your brother there is a similar example that has even made the news. Twin boys were born in July 2008. One had white skin color and the other had black skin color!

How did this happen? First the twins are fraternal. This means their genes are as similar as non-twin brothers. Second they have a white German father and a black African mother. Children of similar mixed-race parents usually have a blended skin tone.

However examples such as these German twins happen from time to time. This is because there are at least seven different genes that affect skin tone. It's all up to chance which combinations the two brothers got. And in this case they got two very different combinations!

Is it true you lose depth perception when looking out of one eye?

If one uses one eye, the brain does try to compensate.

Try covering one eye and, perhaps with the help of a friend, gauge the distance to an object. Or alone, close one eye and reach for something about 18 inches or 0.5 m away - and see the difference between using one eye and two.

Yes, if you're in a familiar surrounding, you already know where things are relative to one another, and can judge distance based on the apparent size of objects, because you already know their size, so your brain can compensate for the loss of visual information (you could navigate a familiar room fairly well with both eyes closed. blind people rely on this). Also, as you get closer to an object and move your head around to see different sides, you can compensate somewhat for lack of binocular vision.

If you really want to test this, go to an unfamiliar location and try to judge distance. Or, sit in an empty room or open field (where you won't have other clues about distance), and have a friend place objects at different distances when you aren't looking, and then with one eye closed, see how well you can identify which is the closer or farther object. Or, close your eyes, have someone choose a distance to stand from you, then open only one eye and try to throw a ball to them (choose something soft so you don't hurt them when your aim is off).

I can't do this with both eyes open either :uhh:

I'll try this later (except for the throwing one, which I can't do with both eyes open either :rofl: )

Thanks for the suggestions, although I still think everything looks the same. Are you supposed to be able to *see* a difference?

you see depth with one eye since your memory can fill in the information. its impossible to tell the position is without two stationary sensors(i think it applies only for 2 dimension space, for 3 dimensions is take 3 sensors)

ask a friend to hide his whole body behind an object, and only show his two fingers(at a small distance) when one is behind the other, and try to observe which finger is closer(try this for a couple of times, since its a 50-50 chance for each observation to be true) and then try it with two eyes, youll see the difference

Classic example. Open both eyes.

Now 'point' with your left and right hands. Hold your hands out infront of you and touch the tips of your 'pointing fingers'

Now close one eye and see if you can do it, you cant.

I am not 100% sure about this but this simple thought experiment may account for it a little bit. I don't know how the brain works. but this is one reason why it may work for two eyes but not for 1 (it definitely won't work for one)

If you hold a pencil infront of your eyes and close one eye, and then take turns and close the other eye, you will notice that the two images that your eyes are recieving differ. In fact, the closer an object is, the more the two images differ (in respect to that object). could this be how our brain judges depth? Maybe someone with a degree in biology can explain how it works. I would research into it but I can't right now.

The reason that you can't judge depth with only one eye is blatant, there is nothing for the brain to compare with. Think about holding two unsharpened pencils, one quite a bit further from the other, but it is bigger (the viewer does not know this) and it is held in such a position that the sizes look the same. If they were being hovered in the air so that you couldn't use other factors to judge, wouldn't they look to be the same distance? But with two eyes your brain could use the two seperate images to compare them.

I am not 100% sure about this but this simple thought experiment may account for it a little bit. I don't know how the brain works. but this is one reason why it may work for two eyes but not for 1 (it definitely won't work for one)

If you hold a pencil infront of your eyes and close one eye, and then take turns and close the other eye, you will notice that the two images that your eyes are recieving differ. In fact, the closer an object is, the more the two images differ (in respect to that object). could this be how our brain judges depth? Maybe someone with a degree in biology can explain how it works. I would research into it but I can't right now.

The reason that you can't judge depth with only one eye is blatant, there is nothing for the brain to compare with. Think about holding two unsharpened pencils, one quite a bit further from the other, but it is bigger (the viewer does not know this) and it is held in such a position that the sizes look the same. If they were being hovered in the air so that you couldn't use other factors to judge, wouldn't they look to be the same distance? But with two eyes your brain could use the two seperate images to compare them.

well, it can be mathematically proven that in two dimensions, you may measure distance with two dot sized "angle sensors" placed in two points in space at a known distance between them. though i do not know if the brain uses the same way to calculate things(i believe not, since the eyes are very memory dependent, also the brain seem to prefer wacky ways to function, it never favored symmetry, unlike our contiuos mind)

one sensor can not do such thing, if it could, how would it determine whether an object is closer, or bigger? heh, i think that if we were not able to determine if an object is closer or bigger, then we would not know what length is, we would only see space as angles, quite odd it would be.

What Makes Pi So Special?

No number can claim more fame than pi. But why, exactly?

Defined as the ratio of the circumference of a circle to its diameter, pi, or in symbol form, π, seems a simple enough concept. But it turns out to be an "irrational number," meaning its exact value is inherently unknowable. Computer scientists have calculated billions of digits of pi, starting with 3.14159265358979323…, but because no recognizable pattern emerges in the succession of its digits, we could continue calculating the next digit, and the next, and the next, for millennia, and we'd still have no idea which digit might emerge next. The digits of pi continue their senseless procession all the way to infinity.

Ancient mathematicians apparently found the concept of irrationality completely maddening. It struck them as an affront to the omniscience of God, for how could the Almighty know everything if numbers exist that are inherently unknowable?

Whether or not humans and gods grasp the irrational number, pi seems to crop up everywhere, even in places that have no ostensible connection to circles. For example, among a collection of random whole numbers, the probability that any two numbers have no common factor &mdash that they are "relatively prime" &mdash is equal to 6/π 2 . Strange, no?

But pi's ubiquity goes beyond math. The number crops up in the natural world, too. It appears everywhere there's a circle, of course, such as the disk of the sun, the spiral of the DNA double helix, the pupil of the eye, the concentric rings that travel outward from splashes in ponds. Pi also appears in the physics that describes waves, such as ripples of light and sound. It even enters into the equation that defines how precisely we can know the state of the universe, known as Heisenberg's uncertainty principle.

Finally, pi emerges in the shapes of rivers. A river's windiness is determined by its "meandering ratio," or the ratio of the river's actual length to the distance from its source to its mouth as the crow flies. Rivers that flow straight from source to mouth have small meandering ratios, while ones that lollygag along the way have high ones. Turns out, the average meandering ratio of rivers approaches &mdash you guessed it &mdash pi.

Albert Einstein was the first to explain this fascinating fact. He used fluid dynamics and chaos theory to show that rivers tend to bend into loops. The slightest curve in a river will generate faster currents on the outer side of the curve, which will cause erosion and a sharper bend. This process will gradually tighten the loop, until chaos causes the river to suddenly double back on itself, at which point it will begin forming a loop in the other direction.

Because the length of a near-circular loop is like the circumference of a circle, while the straight-line distance from one bend to the next is diameter-like, it makes sense that the ratio of these lengths would be pi-like.

Follow Natalie Wolchover on Twitter @nattyover or Life's Little Mysteries @llmysteries. We're also on Facebook & Google+.

Different Types of Heterochromia

‘Heterochromia’ is a Latin term meaning ‘different colors,’ which perfectly describes this trait. There are actually three distinct categories of heterochromia, although some people may have a combination of two or three:

  1. Complete (each eye is a different color).
  2. Sectoral (a segment of one or both irises is a different color).
  3. Central (a different color surrounds the pupil).

Senses: Eyes & Sight for PreK-2

As you explore the amazing senses with your children, encourage them to ask you questions and also ask them some of the following questions. Can you chew food with your nose or smell a flower with your ears? No, of course you can’t. But why not? Because God made each of our body parts with certain jobs to do and the ability to get their jobs done. For example, it is easy to pick up a sheet of paper using your fingers, but do you think you could do it using only your teeth and lips? This would probably look funny, but it also shows you that you can use your mouth and other parts of your body to pick things up, but it is harder than using your hands.

Humans have five different senses do you know what they are? Do you know what parts of your body you use to do each of those things? We see with our eyes, we hear with our ears, smell with our nose, taste with our tongue, and feel with our hands and skin.

Eyes & Vision

As your children look at their eyes in a mirror, talk about the visible eye parts. Then let them look at yours or their siblings’ eyes and compare what they see. Keep the discussion simple for little kids by talking about how eyelids, eyelashes, and tears protect our eyes by keeping dust and other harmful things from getting in.

How do our eyes work? The little dark circle in the center of each of your eyes lets light in. It is called a pupil. If you are in a dark place where no lights at all are on, can you see anything? No, you can’t because our eyes need light to be able to see! Once the light goes in, it hits a part inside at the back of your eye that is very sensitive to light. This part is called the retina. When light touches the retina, it makes an upside-down picture of whatever you are looking at. A large nerve called the optic nerve carries the image to your brain where it gets turned around so that you see it the right way instead of upside-down!

Thinking Scientifically: Your eyes and your brain work together very quickly to flip images around so that you see them right side up. It happens automatically whenever your eyes are open. Seeing is like breathing, you don’t even have to think about it, but you do it all the time!

Plan to get your children’s eyes checked around the time you are studying eyes and ask if you and your kids can go on a tour of the eye doctor’s office while you are there. You can learn all about caring for your eyes, eye diseases and vision problems, the equipment eye doctors use to check and treat your eyes, and how glasses and contact lenses help correct vision. This trip is a fun learning experience for all ages, but will be most beneficial to older children.

Activity 1

Standing several feet away from your child, toss a bean bag back and forth (or two kids can do it together). After a few tosses, blindfold your child (or children) and yourself. Then try tossing the bean bag back and forth again. Talk about what senses you had to use when your eyes were covered and what was harder to do when you couldn’t see. How many more times did you catch the bean bag with your eyes open than when you had the blindfold on?

For younger children: Sit on the floor and roll a large ball instead. Cover only one eye and discover how both eyes work together.

Activity 2

Introduce the Braille alphabet. Braille is what people who cannot see use to read and write! They ‘see” the letters by feeling them with their finger tips instead of looking at them with their eyes. Print out a copy of the (Braille alphabet): To make the letters raised like Braille, put a drop of white glue over each black dot. When the glue is dry you will be able to feel the raised dots with your fingers.

For older children: Encourage your kids memorize the letters (all or some) of the Braille alphabet. Then write a word with dots of glue, let it dry, and have your child ‘read” it with his or her eyes closed. Discuss how and why it is much easier to read with your eyes than feeling letters with your fingers.

Activity 3

Kids love to explore the world around them. Make good use of their curiosity by equipping them with a magnifying glass so they can look at things up close. Even a pesky house fly is fun to look at under a magnifying glass! As you talk to them about sight, give your kids some objects to look at for a few minutes and then give them each a magnifying glass to compare how much different things look through a lens than through our eyes alone.

Ask evolution: Why do we have butt cheeks?

It’s quite natural to be curious about butts. We all have them, and many of us think about them pretty frequently. But have you ever considered why we have such sizeable butt cheeks? Sure, they’re useful for sitting on, but do they serve any other purpose?

Human butts are pretty special: modern society makes a good deal of fuss about the size of people’s butts, but big or small, yours is almost guaranteed to be larger than that of any non-human primate. Gorillas are pretty flat back there, and the only way a chimp could “break the Internet” is if it were let loose in one of Google’s server farms.

Muscle butts

The anatomical basis for the exceptional size of human butts is due to both a large amount of fat and a large amount of muscle. The latter — the gluteus maximus — adds most of the default bulk, while the layer of fat that sits over it varies a lot more from person to person.

Explaining the size of our butt muscles is reasonably straightforward, at least for evolutionary anthropologists like Associate Professor Darren Curnoe from the University of New South Wales.

“The major differences between humans and other apes are the result of our evolution as bipeds, or two-footed apes,” he says. “The muscles we have in common with apes actually often function quite differently in humans, moving our bodies about on two feet instead of four.”

Large, thick gluteal muscles help us remain stable while walking upright, and our pelvises have been moulded by evolution (wider side-to-side, but also shallower front-to-back) to ease the transition to moving about on two legs, which combine to produce a distinctive curve to our posterior, as well as give us much wider hips.

Fat butts

The abundance of fat on human butts is a little harder to explain. There’s no clear connection between walking upright and needing a thicker layer of fat on the behind, so anthropologists have turned to other hypotheses.

One idea is that “fat around the hips, buttocks and thighs represent a safe storage space to help humans survive episodes of food shortage, which were probably regular for our hunter-gatherer ancestors,” says Curnoe. “But also, because breastfeeding is very demanding in terms of energy consumption, the extra fat is probably a kind of insurance policy for women to ensure both their survival and that of their vulnerable infants during the first few years of life.”

In other words, big butts might be a byproduct of the general fattiness of humans — we’re the some of the fattest primates around, (although mind the fat-tailed dwarf lemur, which stores fat in its tail to get through the winter) even before you consider the world’s current obesity crisis.

That tail is fat, not fluff. (David Haring/DUPC/Getty Images)
Source: Getty Images

Unfortunately, we might never know for sure why our ancestors evolved to have so much fat. “Theories about the role fat may have played in this area in our evolution are terribly difficult to establish,” says Curnoe. “We don't have evidence from the past, of course, because we don't have any soft tissue — only bones fossilise.”

“My hope is that maybe with the remarkable new science of ancient human DNA, we may be able to trace the evolution of human body fat in other species, like the Neanderthals and Denisovans,” says Curnoe. Finding genetic information linked to fat deposition in the buttocks in the ancient DNA of our extinct cousins might be the only way of shedding any light on the issue. The history of our cheeks might be obscured until then.

But regardless of what happens, sit down and consider your butt with pride. It turns out to be a rather big part of what makes you human.


  1. Butrus

    Now everything is clear, thanks for the help in this matter.

  2. Kajirr

    I do not have

  3. Kasia

    I love it very much!

  4. Galvarium

    Wonderful, this is a funny answer

  5. Adriaan

    Like attentively would read, but has not understood

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