Despite the apparent prevalence of the traitamong swanky crooners and British spies and creepy old creatures who just hung onto thering for too long, only about 8% of the world's human population has blue eyes. Here in theU.S. it's about twice that, but that's still a lot less than it was at the turn of thetwentieth century when nearly half of all Americans had the trait. So what happenedéAre blueeyed humans going the way of the dodoé Well, no. We cleared up that rumor about redheadsgoing extinct; the same goes for blue eyes. While the trait is becoming more rare, it'sunlikely it will disappear all together. Which
is crazy when you consider that 10,000 yearsago blue eyes didn't even exist. In fact, there's a good chance that blueeyed peoplemay all share one common ancestor. Studies over the past decade have actually tracedthe trait to a mutation that most likely arose among browneyed people in a single humanin the Black Sea region of southeastern Europe between 6,000 and 10,000 years ago. The mutation affected what's known as theOCA2 gene, which helps our bodies produce melanin, the brownish pigment that gives colorto our hair and eyes and skin. The mutation created kind of a dimmer switch for the pigmentation,but it didn't affect the entire gene. Instead,
it only affected the production of melaninin the iris, the ring structure around the eye that regulates how much light gets in. The bulk of the iris is a thick layer of melaninproducingcells called the stroma. And the OCA2 mutation turns the production of melanin in the stromaway down, but the pigmentation still shows up elsewhere, like in the hair and skin andother parts of the iris. Basically, the mutation keeps the stroma from being brown. So why blue thené Well, strictly speaking,blue eyes aren't actually blue. Instead, in people with this mutation, the stroma is fullof nearly colorless cells. And when the light
strikes them, they scatter the wavelengthsback out, in a process similar to what makes smoke or fog look blue when light passes throughthem. If there is some yellowish pigment in thestroma, then the blue light will combine with that to make green. Throw in a little bitof brown, and you have hazel. So how did we go from having no blueeyedpeople, to hundreds of millions of them in less than 10 millenniaé No one's entirelysure why the trait spread so quickly through Europe. Some scientists think the mutationcould have helped prevent certain eye disorders related to long, dark northern winters.
But another factors appears to be that, forwhatever reason, lots of blueeyed people simply mated with other blueeyed people inthe past, which kept the trait in circulation. Because for each of your genes, you have twodifferent versions, called alleles, one from your father and one from your mother. If youhave at least one dominant allele for a gene, that's the trait you have. The blue eyes come from a recessive allele,which means if you inherit one allele for blue eyes and another for brown, you're goingto have brown eyes. But you still carry the recessive blue allele, which can be passedon. Which means that that first person that
had that blueeyed mutation didn't have blueeyes. They had to pass that onto their children, and their children had to pass it onto theirchildren, until eventually they came back together to make someone with two blueeyedalleles. When both parents have blue eyes, they bothhave two recessive blueeyed genes, which means their children will also have blue eyessince there's no dominant gene to mask the recessive one. This is how you end up withScandinavian countries that are 95% blueeyed, and it also explains why the percentage ofblue eyeers is dropping in much of the western world.
The Teenage Brain Explained
Being a teenager is hard. And so is living with one, I'm told. No human gets to escape this moody, angsty, confusing phase And interestingly, such an extended adolescence is unique to humans. Other animals grow up a lot faster than we do. And you may think our teen years are just about streamlining bodies for baby making, but as it turns out, the storm of sex hormones that we associate with the teenage years, are only a small part of what's really going on in the teenage body. Most of the action, it turns out, is happening in the brain.
Until fairly recently, we thought that the brain finished the nuts and bolts of its development, by the time we started kindergarten. But really, when puberty starts, it undergoes massive remodeling. This amounts to several years of neural growing pains, as well as the other more visible growth that's going on all over your body. So take heart! Whether you're going through it now, or about to go through it, or count yourself among the veterans of that turbulent decade,
know that the result of the teen years is a stronger, faster, more sophisticated brain. If there were someone that told me twenty years ago. Let's start with that obvious scapegoat of adolescent anguish, hormones. That word itself, is kind of a lazy shorthand that people use to describe the chemicals that some glands secrete, that can affect our behavior. But the fact is, hormones have all kinds of jobs that have nothing to do with where you grow hair, or what turns you on, or whether you feel glum for no apparent reason Hormones keep your heart beating, and your body hydrated, and they make your organs grow,
and make you grow bone, and muscle, and skin! What people actually mean when they talk about 'teenage hormones', are sex hormones. And yes, puberty involves a whole series of sex hormones storms, the first of which actually kicks in before you're outta Primary School. That's when the Adrenal glands star secreting androgens, which triggers the growth in activity of the skin's sebaceous glands, making skin more oily. Soon enough, more apocrine or sweat glands get activated increasing body odor. Then comes the waves of hormonal agents that start activating the gonads.
For boys, this influx of luteinizing hormones from the pituitary gland, get testosterone from the testes, and suddenly, that guy has up to fifty times more testosterone than he did before puberty. This also changes the shape of the male body, promoting hair growth, and building up lean muscle mass, just as the increased presence of estrogen in girls rearranges the deposition of their fats, stimulating the growth of breasts. Humans are actually lucky to experience the craziness of puberty only once, many other animals undergo multiple similarly intense hormonal rodeos as they enter sexually active periods,
sometimes called the rot or heat, every new breeding season. Some male species completely stop eating during their breeding period, because they're just that sex crazed. And yet all that said, teen are far less ruled in their hormones than you might think. There are other factors that play here. For example, your favorite moody teen may be by turns punchy, angry, depressed, or in a zombie like fog, because of their chronic lack of sleep. Sleep is vital to everyone, but it's specially important for kids and teens, because it's during sleep