Hey smart people, Joe here. Bodies. We've all got 'em, couldn't live without 'em, but why are they so dumb（哑的） and unreliable（ 不可靠的）? I don't know if it's because I just got over a sinus infection（感染）, or because my wife just had knee surgery（外科）, or maybe it's just me getting older and hurting more. I'm a millennial, but I have been noticing lately（近来） that the human body, it's got a lot of problems, and this video is a big long rant about a bunch（束） of 'em. I mean, don't get me wrong.

  Humans are really awesome. I mean, look at all that we can do. It's just that there's so much about our bodies that is flawed（ 有缺点的）, like so many of our parts wear down or are easy to break, and others look like Ikea furniture would look if you accidentally（偶然地） threw away the instructions before putting it together, and that it basically（基本上） functions（运行）, but you're pretty sure something's backwards and somehow（由于某种原因） you have like three of those little twisty things left over? The great American poet John Mayer once said, "Your body is a wonderland, but I think you meant your body is a blunderland. From eyes that don't work right, and backs（支持） that ache to needy（贫困的） diets and extra bones, what I'm saying is, sure, our bodies look cool, especially if they're wearing an awesome shirt, but who the heck designed these things?" Well, no one did. We'll get back to that, but first, instead of talking about how great we are, let's talk about some of our critical（决定性的） weaknesses（虚弱） for a change. The first example is staring（ 盯着看） right at ya.

  I mean, I don't wear these things to look cool and smart. I mean, they do make（使） me look cool and smart, but I wear them because I can't see. Like nearly half of Americans and Europeans, or nearly seven in ten people in Asian countries, my peepers don't peep（ 瞥见） right. I've worn glasses since elementary（基本的） school. Space shuttle. Nice.

  Very on brand, younger me. Anyway, before the invention of corrective lenses a few centuries ago, people who couldn't see just couldn't see. Back in our prehistoric（ 史前的） hunter-gatherer（收集者） days, that could have meant starvation and death. Bad eyes, empty stomachs, you lose. Thing is, even if you don't wear glasses, you have eye problems. While looking at this image, cover your left eye and look at the dot（点） while keeping your face centered（居中） in front of the screen.

  Slowly move closer or farther from your screen and the cross will disappear. Did it work? Around 30 centimeters or 12 inches（英寸） away works for me. Pretty weird（怪异的）, huh? You can try it with the other eye too. Cover your right one, stare at the cross, and move until the dot disappears.

  That's your blind（瞎的） spot, and every animal with a backbone（支柱） has a blind（瞎的） spot in each eye because of how the eye is built. The light-sensitive（敏感的） layer inside your eye is filled with tiny cells（细胞） called photoreceptors. They're like little microphones（扩音器）. One end turns photons of light into electrical signals, and the other end is a wire（金属线） that carries the signal away. Except our retinas（[解剖] 视网膜） are built so the cables（缆） are pointed towards the light, like talking to the back of a microphone. The cables from all those little microphones have to pass through a hole in the retina to get to the brain.

  And where that hole is, we have a blind spot. We just don't usually notice it because our brain lies to us and fills in the image. Why do we have it? Because at some point, way（大大地） back in evolution（演变）, when our ancestors（祖宗） started to evolve（发展） the first light-sensitive（敏感的） tissues, that's just the direction the cells were facing. And later, when those patches（片） morphed into actual（实际的） eyes, it was too late, the backwards pattern was already set. When you can't suddenly flip（掷） a whole eye around, it can only make tweaks to what's already there.

  But cephalopods, like octopuses（章鱼）, squid, and cuttlefish, they don't have a blind spot. This branch of animals evolved eyes completely on their own, and in early octopus ancestors, the cables on all their microphone-shaped（形成） light-sensing（感觉到） cells（细胞） pointed towards the back, so their retina is unbroken（ 完整的）. Am I saying that cephalopods have better eyes than us? Yes, point cephalopods, and another point for having eight legs.