Evolution（演变） spent nearly 400 million years crafting（精巧地制作） these works of art, two of the most important pieces of the human evolutionary（进化的） puzzle（难题）, yet（但是） 99% of us end up being good with one hand and not the other for common tasks like writing, high-fiving, and the all-important one-handed texting. Even life itself seems to have chosen sides. Our amino（ 氨基的） acids（酸） are said to be left-handed. Our DNA turns into a right-handed helix. Both of our hands seem fully（充分地） capable（有能力的）, they're both connected to fully functional（ 实用的） arms（臂）, so we'd expect an even split（劈开） when it comes to which hand we prefer. In the animal kingdom（王国） where we find（找到） creatures（生物） with a preference for one paw, hoof（蹄）, or wing, it's usually 50/50, yet on average only one out of ten humans are south paws（爪）.

  I guess the real question is why are so few people left-handed? Maybe it's because the world is conspiring against them, I mean spiral notebooks（笔记本）, scissors, e-readers, video game controllers, zippers, even can-openers. Only somebody would open a chain of stores that's specifically stocked（进货） with products for left-handers（支持器）. That's a moneymaker. Even language hates lefties. To be correct is to be right, and correct is also a word that comes from right.

  Gods are full of righteousness（正义）. To be skillful is to have dexterity. Kings are regal. Left itself comes from Old English, meaning weak, and words for left give us sinister, gauche（ 笨拙的）, maledroit. Bad dancers have two left feet. But language turns out to be an important part of the puzzle.

  Our brains are cross-wired, meaning that the right hand is controlled by the left side, and vice versa. But that doesn't mean our brains symmetrical（对称的）. In the 1860s French scientist Paul Broca discovered that a region of our brains used for speech processing（ 处理） usually only located on one side. Language is a complex process, and the move to one hemisphere（半球） probably helps the brain deal with it more efficiently（有效率地）. Today we know that in 99% of right-handers brains, Broca's area is located only in the left hemisphere, and in left-handers brains, it's also on the left. 70% of the time.

  Only 19% of lefties process（加工） speech in the right hemisphere（半球）, and 20% of them use both. So language in hands, not directly linked. That's confusing. Two similar theories have been proposed（提议） to explain this. They suggest that as early humans evolved（发展）, a gene（[遗] 基因） mutant popped（ 发出“砰”的一声） up that threw a whole bunch（束） of our brains functions to one side, and allowed them to specialize（成为…专家）. If you have two copies of the so-called right-shift mutation（变化）, language and hand dominance（优势） move to the left hemisphere（半球）.

  That shift is slightly less if you carry one copy, and if you don't carry the right-shift mutation at all, language and handedness（用右手或左手的习惯） could go to the right or the left, just by chance. This means instead of there being a gene for left-handedness, it's really the lack of a gene. A genetic（遗传的） influence makes sense, since parents seem to sometimes pass on handedness to their kids. Two right-handed parents only have a 9% chance of having a left-handed child. One parent being left-handed raises that to 19%, and two to 26%. But research has found evidence of right-hand preference（偏爱） in captive（被俘虏的） chimps（（非洲）黑猩猩）, and yes, I'm talking about throwing poop.

  This would mean that a brain shift for coordination（协调） came first, before we became the talking, throwing geniuses（天才） that we are today. There's even another theory, which goes back to the video we did on how our bodies tell right from left when it comes to our guts（内脏）, that this whole brain hand-sidedness can be traced（跟踪） all the way back to our embryonic（[生]胚胎的） days. And in 2013, researchers reported that some of the same genes that tell our spleens and stomachs which way is which, are linked to brain asymmetry. I know you guys hate this, but the answer is, we still don't really know for sure why more of us aren't left-handed. Sorry. As you've seen though, we've got some great ideas that people are still researching, but that's how science works.

  Of course, if being right-handed and shifting（替换） our speech and coordination（协调） to one side was such a great thing for evolution（演变）, why do lefties still exist? One idea says that whether it's on the baseball mound（堆） or the tennis court, we right-handers do worse when facing lefties. We're just not used to all your strange southpaw ways. And if you happen to be holding a sword（剑） instead of a tennis racket, being lefty might be a great survival（ 幸存） tool. The simpler theory is just that nature enjoys variety. Chris McManus believes that left-handers may have what he calls random（[数] 随机的） cerebral（脑的） variation（变化）.

  Their brains are just more shuffled up. This might lead to more unexpected（想不到的） connections, more crosstalk between the right and left hemispheres, and maybe more creativity（创造力）. What's clear is that having a diverse（不同的） set of brains have made humans a much more interesting species. We may not have all the answers yet, but these questions have taught us a lot of important lessons about the brain along the way. And hey, at least we're not burning them at the stake（树桩）. Stay curious（好奇的）.