If you know an older left-handed person, chances are they had to learn to write or eat with their right hand. And in many parts of the world, it's still common practice to force children to use their "proper" hand. Even the word for right also means correct or good, not just in English, but many other languages, too. But if being left-handed is so wrong, then why does it happen in the first place? Today, about 1/10 of the world's population are left-handed. Archeological evidence shows that it's been that way for as long as 500,000 years, with about 10% of human remains showing the associated differences in arm length and bone density, and some ancient tools and artifacts showing evidence of left-hand use. And despite what many may think, handedness is not a choice. It can be predicted even before birth based on the fetus' position in the womb. So, if handedness is inborn, does that mean it's genetic? Well, yes and no. Identical twins, who have the same genes, can have different dominant hands. In fact, this happens as often as it does with any other sibling pair. But the chances of being right or left-handed are determined by the handedness of your parents in surprisingly consistent ratios. If your father was left-handed but your mother was right-handed, you have a 17% chance of being born left-handed, while two righties will have a left-handed child only 10% of the time. Handedness seems to be determined by a roll of the dice, but the odds are set by your genes. All of this implies there's a reason that evolution has produced this small proportion of lefties, and maintained it over the course of millennia. And while there have been several theories attempting to explain why handedness exists in the first place, or why most people are right-handed, a recent mathematical model suggests that the actual ratio reflects a balance between competitive and cooperative pressures on human evolution. The benefits of being left-handed are clearest in activities involving an opponent, like combat or competitive sports. For example, about 50% of top hitters in baseball have been left-handed. Why? Think of it as a surprise advantage. Because lefties are a minority to begin with, both right-handed and left-handed competitors will spend most of their time encountering and practicing against righties. So when the two face each other, the left-hander will be better prepared against this right-handed opponent, while the righty will be thrown off. This fighting hypothesis, where an imbalance in the population results in an advantage for left-handed fighters or athletes, is an example of negative frequency-dependent selection. But according to the principles of evolution, groups that have a relative advantage tend to grow until that advantage disappears. If people were only fighting and competing throughout human evolution, natural selection would lead to more lefties being the ones that made it until there were so many of them, that it was no longer a rare asset. So in a purely competitive world, 50% of the population would be left-handed. But human evolution has been shaped by cooperation, as well as competition. And cooperative pressure pushes handedness distribution in the opposite direction. In golf, where performance doesn't depend on the opponent, only 4% of top players are left-handed, an example of the wider phenomenon of tool sharing. Just as young potential golfers can more easily find a set of right-handed clubs, many of the important instruments that have shaped society were designed for the right-handed majority. Because lefties are worse at using these tools, and suffer from higher accident rates, they would be less successful in a purely cooperative world, eventually disappearing from the population. So by correctly predicting the distribution of left-handed people in the general population, as well as matching data from various sports, the model indicates that the persistence of lefties as a small but stable minority reflects an equilibrium that comes from competitive and cooperative effects playing out simultaneously over time. And the most intriguing thing is what the numbers can tell us about various populations. From the skewed distribution of pawedness in cooperative animals, to the slightly larger percentage of lefties in competitive hunter-gatherer societies, we may even find that the answers to some puzzles of early human evolution are already in our hands.
Ako poznajete stariju levoruku osobu, velike su šanse da su morali da uče ili jedu svojom desnom rukom. U mnogim delovima sveta, i dalje je ustaljen slučaj da teraiju decu da koriste svoju "odgovarajuću" ruku. Čak i reč "right" (eng. desno) takođe znači tačno ili dobro, ne samo na engleskom već i na mnogim drugim jezicima. Ali ako je biti levoruk toliko pogrešno, zašto se onda to uopšte dešava? Danas je oko 1/10 svetske populacije levoruko. Arheološki nalazi pokazuju da se ovo dešava već 500 000 godina, da oko 10% čovečanstva i dalje pokazuje povezane razlike u dužini ruke i gustini kostiju, a neki prastari alati i artefakti dokazuju levoruko korišćenje. Uprkos onome što mnogi misle, upotreba ruke nije izbor. Može se predvideti čak pre rođenja na osnovu položaja fetusa u materici. Ako je upotreba ruke nasledna, da li to znači da je genetski uslovljena? I da i ne. Identični blizanci koji imaju iste gene mogu da imaju različite dominantne ruke. Zapravo, ovo se javlja jednako često koliko i sa svakim drugim parom. Ali šanse da se postane levoruk su određene upotrebom ruke vaših roditelja u iznenađujuće ustaljenom rasponu. Ako je vaš otac bio levoruk, a vaša majka desnoruka, imate 17% šansi da se rodite levoruki, dok će dva dešnjaka imati levoruko dete samo u 10% slučajeva. Izgleda da se upotreba ruke određuje bacanjem kockice, ali šanse su uslovljene vašim genima. Sve ovo naznačava da postoji razlog da je evolucija proizvela ovu malu proporciju levorukih, i održala je tokom milenijuma. Iako je postojalo nekoliko teorija koje su težile da objasne zašto izbor ruke uopšte postoji, ili zašto je većina ljudi desnoruko, skorašnji matematički model objašnjava da trenutna količina predstavlja balanas između takmičarskih i sarađujućih pritisaka na ljudsku evoluciju. Dobiti od levorukosti su naistaknutije u aktivnostima koje uključuju suparnika, poput borbe ili takmičarskih sportova. Na primer, oko 50% najboljih udarača u bejzbolu bilo je levoruko. Zašto? Posmatrajte to kao iznenađujuću prednost. Pošto su levaci manjina, levoruki i desnoruki takmičari provesšće najviše svog vremena upoznavajući i vežbajući protiv levaka. Kada se dvojica suoče, levoruki će biti bolje spremljen protiv svog desnorukog takmaca, dok će dešnjak biti izbačen. Ova borbena pretpostavka, gde disbalans u stanovništvu za posledicu ima prednosti za levoruke borce ili sportiste, primer je negativne selekcije zavisne od učestalosti. Ali prema principima evolucije, grupe koje imaju relativne prednosti teže da rastu dok ta prednost ne nestane. Ako su se ljudi samo tukli i nadmetali kroz ljudsku evoluciju, prirodna selekcija bi dovela da više levaka pobeđuje dok ne bude mnogo njih, da na kraju to ne bi ni bilo redak slučaj. U potpuno takmičarskom svetu, 50% stanovništva bilo bi levoruko. Ali ljudska evolucija je oblikovana saradnjom, ali i nadmetanjem. Takmičarski pritisak tera podelu korišćenja ruke u suprotnom pravcu. U golfu, gde nastup ne zavisi od suparnika, samo je 4% najboljih igrača levoruko, kao primer šireg fenomena podele alata. Kao što mladi potencijalni golferi mogu lako da pronađu set štapova za desnoruke, mnogi od važnih instrumenata koji su oblikovali društvo bili su dizajnirani za desnoruku većinu. Levaci su lošiji u korišćenju ovih alata, i pogađa ih veći nivo nesreća i bili bi manje uspešni u čisto kooperativnom svetu, i na kraju bi nestali iz stanovništva. Pravilnim predviđanjem podele levorukih ljudi u opštoj populaciji, kao i usklađivanjem podataka iz različitih sportova, model pokazuje da upornost levaka, male ali stabilne manjine, odaje jednakost koja potiče iz takmičarskih i sarađivačkih efekata koji se vremenom paralelno smenjuju. Najintrigantnija stvar je šta brojevi mogu da nam kažu o različitim stanovništvima. Od iskrivljene podele upotrebe šapa kod društvenih životinja, do malo manjeg procenta levaka u takmičarskim društvima koja lovaca i sakupljača, možemo čak da otkrijemo da su odgovori na neke nedoumice rane ljudske evolucije uveliko u našim rukama.