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.
Jika kau kenal orang dewasa yang kidal, kemungkinan mereka harus belajar menulis atau makan dengan tangan kanannya. Di banyak tempat di dunia, sangat lazim anak disuruh memakai tangan yang “pantas”. Bahkan kata right (kanan) juga bermakna benar atau baik. Bukan hanya dalam bahasa Inggris, tetapi juga banyak bahasa lainnya. Namun jika menjadi kidal itu salah, bagaimana awalnya ini terjadi? Saat ini, sekitar sepersepuluh populasi dunia adalah kidal. Bukti arkeolog menyebut hal ini sudah terjadi selama 500.000 tahun. Sekitar 10% sisa manusia purba menunjukkan perbedaan tersebut pada panjang tangan dan kepadatan tulang. Beberapa peralatan kuno dan artefak menunjukkan bukti penggunaan tangan kiri. Terlepas pendapat umum, dominansi tangan bukanlah pilihan. Hal ini bisa ditelaah bahkan sebelum lahir sesuai kondisi janin dalam rahim. Jika dominansi tangan bawaan lahir, apa artinya ini bersifat keturunan? Bisa ya atau tidak. Kembar identik yang punya kesamaan gen dapat memiliki tangan dominan berbeda. Bahkan ini sering terjadi dengan pasangan saudara lainnya. Namun, peluang menjadi kinan atau kidal ditentukan dominansi tangan orang tuamu. Rasionya ternyata selalu tetap. Bila ayahmu kidal tapi ibumu kinan, kau punya 17% peluang terlahir kidal. Sedangkan dua orang tua kinan hanya punya peluang 10% beranak kidal. Dominansi tangan kelihatannya ditentukan oleh kemungkinan, tetapi sebenarnya didasari gen milikmu. Semua ini menyiratkan adanya alasan evolusi menghasilkan sedikit orang kidal dan mempertahankannya selama ribuan tahun. Ada beberapa teori yang mencoba menjelaskan kenapa dominansi tangan ada atau kenapa kebanyakan orang kinan. Dan model matematika baru-baru ini menyimpulkan rasio sebenarnya mencerminkan keseimbangan antara tekanan bersaing dan bekerja sama dalam evolusi manusia. Keuntungan menjadi kidal terlihat jelas di aktivitas yang melibatkan lawan seperti perkelahian atau olahraga kompetitif. Contohnya, sekitar 50% pemukul terbaik di bisbol adalah orang kidal. Alasannya? Anggaplah ini sebagai keunggulan kejutan. Karena orang kidal sedikit jumlahnya, pemain kinan dan kidal sering kali bertemu dan berlatih melawan orang kinan. Saat keduanya bertemu, orang kidal akan lebih siap menghadapi lawan kinan sementara orang kinan dibuat tak berkutik. Hipotesa pertarungan ini, yang menunjukkan ketimpangan dalam populasi berujung pada keunggulan petarung atau atlet kidal, adalah contoh seleksi yang bergantung frekuensi negatif. Namun, berdasarkan hukum evolusi, kelompok yang punya keunggulan relatif cenderung berkembang hingga keuntungan itu hilang. Jika orang-orang hanya bertarung dan bersaing sepanjang evolusi manusia, seleksi alam akan membuat lebih banyak orang kidal bertahan. Pada akhirnya, ada banyak orang kidal, sehingga itu tak lagi jadi aset langka. Jadi, di dunia yang murni persaingan, 50% populasi akan menjadi kidal. Namun evolusi manusia telah menciptakan kerja sama, begitu juga persaingan. Tekanan bekerja sama mendorong penyebaran dari dominansi tangan ke arah sebaliknya. Dalam golf, ketika prestasi tak jadi tolak ukur lawan, hanya 4% pemain terbaik yang kidal. Ini contoh kecil dari peristiwa peralatan massal. Seperti halnya pemain golf muda berbakat yang mudah menemukan set stik golf kinan, banyak peralatan penting yang membentuk masyarakat dirancang untuk mayoritas orang kinan. Karena orang kidal kesusahan memakai alat-alat itu dan menderita dengan tingkat kecelakaan tinggi, mereka jadi sulit berhasil di dunia yang murni kerjasama ini. Mereka pun akhirnya menghilang dari populasi. Dengan mengkaji distribusi orang kidal di populasi yang luas secara tepat serta mencocokkan data beragam olahraga, model tersebut menunjukkan bahwa kegigihan orang kidal sebagai minoritas yang selalu ada memunculkan kesetimbangan. Hal itu muncul dari dampak persaingan dan kerja sama yang terjadi bersamaan dalam waktu lama. Bagian paling menarik adalah bagaimana angka bisa menjelaskan beragam populasi. Dari persebaran tak merata pada dominansi cakar hewan kooperatif hingga persentase kidal agak besar pada masyarakat pemburu peramu yang kompetitif, mungkin jawaban dari beberapa teka-teki di awal evolusi manusia sudah berada di tangan kita.