我爲什麼存在? 祖母纔是人類進化的關鍵

Why do I exist? This isn't a philosophical cri de coeur; it's an evolutionary conundrum. At 58, I'm well past menopause, and yet I'll soldier, on, with luck, for many years more. The conundrum is more vivid when you realize that human beings (and killer whales) are the only species where females outlive their fertility. Our closest primate relatives--chimpanzees, for example--usually die before their 50s, when they are still fertile.
我爲什麼存在？這不是一句哲學的吶喊，而是一個進化方面的謎題。58歲的我早已過了更年期，但如果幸運的話，我還會再活很多年。當你認識到人類（以及虎鯨）是雌性在生育期過後繼續存活的唯一物種之後，這個謎題就顯得更加突出了。與我們血緣最近的靈長類親戚――比如黑猩猩――一般在還有生育能力的50多歲之前就死去了。

This isn't just a miracle of modern medicine. Our human life expectancy is much longer than it used to be--but that's because far fewer children die in infancy. Anthropologists have looked at life spans in hunter-gatherer and forager societies, which are like the societies we evolved in. If you make it past childhood, you have a good chance of making it into your 60s or 70s.
這不只是現代醫學創造的奇蹟。我們人類的平均壽命比過去長了很多――但這是因爲早夭的兒童大大減少。人類學家考察過狩獵採集社會的人類壽命。這些社會跟我們最終進化形成的各種社會有相似之處。如果活到童年過後，那麼你就很有可能能夠活到60多歲或70多歲。

It turns out that my existence may actually be the key to human nature. This isn't a megalomaniacal boast but a new biological theory: the 'grandmother hypothesis.' Twenty years ago, the anthropologist Kristen Hawkes at the University of Utah went to study the Hadza, a forager group in Africa, thinking that she would uncover the origins of hunting. But then she noticed the many wiry old women who dug roots and cooked dinners and took care of babies (much like me, though my root-digging skills are restricted to dividing the irises). It turned out that these old women played an important role in providing nutrition for the group, as much as the strapping young hunters. What's more, those old women provided an absolutely crucial resource by taking care of their grandchildren.
原來，我的存在實際上可能是人類本性的關鍵。這不是狂妄自大的吹牛，而是一套新的生物學理論：“祖母假說”(grandmother hypothesis)。20年前，猶他大學(University of Utah)的人類學家克里斯滕・霍克斯(Kristen Hawkes)去非洲研究狩獵採集族羣“哈扎”(Hadza)，以爲她會揭開狩獵的起源。但去了之後她注意到很多身材精瘦的老婦人負責挖植物的根、做飯、照顧孩子（跟我很像，不過我挖植物根的技能僅限於掰鳶尾根）。原來這些老婆婆對於爲集體提供營養髮揮着重要的作用，不亞於那些年輕強壯的狩獵者。另外，老婆婆們照管孫輩，提供了一種絕對重要的資源。

There are many controversies about what happened in human evolution. But there's no debate that there were two dramatic changes in what biologists call our 'life-history': Besides living much longer than our primate relatives, our babies depend on adults for much longer.
有關人類進化史上發生過哪些事情的爭議有很多，但沒有爭議的是，生物學家所說的“生命史”上曾經有過兩次重大變化：除了壽命在靈長類親戚基礎上大大延長以外，人類嬰兒依賴成年人的時間也大大延長了。

Young chimps gather as much food as they eat by the time they are 7 or so. But even in forager societies, human children pull their weight only when they are teenagers. Why would our babies be helpless for so long? That long immaturity helps make us so smart: It gives us a long protected time to grow large brains and to use those brains to learn about the world we live in. Human beings can learn to adapt to an exceptionally wide variety of environments, and those skills of learning and culture develop in the early years of life.
幼年黑猩猩到七歲左右時採集的食物已經足夠自己吃，人類兒童即便是在狩獵採集社會中，也要到十幾歲的時候才能自給自足。我們的孩子爲什麼這麼長的時間無法自立？這一漫長的未成熟期幫助我們變成了如今這般的聰明，因爲它給我們很長的受保護的時間來長成碩大的大腦，並用這些大腦去了解周圍的世界。人類可以學會適應各種各樣的環境，而這些學習以及文化技能是在生命的早年形成的。

But that immaturity has a cost. It means that biological mothers can't keep babies going all by themselves: They need help. In forager societies grandmothers provide a substantial amount of child care as well as nutrition. Barry Hewlett at Washington State University and his colleagues found, much to their surprise, that grandmothers even shared breast-feeding with mothers. Some grandmoms just served as big pacifiers, but some, even after menopause, could 'relactate,' actually producing milk. (Though I think I'll stick to the high-tech, 21st-century version of helping to feed my 5-month-old granddaughter with electric pumps, freezers and bottles.)
但這種不成熟有其代價。它意味着生物學意義上的母親無法完全靠自己帶孩子：她們需要幫手。在狩獵採集社會，祖母不僅提供營養，也做了大量的育兒工作。華盛頓州立大學(Washington State University)的巴利・休利特(Barry Hewlett)及其同事非常意外地發現，祖母甚至和母親共同哺乳。一些祖母僅僅是充當大奶嘴的作用，但有些祖母甚至在更年期過後都能“重新哺乳”(relactate)，真的會分泌乳汁。（不過我想我自己會堅持採用21世紀的高科技方法，用電泵、冰箱和奶瓶來幫助餵養五個月大的孫女。）

Dr. Hawkes's 'grandmother hypothesis' proposes that grandmotherhood developed in tandem with our long childhood. In fact, she argues that the evolution of grandmothers was exactly what allowed our long childhood, and the learning and culture that go with it, to emerge. In mathematical models, you can see what happens if, at first, just a few women live past menopause and use that time to support their grandchildren (who, of course, share their genes). The 'grandmother trait' can rapidly take hold and spread. And the more grandmothers contribute, the longer the period of immaturity can be.
霍克斯博士的“祖母假說”提出，祖母現象是跟我們的漫長幼年期是一起形成的。她認爲，事實上正是祖母的進化使得我們的漫長幼年期以及與之伴隨的學習與文化得以出現。在數學模型中，你可以看到，如果最開始只有幾個婦女活過更年期，用這些時間來撫養孫輩（孫輩當然也帶有她們的基因），那麼會出現什麼情況。“祖母遺傳特性”可以迅速形成並擴散。祖母貢獻越大，未成熟期可能就越長。

So on Mother's Day this Sunday, as we toast mothers over innumerable Bloody Marys and Eggs Benedicts across the country, we might add an additional toast for the gray-haired grandmoms behind the scenes.
所以在這個週日的母親節，在全國各地人們用無數的血腥瑪麗和班尼迪克蛋向母親們表達祝福之際，我們不妨也向默默奉獻的銀髮祖母們致以敬意。