【鹦鹉螺】祖辈带孙，“理”所当然

物竞天择的重点并非只是繁殖出尽可能多的后代。“适应”的意义在于让基因延续到未来，或者更准确地说，是让基因成功推进对自身的复制。

祖辈带孙，“理”所当然

作者：David P. Barash

译者：王津雨 & 邵海灵

校对：朱明艾

编辑：邹世昌

The growing role of grandparents in raising children is right in line with human biology.

在养娃过程中祖父母起到的作用愈加重要，这与人类生物学的理论不谋而合。

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Grandparents are busy these days. The number of juveniles in the United States under 18 living in a grandparent’s household rose from 4.6 million in 2005-07 to 5.2 million in 2008-10, probably because of economic stress associated with the Great Recession. Thirteen percent of grandparents provide regular care for at least one grandchild, and 62 percent have provided financial support of some kind to their grandchildren within the past five years.

有孙辈的老人们如今是越来越忙了。在美国，18岁以下与祖父母同住的青少年，从2005年7月的460万攀升到了2008年10月的520万，这可能是经济大萧条给家庭带来的经济压力所致。13%的祖父母要定期照顾至少一个孙辈，62%的人在过去5年内给孙辈提供过某些形式的经济支持。

These circumstances can be stressful for youngsters, their parents, and their grandparents. But there is another way of looking at it: Today’s grandparents are doing exactly what their biology has prepared them for.

无论对青少年及其父母还是祖父母来说，上述情况都会给他们造成压力。但我们也能换个角度看问题：祖父母帮的这些忙，恰恰是人类的生命机体预备他们要做的。

In pretty much all other species, individuals reproduce until they can’t anymore, and nearly always, this coincides with the end of their lives. Human beings are extraordinary, by contrast, in that long after we’ve stopped reproducing, we just keep on living. In particular, we appear to be the only animals in which half the population loses the ability to reproduce, through menopause, while they still have roughly one-third of their lives ahead of them, much of it quite healthy.

在几乎所有其他物种中，个体都会不断繁殖，而且几乎总是要到生命结束之时才会停止。相比之下，人类是比较特殊的，因为在停止生育下一代之后，人类还能活很久。特别是人类中有一半人口（即女性）会因绝经而失去生育能力，但此后这些人还要度过余下约三分之一的生命，且大部分时间都是身体健康地生活着，从整个动物界来看，这是绝无仅有的。

Natural selection rewards reproduction, and the mathematics show that a woman who produces an additional child will be favored over one who stops ovulating. (Reproduction offers the biological equivalent of compound interest; having just one more offspring means the potential of a whole lot of enhanced fitness, so selection should always favor giving it one more try, even if the would-be mother dies in the attempt.) As a result, the existence of menopause as a species-wide trait presents a species-wide mystery. Why don’t people, like other perfectly good mammals, keep on reproducing until they die? Or, another way of looking at it, why do women stop reproducing in late middle age, and yet keep on living? The answer that is emerging from a productive confluence of evolutionary theory and anthropology is that these old people—too old to reproduce, too young and healthy to kick the bucket—are highly biologically relevant.

物竞天择的过程会鼓励繁殖行为，也有数学计算表明，比起停经的女性，能多生一个孩子的女性将更受人们的青睐。（从生物学角度讲，繁殖行为相当于产生了“利滚利”的效果；哪怕只多生出一个后代，也意味着显著增强适应值的可能，所以适者生存的法则应该更希望让个体再试一次，哪怕准妈妈会死于这种尝试。）因此，绝经作为一整个物种范围内出现的特征，就成了全人类的一个谜。为什么人类不像其他完美的哺乳动物那样持续繁殖“至死方休”？或者换个角度看，为什么女性在中年晚期就停止生育了，但仍然会继续活下去？如果将进化理论和人类学进行有效结合，答案就水落石出了：这些老年人——年纪过大无法生育，但其年龄和健康程度离去世尚早的人群——在生物学上是具有重大意义的。

natural selection：Natural selection is a process by which species of animals and plants that are best adapted to their environment survive and reproduce, while those that are less well adapted die out. 达尔文提出的进化理论，又称为“物竞天择”或“适者生存”

compound interest：is interest that is calculated both on an original sum of money and on interest which has previously been added to the sum. [金融学]复利，是指一笔资金除本金产生利息外，在下一个计息周期内，以前各计息周期内产生的利息也计算利息的计息方法。

kick the bucket：pass from physical life and lose all bodily attributes and functions necessary to sustain life [俚语]死了；一命呜呼

Why, specifically, do our bodies begin wearing out so early compared to their potential lifespan, and why is women’s reproductive function particularly targeted? Why don’t kidneys similarly give up and shut down at age 50? Or lungs? The targeting of reproduction is especially perplexing since evolution rewards breeding above all else. Baby-making should be among the last things to go. Not only that, but modern techniques of artificial fertilization have made it possible for women in their 50s and even 60s to sustain pregnancy and bear healthy babies, showing that an aging female body can be up to the task.

与人类的潜在寿命相比，身体机能却这么早就开始衰退，这究竟是为什么？且为何这种衰退机制尤其作用于女性的生育功能呢？为什么肾脏或肺部就不会在人类50岁时就停工？既然进化过程对繁殖功能的重视程度高于一切，那上述针对繁殖功能出现的衰退现象就特别令人费解了。照理说，生娃应该是人类到死都应具备的能力才对。不仅如此，现代人工受精技术已经能让50甚至60多岁的女性仍然保持妊娠能力并生育健康的婴儿，这说明老龄的女性身体仍能胜任生育的任务。

Maybe women stop ovulating and menstruating around age 50 because they simply run out of eggs. After all, every girl is born with about 400 eggs, which are ovulated—roughly one per month—for approximately 35 years. But each XX embryo starts off with about 6 million primordial germ cells, and natural selection has generated all sorts of remarkable adaptations in other respects. There’s no reason to think that evolution wouldn’t be capable of endowing a woman with 500 or even 1,000 viable eggs if she could make good use of them.

女性在50岁左右停止排卵和月经来潮，可能只是因为她们排光了卵子。毕竟，每个女孩天生只携带有400枚左右的卵子，基本每月排卵一次的话，差不多35年就排完了。但每个女性胚胎（原文XX是女性的染色体类型）在形成伊始都会带有约600万个原始生殖细胞，而且自然选择的过程已让人体在除繁殖功能外的各个方面都进化出了非凡的适应能力。所以，认为女性即使能充分利用所有卵子进化过程也不能让她们拥有500甚至1000枚可用的卵子，这种想法是站不住脚的。

primordial germ cells：原始生殖细胞，是产生雄性和雌性生殖细胞的早期细胞。

We’re left with the likelihood that menopause is almost certainly adaptive.

那么只剩下一种可能：绝经现象十有八九也是进化出来的能力。

Or maybe old is new and evolution just hasn’t had a chance to keep up with our newfound longevity. After all, the average lifespan during most of our evolutionary past was considerably shorter than it is today. The truth is more complex, however. Most of the increased average lifespan that modern human beings enjoy today is a result of reductions in infant mortality. If one baby died at birth for every one who made it to age 60, the average lifespan would be 30, but there would still be plenty of 60-year-olds. Even some octogenarians. Moreover, for contemporary hunter-gatherer-forager people today, life expectancy at age 20 is about 70 years, two decades or so after menopause. Almost certainly, there were old people in our past, just as there are today.

或许还可能因为，老年人是新近出现的群体，进化过程只是还来不及跟上人类寿命在短期内的大幅延长。毕竟在人类大部分的进化过程中，我们的平均寿命比现在要短得多。然而，真相其实更为复杂。现代人类的平均寿命有所延长，大部分是因为婴儿死亡率的降低。假设有一个人活到了60岁，但有一个婴儿一出生就死亡了，那这两人的平均年龄就是30岁，但即便如此，60岁的老者仍大有人在——甚至还有一些年过八旬的老人。而且，对于在当今环境中“狩猎-采集-觅食”的人类来说，一个20岁的人能预计活大约70岁，也就是在绝经后还能差不多再活20年。几乎可以肯定的是，过去也存在老年人口，就像现在一样。

If only by process of elimination, we’re left with the likelihood that menopause—the persistence of non-reproductive, elderly life—is not an illness, or something random, or a consequence of long modern lifespans. Nor is it testimony to evolution’s lack of foresight, with natural selection somehow forgetting to fill ’er up with enough follicles. Rather, it is almost certainly adaptive, something that has been produced by natural selection because it benefits its possessors. But what’s the payoff?

如果只结合自然淘汰的机制来看，我们就只剩下这样一种可能性：绝经现象——即不再生育但仍能生存的老年阶段——并非一种疾病，也非某种随机事件，亦非当今人类长寿的结果。另外，它也不是进化过程缺乏远见的证明，不是物竞天择的过程莫名其妙地忘了给人体填充足够的卵泡。相反，我们几乎可以肯定，该现象是一种为适应环境而进化出来的能力，是由自然选择的过程产生的，因为它能使人类受益。但回报究竟是什么呢？

Consider, first, that natural selection isn’t simply focused on producing as many children as possible. “Fitness” is a matter of projecting genes into the future, or, to put it more accurately, genes successfully promoting copies of themselves. Putting those genes in one’s offspring is an efficient route, but so would be employing other bodies, with the importance of such bodies decreasing as they are more distantly related. This is the basic principle underlying “inclusive fitness theory,” first clearly articulated by the late William D. Hamilton, and now one of the cornerstones of modern evolutionary biology.

首先我们要考虑到的是：物竞天择的重点并非只是繁殖出尽可能多的后代。“适应”的意义在于让基因延续到未来，或者更准确地说，是让基因成功推进对自身的复制。把基因植入后代体内是一个有效途径，但也能通过植入其他生物体来实现，不过要清楚，由于基因所在个体与其他生物体的关联不如与后代之间那么紧密，其重要性也就有所降低。这就是“内含适应性理论”的基本原理，该理论由已故的威廉•D•汉密尔顿首次做了清晰阐述，现已成为现代进化生物学的一大基石。

inclusive fitness theory：内含适应性理论，自然选择将倾向于那些能促使有机体的基因得以传播的特性，而不管有机体是否能直接繁殖出后代。

Even if we consider only traditional Darwinian fitness—offspring—the issue isn’t simply how many a woman generates, but how many are actually successful. Successful at what? Reproducing. Accordingly, offspring are evolutionarily important insofar as they, in turn, make their own offspring—that is, grandchildren. (Of course, this keeps going indefinitely into subsequent generations—but for the sake of understanding the process, grandchildren are enough.)

即使我们只考虑传统的“达尔文适合度”参数——即“后代”数量的多少——问题也不仅在于一个女性能生出多少后代，而在于有多少后代能取得成功。在哪个方面取得成功呢？就是她的后代能否继续繁殖。因此，从进化角度看，后代之所以重要，是因为他们转而还能生育自己的后代——也就是孙辈。（当然，这种生育行为代代相传，永无止境——但为了理解上述过程，我们只考虑到孙辈就够了。）

Darwinian fitness：达尔文适合度，一般用来度量自然选择的参数是达尔文适合度(Darwinian fitness),也叫做相对适合度(relative fitness)、选择值(selective value )或适应值(adaptive value)。适合度是一种基因型相对的生殖效率的大小。不管一个基因型的表现如何，只要在同样的环境下和其他基因型相比较能够产生较多的后代，它的适合度就是高的。

Let’s consider, now, one of our great-great-great ... grandmothers. She is middle-aged, having produced, say, four children of whom two have survived to young adulthood. These two children are now about to become parents. What is the best, most fitness-enhancing thing that our middle-aged ancestral woman, their mother, can do? One possibility is that she could keep on having more children, which is what essentially all other mammals do. They breed ‘till they drop. A problem with this strategy is that pregnancy, childbirth, and lactation are physiologically taxing, stresses that younger individuals can usually deal with quite well, but that generate increased morbidity and mortality as any would-be mother ages. (Even with the benefit of modern obstetrics, a woman over the age of 40 has a seven times greater risk of dying in childbirth than a 20-year-old does.)So, our middle-aged momma could keep at it, maybe succeeding and maybe dying in the attempt. A naive view, based simply on reproductive arithmetic, suggests that she’s better off (more fit, evolutionarily) if she tries than if she doesn’t, simply because sometimes she’d succeed. But she might not just die trying; her loss might also mean that any of her young, dependent offspring would be deprived of her mothering, and this would actively diminish her fitness compared to a mother who refrained from having just one more child.

现在让我们试想一下人类的某一位曾曾曾……祖母吧。她人到中年，假设已生育了4个孩子，其中2个已长大成人，现在即将为人父母。此时此刻，我们这位中年女性、即他们的母亲，能采取的最优化、最有利于增强自身“适合度”的举措是什么呢？一种可能方式是，她能继续生育更多的孩子，就像所有其他哺乳动物那样“至死方休”。这种策略的问题在于，怀孕、分娩和哺乳都是生理负担和压力，年轻的个体一般可以很好应对，但随着准妈妈们年龄的增长，上述行为会增加发病率和死亡率。（就算得益于现代产科的技术，40岁以上的女性死于分娩的风险也会比20岁女性高出6倍。）因此，这位中年母亲可以坚持生育，也许她会成功，但也可能会死于这种尝试过程。有一种单纯基于繁殖算法的朴素观点认为，比起不再尝试生育，如果该母亲还是坚持尝试，结果对她更有利（从进化角度讲会提高她的“适合度”），原因仅仅是她还有成功的可能。但后果可能并不只是她本人死于生育尝试这么简单：她的死亡还意味着她所有年幼的、需要养育的后代都无法再享受母亲的照顾，所以比起其他不再多生孩子的母亲，她的“适合度”会被明显削弱。

Among Jane Goodall’s famous chimpanzees, the gloriously maternal Flo produced a youngster, Flint, in her old age. Flint wasn’t quite weaned when Flo did it yet again, becoming mother to Flame. Shortly after, Flame died and so did Flo, perhaps because of the stress pregnancy and lactation imposed on her. In any event, Flint was left an orphan and he died within a month of losing his mother.

在珍妮·古道尔观察的那些著名的黑猩猩之中，有一位了不起的母猩猩Flo在年老时生下了小猩猩Flint。Flint还没有断奶，Flo就再次生产，生下了Flame。不久之后，Flame夭折，Flo也跟着去世了，也许是怀孕和哺乳的巨大压力让它无法承受吧。不管怎样，Flint变成了孤儿。在失去母亲后不到一个月，Flint也死了。

Why don’t people, like other perfectly good mammals, keep on reproducing until they die?

为什么人类不像其他完美的哺乳动物那样持续繁殖“至死方休”呢？

Flo wasn’t an evolutionary failure. She had already produced three flourishing offspring, themselves reproductive. But she would have been even more successful if her body had “just said No” after Flint and before Flame. Primatologist Sarah Hrdy contemplated the sad case of Flo and her too-persistent reproduction, coming up with the “Prudent Mother Hypothesis.” It suggests that menopause is a way that natural selection ensures that human beings, at least, don’t make the same mistake. “Stopping early,” according to Hrdy, “guarantees that old mothers who give birth to one last baby will care for it long enough to see this reproductive enterprise through to a successful conclusion.”

Flo并不是进化失败的结果。它已经生下了三个茁壮成长的后代，这些后代也在继续繁衍。但如果它的身体能在生完Flint以后拒绝再生下一胎，它的“适合度”就更高了。灵长类动物学家莎拉·海尔蒂在考量了Flo过于持久的生育行为酿成的悲剧后，得出了“谨慎母亲假说”。根据这一假设，女性停经是自然选择的过程为确保人类至少不会犯下同样错误而采取的手段。海尔蒂认为：“早日停经能够确保那些生过孩子的年长母亲有足够长的时间来照顾孩子，直到繁衍的进程能顺利延续到这些子女的生命中。”

But why does evolution impose this order only upon human beings, and not chimpanzees, or other primates? These days, evolutionary biologists are increasingly convinced that the answer has a lot to do with another special characteristic of our species: our very long period of childhood dependency, which, in turn, is associated with how much we have to learn. Part of being a prudent mother, if you’re of the species Homo sapiens, is that your job isn’t done simply when you’ve popped out some offspring. Moreover, it isn’t finished when those offspring have grown up, or even when they’ve begun to have their own children. Rather, those children—your grandchildren—also need to be successful, and for that, it evidently helps if someone (beyond the biological parents) is around to help rear that next generation.

但为什么进化过程只为人类设置了这一法则，而没有对猩猩或其他灵长类动物一视同仁呢？如今，进化生物学家们愈发确信，这个问题的答案与人类的另一个特性密切相关：我们无法独立生活的童年时期相当之长，而这又与我们需要学习多少知识有关。如果你是现代人的一员，那身为一个谨慎的母亲就意味着你的职责不仅仅是把孩子生出来，还要等他们长大成人，甚至要等到他们也有了自己的孩子——且这些孙辈也成功繁衍了下一代——你的活儿才算干完了。出于这一目的，如果（除亲生父母之外）还能有人来帮忙照顾刚出生的第三代，那显然是很有帮助的。

The Grandmother Hypothesis helps make sense of a recent and intriguing finding: A particular variant of the immunoregulatory receptor gene known as CD33 has been adjusted in human beings (compared with chimps) to be protective against late-onset Alzheimer’s disease. It seems reasonable that the chimpanzee equivalent of this illness, if it existed, would be disadvantageous to chimps, too, as it is in human beings. But chimpanzees don’t experience a prolonged post-reproductive lifespan like we do, and so, they haven’t undergone comparable selection to ameliorate the devastating effects of Alzheimer’s or its parallel among chimps. A fundamental evolutionary insight is that selection is relaxed after reproduction has been achieved, and indeed, this helps make sense of the phenomenon of senescence generally.

祖母假说也有助于解释最近一项令人疑惑的新发现：与猩猩相比，人体中一种名为CD33的免疫控制受体基因的某种变体在进化过程中被调整了一番，以保护人体免于罹患迟发性老年痴呆症。可以想见，如果猩猩也患上同类病症（如果猩猩痴呆症也存在的话），那对猩猩而言同样是非常不利的。但猩猩不像我们这样在停止生育后还能活很久，所以它们的基因没有在进化过程中出现相似的调整，以减轻老年痴呆或其他类似病症所产生的痛苦。一个最基本的进化论的视角是：繁衍的目标达到后，物竞天择的作用力就放松下来了。这也的确让普遍存在的老年期显得比较合理了。

Natural selection is less potent as aging proceeds, simply because older individuals are less relevant as they have less reproductive future. This is almost certainly true for human beings, too, but apparently less so for us than for other critters. And according to the Grandmother Hypothesis, this is because, more than for other critters, old people can make a contribution to their own fitness by enhancing that of their offspring, via their grandchildren. By the same token, the fact that our species has apparently experienced selection that is at least somewhat protective against the ravages of old age suggests that even the elderly are valuable enough—purely in evolutionary terms—to warrant keeping them functional.

随着生命日渐衰老，物竞天择的作用力也会渐渐变弱，原因很简单：衰老的个体对未来的繁衍进程来说逐渐失去了意义。几乎可以肯定的是，这条原则同样适用于人类，但看起来在我们身上没有在动物身上那么明显。根据祖母假说，这是因为与其他物种相比，人类中的长者可以通过照顾孙辈来提高后代的“适合度”，从而也提高了自己的“适合度”。同理可证，我们族类的进化历程至少能在一定程度上保护我们免受衰老的摧残，这一事实表明，从纯粹的进化角度看，即使是无法生育的老年人，也有足够的价值，所以物竞天择的进程仍要确保他们能够继续活下去。

The Hadza are hunter-gatherers currently living in modern-day Tanzania. Although there aren’t many Hadza left (perhaps 1,000 or so), they’ve persisted in a lifestyle in which women gather and forage, while men hunt—harkening back, to some extent, to what paleo-anthropologists believe our ancient ancestors did for millennia on the African savannah. Kristen Hawkes, an anthropologist at the University of Utah, and her colleagues have found that among the Hadza, women bring in considerably more calories by foraging than men do by hunting. Not only that, but the hardest-working foragers—the ones who bring in the most calories of all—are post-menopausal women.

生活在坦桑尼亚的哈扎族人至今仍过着狩猎和采集的生活。尽管他们的余数已经不多了（大概1000人左右），但仍然坚守着女人采集觅食、男人出门狩猎的生活方式——这种生活在一定程度上回归了古人类学家所相信的、我们的祖先在非洲大草原上延续千年的生活方式。克里斯滕·霍克斯是犹他大学的人类学专家，她和她的同事们发现，在哈扎族人中，女性通过觅食而获得的卡路里，远远高于男性打猎的收获。不仅如此，工作最卖力的觅食者——为族人带来最多卡路里的人——都是已经停经的妇女。

It’s the grandmothers who provide edible roots, who gather more than their share of fruit, and who obtain gobs of honey, which is especially valued. Teenagers and newly-weds forage for roughly three hours per day; married women with young children pitch in at an average of four and a half hours (it’s difficult to dig and forage when you’re also carrying a baby, never mind nursing as well). Grandmothers tip the scales at seven hours. The resulting “income” is far more than they need for themselves, and not surprisingly, they share the bounty with their children and grandchildren. Also not surprising: The body weight of grandchildren increases in proportion as their grandmothers contribute. Moreover, among the Hadza studied by Hawkes and her team, every nursing mother is supported by a post-menopausal helper, nearly always her mother or mother-in-law.

是祖母们摘回了可吃的根茎，采集了她们自己吃不完的水果，带回了成块的蜂蜜——蜂蜜的价值可是非比寻常。少年人和新婚妇女每天大概出去觅食三小时，已婚已育的妇女平均每天劳作四个半小时（如果你背上还有一个小宝宝，那挖掘和觅食就会更困难，更不要说还要喂奶了）。祖母们的工作时间则长达七个小时。她们带回的“收入”远超过自己的需要，所以当然，她们会与孩子和孙辈分享这些吃不完的食物。孙辈的体重增加与祖母的贡献成正比，也就不足为奇了。不仅如此，在霍克斯及其团队所研究的哈扎人中，每一个乳养婴孩的母亲都有一个已经停经的女性来帮忙，通常不是她的母亲，就是她的婆婆。

What’s true of the Hadza isn’t necessarily true for all people currently living a traditional lifestyle. Furthermore, it isn’t guaranteed that today’s gatherers and hunters offer an unobstructed view of how our early ancestors functioned. But it’s the best that we currently have, and it strongly suggests that natural selection has favored mandatory non-reproduction among middle-aged women so that they could invest in their grandchildren.

哈扎族的情况也许并不适用于所有如今仍然保持传统生活方式的人们。此外，我们也不能保证如今的觅食者和狩猎者能完整重现人类先祖的运作方式。但这是我们目前为止所能找到的最好素材，而且它显著表明了物竞天择的法则倾向于强制中年妇女停止生育，好让她们能把精力花在孙辈上。

For today’s shrinking and increasingly stressed middle class, it seems that extended families are more needed than ever, as Mom and Dad have to work full time, leaving the grandparents to take up at least some of the slack when it comes to child-rearing: a modern day Hadza scenario, right here in high-tech America. The silver lining is that, in a sense, this is a return to form for the grandparent. According to the Pew Research Center, older Americans report that the best part of growing old, by far, is spending more time with family, especially grandchildren. A 2013 study by Boston College found that emotionally close relationships between grandparents and grandchildren are “associated with fewer symptoms of depression for both generations.” There’s also a growing granny labor pool: The numbers of U.S. grandparents are at record highs, and growing faster than the general population. By 2020, they are projected to represent nearly 1-in-3 adults. They are also close by, with 69 percent living within 50 miles of their closest grandchildren.

对于今天数量缩减且压力山大的中产阶级来说，这意味着他们从来没有像现在这样如此需要过近亲提供的帮助。因为父母都有全职工作，照顾孩子的任务多多少少就得由老人代劳了：在高科技时代的美国，这可以说是一幅现代版哈扎族的生活场景了。但从某种角度来说，事情也有好的一面：祖父母们又能回归常态了。皮尤研究中心的调查显示，美国的老年人普遍认为，步入老年后最开心的一件事，就是能花更多时间陪伴家人，特别是和孙辈在一起。波士顿大学于2013年进行的一项研究发现，祖父母与孙辈之间密切的情感联系与“两代人出现的抑郁症状都有所减轻”有关。祖母级的劳动力人口也在增加：美国祖父母的数量已经达到历史巅峰，且这一群体的增长速度超过了人口总体的增长。预计到2020年，老年人口将占成年人口比例的三分之一。而且他们也近在咫尺，69%的老人和他们住得最近的孙辈之间只相距不到50英里（约80公里）的路程。

Thank goodness that they aren’t burdened with their own young children and are available to pitch in, Hadza-style.

感谢上帝，他们已经停止生育，没有给被更多年幼的孩子拖累，所以才能参与这场哈扎族式的家庭大接力。

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<原文链接：http://aging.nautil.us/feature/175/what-good-is-grandma>