經濟學人 | 嬰兒和大腦進化的秘密（2016.5.28）

01-29

Human evolution

人類進化
Of bairns and brains

嬰兒和大腦

Source: The Economist, May 28th 2016

Babies are born helpless, which might explain why humans are so clever
嬰兒生而無助，這可能可以解釋為什麼人類那麼聰明

HUMAN intelligence is a biological mystery. Evolution is usually a stingy process, giving animals just what they need to thrive in their niche and no more. But humans stand out. Not only are they much cleverer than their closest living relatives, the chimpanzees, they are also much cleverer than seems strictly necessary. The ability to do geometry, or to prove Pythagoras』s theorem, has turned out to be rather handy over the past few thousand years. But it is hard to imagine that a brain capable of such feats was required to survive on the prehistoric plains of east Africa, especially given the steep price at which it was bought. Humans』 outsized, power-hungry brains suck up around a quarter of their body』s oxygen supplies.

人類的智力是生物學上的一個謎。生物進化通常是一個十分吝嗇的過程，只給動物供他們生存所需的量，不再多給。但人類卻從中脫穎而出。人類不僅比最近的親緣物種黑猩猩要聰明得多，而且他的智力程度似乎遠超嚴格意義上「必須」的範疇。僅僅經過幾千年的進化，人類已經能駕輕就熟地做幾何題，或者證明畢德哥拉斯定理。但是，令人難以置信的是，能夠完成這些壯舉的大腦，曾在史前的東非大平原上掙扎求生，特別是為此付出了巨大的代價。人類的特大號、高耗能的大腦，要消耗全身約1/4的需氧量。

Sexy brains

性感的大腦

There are many theories to explain this mystery. Perhaps intelligence is a result of sexual selection. Like a peacock』s tail, in other words, it is an ornament that, by virtue of being expensive to own, proves its bearers』 ?tness. It was simply humanity』s good fortune that those big sexy brains turned out to be useful for lots of other things, from thinking up agriculture to building internal-combustion engines. Another idea is that human cleverness arose out of the mental demands of living in groups whose members are sometimes allies and sometimes rivals.

有很多理論來解釋這一現象。也許，智力發達是性選擇的結果。就像孔雀的尾巴，換句話說，這是一種裝飾品，因為成本高昂，所以顯示出擁有者的能力。這些大而性感的大腦恰好對很多事情有用（無論是開發農業還是建造內燃機），這只是人類撞大運了。另一種觀點是，人類智力之所以發達，源於群居產生的心理需求。因為人們需要搞清楚，哪些成員是盟友，哪些是敵人。

Now, though, researchers from Rochester University, in New York, have come up withanother idea. In Proceedings of the National Academies of Science, Steven Piantadosi and Celeste Kidd suggest that humans may have become so clever thanks to another evolutionarily odd characteristic: namely that their babies are so helpless.

但是，現在，來自紐約羅切斯特大學的研究者們提出了新的觀點。學術期刊《美國國家科學院報》顯示，史蒂芬?皮昂達多斯和塞萊斯特?基德認為，人類的聰明得益於另外一項進化中非常特殊的特點：人類的小孩都非常弱小。

Compared with other animals, says Dr Kidd, some of whose young can stand up and move around within minutes of being born, human infants take a year to learn even to walk, and need constant supervision for many years afterwards. That helplessness is thought to be one consequence of intelligence—or, at least, of brain size. In order to keep their heads small enough to make live birth possible, human children must be born at an earlier stage of development than other animals. But Dr Piantadosi and Dr Kidd, both of whom study child development, wondered if it might be a cause as well as a consequence of intelligence as well.

基德博士表示，與其他動物相比，有些動物在出生幾分鐘後就能站立，四處走動，而人類嬰兒需要花一年時間去學會走路，並在此後數年一直需要他人監護。這種無助性被認為是智力發達的結果之一——或者至少說，大腦體積的大小。為了使胎兒腦袋足夠小，確保順利分娩，人類嬰兒在發育極早期就出生了。但是，皮昂達多斯博士和基德博士這兩位兒童發展學家猜測，這可能既是因，又是果。

Their idea is that helpless babies require intelligent parents to look after them. But to get big-brained parents you must start with big-headed—and therefore helpless—babies. The result is a feedback loop, in which the pressure for clever parents requires ever-more incompetent infants, requiring ever-brighter parents to ensure they survive childhood.

他們的觀點認為，因為嬰兒十分無助，所以需要聰明的父母去照看他們。而要有大腦袋的父母，你首先得是個大腦袋——因此也很無助——的嬰兒。這形成了一個反饋循環。在這個循環中，因為要得到更聰明的父母，所以他們在嬰兒時就會更加無助，這又需要更加聰明的父母來照料孩子，確保孩子的存活。

It is an elegant idea. The self-reinforcing nature of the process would explain why intelligence is so strikingly overdeveloped in humans compared even with chimpanzees. It also o?ers an answer to another evolutionary puzzle, namely why high intelligence developed ?rst inprimates, a newish branch of the mammals, a group that is itself relatively young. Animals that lay eggs rather than experiencing pregnancy do not face the trade-o? between head size at birth and infant competence that drives the entire process.

這是一個非常精妙的想法。這一過程中，自我強化的特性可以解釋為什麼與黑猩猩相比，人類的智力水平遠超必要水平。它也為另一個進化難題提供了解答：為什麼高等智能首先在靈長類，也就是最新的哺乳動物分支中出現？它們在哺乳類動物中還是屬於較為年輕的。那些卵生動物沒有經歷懷孕期，因此不必在嬰兒頭部大小和適應能力兩方面做權衡，這種權衡選擇也推動了整個進化進程。

To test their theory, Dr Piantadosi and Dr Kidd turned ?rst to a computer model of evolution. This con?rmed that the idea worked, at least in principle. They then went looking for evidence to support the theory in the real world. To do that they gathered data from 23 di?erent species of primate, from chimps and gorillas to the Madagascan mouse lemur, a diminutive primate less than 30cm long.

為了驗證理論，皮昂達多斯博士和基德博士首先在一個進化電腦模型上求證。該模型至少在理論上證實了這一觀點的可行性。然後，他們開始在世界各地尋找支持理論的切實證據。為此，他們搜集了23種靈長類動物的數據，從黑猩猩到大猩猩，再到馬達加斯加狐猴，一種小型的靈長類動物，不到30厘米長。

The scientists compared the age at which an animal weaned its young (a convenient proxy for how competent those young were) with their scores on a standardised test of primate intelligence. Sure enough, they found a strong correlation: across all the animals tested, weaning age predicted about 78% of the eventual score in intelligence. That correlation held even after controlling for a slew of other factors, including the average body weight of babies compared with adults or brain size as a percentage of total body mass.

科學家比較了他們的斷奶年齡（衡量幼崽成熟度的方便的替代指標）和智力水平標準化測試分數。非常肯定的是，兩者存在著強相關性：對於所有受測動物，斷奶年齡可以預測78%的最終智力分數。即使在控制了一系列其他影響因素（包括與成年動物相比的平均嬰兒體重，或大腦占身體總重量的比值）之後，這一顯著相關性仍然成立。

The researchers point to other snippets of data that seem to support their conclusions: a study of Serbian women published in 2008, for instance, found that babies born to mothers with higher IQs had a better chance of surviving than those born to low-IQ women, whichbolsters the idea that looking after human babies is indeed cognitively taxing. But although their theory is intriguing, Dr Piantadosi and Dr Kidd admit that none of this adds up tode?nitive proof.

研究者們指出，其他數據片段似乎證明了這一結論：例如，2008年發布的一項關於塞爾維亞女性的研究表明，高智商女性的孩子比低智商女性的孩子生存率更高，這表明，照顧嬰兒確實有更高的認知要求。但是，儘管他們的理論頗有趣味，皮昂達多斯博士和基德博士也承認，目前這些都不構成決定性證據。

That, unfortunately, can be the fate of many who study human evolution. Any such feedback loop would be a slow process (at least as reckoned by the humans themselves), most of which would have taken place in the distant past. There are gaps in the theory, too. Even if such a process could drastically boost intelligence, something would need to get it going in the ?rst place. It may be that some other factor—perhaps sexual selection, or the demands of a complex environment, or some mixture of the two—was required to jump-start the process. Dr Piantadosi and Dr Kidd』s idea seems a plausible addition to the list of explanations. But unless human intelligence turns out to be up to the task of building a time machine, it is unlikely that anyone will ever know for sure.

不幸的是，很多研究人類進化的學者們可能就將深陷其中。任意一個這樣的反饋循環都會是一個緩慢的過程（至少對人類自身而言是如此），大部分循環可能在久遠的過去已經發生。理論中也有一些鴻溝。即使這種循環能大幅促進智力提升，首先需要有啟動因素。它可能是一些其他因素——可能是性選擇，或者複雜環境所需，或者兩者兼有——需要這樣一個因素來啟動循環過程。皮昂達多斯博士和基德博士的觀點似乎增加了一種新的解釋。但是，除非人類智力強大到能夠建造時間機器，否則，沒有人夠確切知道究竟是怎麼回事。