万物简史 第643期:神秘的两足动物(21)（在线收听）

Homo habilis ("handy man") was named by Louis Leakey and colleagues in 1964 and was so called because it was the first hominid to use tools, albeit very simple ones. It was a fairly primitive creature, much more chimpanzee than human, but its brain was about 50 percent larger than that of Lucy in gross terms and not much less large proportionally, so it was the Einstein of its day. No persuasive reason has ever been adduced for why hominid brains suddenly began to grow two million years ago. For a long time it was assumed that big brains and upright walking were directly related—that the movement out of the forests necessitated cunning new strategies that fed off of or promoted braininess—so it was something of a surprise, after the repeated discoveries of so many bipedal dullards, to realize that there was no apparent connection between them at all.

能人（“有技能的人”）是由路易斯·利基和他的同事于1964年命名的。之所以取这样的名字，是因为那是第一个会使用工具——尽管是非常简单的工具——的人科动物。他是一种相当原始的动物，看上去更像猿，而不像人类，但是大脑的绝对量大约比露西大50%，按比例也小不了多少，称得上是那一时期的爱因斯坦。没有任何原因足以说明为什，么200万年前人科动物的大脑突然开始增大。在很长一段时期里，人们都认为大脑的发育和直立行走直接相关——-走出森林的古人类不得不制定较为复杂的计划，这促进了大脑的进化——因此，当一再发现有那么多笨蛋两足动物之后，意识到这两者之间并没有任何明显的联系，真的有点儿令人感到意外。

"There is simply no compelling reason we know of to explain why human brains got large," says Tattersall. Huge brains are demanding organs: they make up only 2 percent of the body's mass, but devour 20 percent of its energy. They are also comparatively picky in what they use as fuel. If you never ate another morsel of fat, your brain would not complain because it won't touch the stuff. It wants glucose instead, and lots of it, even if it means shortchanging other organs. As Guy Brown notes: "The body is in constant danger of being depleted by a greedy brain, but cannot afford to let the brain go hungry as that would rapidly lead to death." A big brain needs more food and more food means increased risk.

“为什么人类脑量开始增大，对此我们实在没有完全令人信服的证据来进行解释。”塔特萨尔说。巨大的脑子是高耗能的器官。它只占人类身体总质量的2%，却消耗了20%的能量。它对用做能量的东西还有些挑剔。如果你从不吃油腻的东西，你的大脑不会产生怨言，因为它对此不感兴趣。相反，它对葡萄糖却喜爱有加，而且是多多益善，即使这意味着克扣其他器官。正如盖伊·布朗所描述的那样：“贪食的大脑经常使身体处于枯竭的危险之中，但它也不敢让大脑挨饿，因为那样会迅速走向死亡。”你大脑越大，吃得就越多；吃得越多，危险性就越大。