What is so special about the human brain? Why is it that we study other animals instead of them studying us? What does a human brain have or do that no other brain does? When I became interested in these questions about 10 years ago, scientists thought they knew what different brains were made of. Though it was based on very little evidence, many scientists thought that all mammalian brains, including the human brain, were made in the same way, with a number of neurons that was always proportional to the size of the brain. This means that two brains of the same size, like these two, with a respectable 400 grams, should have similar numbers of neurons. Now, if neurons are the functional information processing units of the brain, then the owners of these two brains should have similar cognitive abilities. And yet, one is a chimp, and the other is a cow. Now maybe cows have a really rich internal mental life and are so smart that they choose not to let us realize it, but we eat them. I think most people will agree that chimps are capable of much more complex, elaborate and flexible behaviors than cows are. So this is a first indication that the "all brains are made the same way" scenario is not quite right.
人類的腦有什麼特別的呢? 為何是我們研究其它動物的腦 而不是我們的被研究呢? 人腦擁有什麼 其它動物的腦沒有的呢? 十幾年前我開始 對這類的問題感興趣 那時的科學家自認已知差異所在 即便可依據的證據非常少 許多科學家認為所有哺乳類的腦 包括人類的頭腦 都有著相同的構造 有著一定數量的神經元 而數量與腦的大小成正比例 這表示同樣大小的兩個腦 例如這兩約400公克的 應有差不多數量的神經元 如果神經元是腦裡面 訊息處理的最小單位 那這兩腦的主人 應該有著差不多一樣的認知能力 然而一個是黑猩猩的 另一個是乳牛的 或許乳牛有著 心靈極豐富的內在美 聰明到不讓我們發現這些 但我們吃牠們 我想大部分的人會同意 黑猩猩比乳牛更有能力處理 複雜、精細且有彈性的行為 所以這意味著 「所有腦都有相同的構造」的想法 不完全正確
But let's play along. If all brains were made the same way and you were to compare animals with brains of different sizes, larger brains should always have more neurons than smaller brains, and the larger the brain, the more cognitively able its owner should be. So the largest brain around should also be the most cognitively able. And here comes the bad news: Our brain, not the largest one around. It seems quite vexing. Our brain weighs between 1.2 and 1.5 kilos, but elephant brains weigh between four and five kilos, and whale brains can weigh up to nine kilos, which is why scientists used to resort to saying that our brain must be special to explain our cognitive abilities. It must be really extraordinary, an exception to the rule. Theirs may be bigger, but ours is better, and it could be better, for example, in that it seems larger than it should be, with a much larger cerebral cortex than we should have for the size of our bodies. So that would give us extra cortex to do more interesting things than just operating the body. That's because the size of the brain usually follows the size of the body. So the main reason for saying that our brain is larger than it should be actually comes from comparing ourselves to great apes. Gorillas can be two to three times larger than we are, so their brains should also be larger than ours, but instead it's the other way around. Our brain is three times larger than a gorilla brain.
但先讓我們繼續下去 如果所有腦的構造都是一樣的 你比較動物之間不同大小的腦 那較大的腦就該有較多的神經元 腦愈大的話 其主人也該有較強的認知能力 所以世上最大的腦 都該有最強大的認知能力 但壞消息來了 我們人類的腦不是最大的 這聽起來有點傷腦筋 我們人類的腦大約是 1.2~1.5 公斤 但大象的腦是 4~5 公斤 而鯨的腦重達 9 公斤 這就是為何科學家總是說 我們的腦一定是特別 這有便於解釋我們的認知能力 我們的腦一定是很非比尋常 排除在一般規則外 牠們的腦或許更大但我們的更好 舉例來說 我們的腦比它應該有的樣子更大 而且依我們身軀的大小來看 它有著超過應有的大腦皮質數量 這些「過多」的大腦皮質讓我們 做出比操控身軀更多有趣的事情 因為通常腦的大小 跟身材的大小有關係 主要會說「我們的腦 比它應有的樣子更大」 是來自於 我們跟大猩猩作的比較 大猩猩的身材比我們大上二到三倍 所以牠們的腦也該比我們的大才對 事實上卻相反 我們的腦比大猩猩的大上三倍
The human brain also seems special in the amount of energy that it uses. Although it weighs only two percent of the body, it alone uses 25 percent of all the energy that your body requires to run per day. That's 500 calories out of a total of 2,000 calories, just to keep your brain working.
從能量消耗的角度來看 人類的腦也是很特別的 雖然腦只佔身體重量的 2% 光是它就消耗了身體 每天需要25% 的能量 也就是一天 2,000 卡路里中的 500 卡 都是花在腦的運作上
So the human brain is larger than it should be, it uses much more energy than it should, so it's special. And this is where the story started to bother me. In biology, we look for rules that apply to all animals and to life in general, so why should the rules of evolution apply to everybody else but not to us? Maybe the problem was with the basic assumption that all brains are made in the same way. Maybe two brains of a similar size can actually be made of very different numbers of neurons. Maybe a very large brain does not necessarily have more neurons than a more modest-sized brain. Maybe the human brain actually has the most neurons of any brain, regardless of its size, especially in the cerebral cortex. So this to me became the important question to answer: how many neurons does the human brain have, and how does that compare to other animals?
人腦不僅比它應有的大小還大 且還消耗掉更多能量 所以說它是特別的 這也是一開始困擾我的地方 在生物學上我們尋找規則 可以大體上適用於所有的動物和生命 但為什麼演化的規則 適用於所有生物,但不包括我們呢? 或許問題就在於 「所有腦的構造都是一樣的」這個假設上 或許兩個大小相近的腦 其實可以有著非常不同數量的神經元 或許一個非常大的腦 並不需要比一般大小的腦 需要更多的神經元 或許不管人腦的大小 都比其他生物擁有最多的神經元 尤其是在大腦皮質這部分 這些讓我想到一個 待解答的重要問題: 人類的腦有多少神經元? 答案可如何與其它動物做比較呢?
Now, you may have heard or read somewhere that we have 100 billion neurons, so 10 years ago, I asked my colleagues if they knew where this number came from. But nobody did. I've been digging through the literature for the original reference for that number, and I could never find it. It seems that nobody had actually ever counted the number of neurons in the human brain, or in any other brain for that matter.
你可能聽過或在哪讀到 我們有一千億個神經元 十前年我問我的同事 知不知這數字哪來的 結果沒人知道 我翻閱所有的文獻 想找出這數字最原始的出處 但我也沒辦法找到 看來似乎沒有人真正算出過 人類的腦究竟有多少個神經元 其它的腦也一樣沒數出來過
So I came up with my own way to count cells in the brain, and it essentially consists of dissolving that brain into soup. It works like this: You take a brain, or parts of that brain, and you dissolve it in detergent, which destroys the cell membranes but keeps the cell nuclei intact, so you end up with a suspension of free nuclei that looks like this, like a clear soup. This soup contains all the nuclei that once were a mouse brain. Now, the beauty of a soup is that because it is soup, you can agitate it and make those nuclei be distributed homogeneously in the liquid, so that now by looking under the microscope at just four or five samples of this homogeneous solution, you can count nuclei, and therefore tell how many cells that brain had. It's simple, it's straightforward, and it's really fast. So we've used that method to count neurons in dozens of different species so far, and it turns out that all brains are not made the same way. Take rodents and primates, for instance: In larger rodent brains, the average size of the neuron increases, so the brain inflates very rapidly and gains size much faster than it gains neurons. But primate brains gain neurons without the average neuron becoming any larger, which is a very economical way to add neurons to your brain. The result is that a primate brain will always have more neurons than a rodent brain of the same size, and the larger the brain, the larger this difference will be. Well, what about our brain then? We found that we have, on average, 86 billion neurons, 16 billion of which are in the cerebral cortex, and if you consider that the cerebral cortex is the seat of functions like awareness and logical and abstract reasoning, and that 16 billion is the most neurons that any cortex has, I think this is the simplest explanation for our remarkable cognitive abilities. But just as important is what the 86 billion neurons mean. Because we found that the relationship between the size of the brain and its number of neurons could be described mathematically, we could calculate what a human brain would look like if it was made like a rodent brain. So, a rodent brain with 86 billion neurons would weigh 36 kilos. That's not possible. A brain that huge would be crushed by its own weight, and this impossible brain would go in the body of 89 tons. I don't think it looks like us.
所以我用自己的方法來數 這方法基本上是 把大腦溶解在一碗湯裡 就像這樣 你拿出一個或部分的腦 用某種溶劑溶解它 破壞掉細胞壁 但保留細胞核完整 這樣你就有自由懸浮的細胞核 看起來就會像這樣 一碗清湯 這碗湯內有一隻鼠腦 所有的細胞核 這碗湯的好處就在於因為它是一碗湯 你可攪動它 讓細胞核均勻散布在液體中 然後只需要在顯微鏡之下 觀察這碗湯中四到五個樣本 數算細胞核的數量 就可得知這個腦有多少個細胞了 這既簡單又直接 而且迅速 所以我們利用這方法 計數十種不同物種的神經元 得到的結果是 所有的腦都不具有相同的構造 以嚙齒動物和靈長類為例 較大隻的嚙齒動物腦中 其神經元的平均大小變大 腦就可迅速變大 腦變大的速度比增加神經元快 但靈長類的腦 可在神經元沒有變大的情況下增加數量 這使得增加腦的神經元數量上 變得非常有效率 結果顯示同樣大小的腦 靈長類的神經元數量都會超過嚙齒類的 而且腦愈大 這差異就顯得愈大 那我們的腦呢? 平均來講 我們有 860億個神經元 其中的 160億都在大腦皮質裡 如果你認為大腦皮質 的功能中心是像 認知 、邏輯 、抽象理解力 那腦皮層中的160億就比其他 任何大腦皮質擁有最多的數量了 我認為這是我們人類卓越的認知能力 最簡單的解釋了 同樣重要的是這860億個神經元意味著什? 因為我們發現到 腦的大小和神經元數量的關係 可以用數學的方式描述 我們可計算出 如果人腦的構造像嚙齒動物的會怎麼樣呢? 若嚙齒動物的腦有 860億的神經元 體重就會重達 36公斤 這是不可能的事情 這麼大的一個腦 會被自己的重量壓垮 要這麼重大的腦 搭配一個 89公噸的身體 是件讓我難以想像的東西
So this brings us to a very important conclusion already, which is that we are not rodents. The human brain is not a large rat brain. Compared to a rat, we might seem special, yes, but that's not a fair comparison to make, given that we know that we are not rodents. We are primates, so the correct comparison is to other primates. And there, if you do the math, you find that a generic primate with 86 billion neurons would have a brain of about 1.2 kilos, which seems just right, in a body of some 66 kilos, which in my case is exactly right, which brings us to a very unsurprising but still incredibly important conclusion: I am a primate. And all of you are primates.
這為我們今天下一個很重要的結論 我們不是嚙齒動物 人腦沒有嚙齒動物搬的大腦 比起老鼠我們的看起來特別多了 但這樣比較並不公平 因為我們知道我們不是嚙齒動物 我們是靈長類 所以跟其它的靈長類動物作比較才正確 所以計算一下的話 你會發現一般靈長類動物 擁有860億的神經元 腦就會重約 1.2公斤 搭配重達 66公斤的身軀 看起來似乎合理 這剛好是我自身的例子 這結果雖然一點也不意外 但還是下了一個很重要的結論: 我是靈長類動物 你們全部也是靈長類動物
And so was Darwin. I love to think that Darwin would have really appreciated this. His brain, like ours, was made in the image of other primate brains.
達爾文也是 我猜想達爾文也會很喜歡這項結論 他的腦跟我們一樣 構造都是與其它靈長類相同的
So the human brain may be remarkable, yes, but it is not special in its number of neurons. It is just a large primate brain. I think that's a very humbling and sobering thought that should remind us of our place in nature.
沒錯,人腦或許較卓越 但在神經元的數量方面就沒什特別的 它不過是大的靈長類的頭腦 我想這是一個非常謙虛和中肯的想法 提醒我們在大自然中扮演的角色
Why does it cost so much energy, then? Well, other people have figured out how much energy the human brain and that of other species costs, and now that we knew how many neurons each brain was made of, we could do the math. And it turns out that both human and other brains cost about the same, an average of six calories per billion neurons per day. So the total energetic cost of a brain is a simple, linear function of its number of neurons, and it turns out that the human brain costs just as much energy as you would expect. So the reason why the human brain costs so much energy is simply because it has a huge number of neurons, and because we are primates with many more neurons for a given body size than any other animal, the relative cost of our brain is large, but just because we're primates, not because we're special.
為何它會消耗如此多的能量? 有人已經估算出 人類和其它動物的腦 所消耗的能量 現在了解每種腦有多少神經元 我們就可做些計算 結果發現人類和其它動物的腦 消耗方式差不多一樣 每天平均每 10億個神經元消耗 6卡 一個腦所消耗的能量 和它的神經元數量 成了一個簡單的線性關係 所以人腦消耗的能量 就跟你估算的一樣多 所以人腦為什麼 會消耗許多能量 單純是因為它有較多神經元的關係 作為靈長類 比起身體大小類似的動物 我們擁有更多的神經元 腦也就消耗較多能量 這單純是因為我們是靈長類動物的關係 不是因為我們特別
Last question, then: how did we come by this remarkable number of neurons, and in particular, if great apes are larger than we are, why don't they have a larger brain than we do, with more neurons? When we realized how much expensive it is to have a lot of neurons in the brain, I figured, maybe there's a simple reason. They just can't afford the energy for both a large body and a large number of neurons. So we did the math. We calculated on the one hand how much energy a primate gets per day from eating raw foods, and on the other hand, how much energy a body of a certain size costs and how much energy a brain of a certain number of neurons costs, and we looked for the combinations of body size and number of brain neurons that a primate could afford if it ate a certain number of hours per day.
最後一個問題: 我們是如何有這麼多的神經元? 明確地說 如果大猩猩身體比我們大 為何沒有較大的腦和更多的神經元呢? 當理解到擁有許多神經元的同時 也就需要燃燒更多的能量時 我推測或許這就是原因- 大猩猩無法同時供給能量 應付龐大的身軀和大量的神經元 我們計算過 一方面計算出 大猩猩一天從食物中 獲得多少能量 另一方面計算 特定大小的身軀要消耗多少能量 還有一定數量的神經元所消耗的能量 然後我們一併查看 如果牠一天進食固定的時數 牠可以提供多少能量 給多大的身軀和多少的神經元數量
And what we found is that because neurons are so expensive, there is a tradeoff between body size and number of neurons. So a primate that eats eight hours per day can afford at most 53 billion neurons, but then its body cannot be any bigger than 25 kilos. To weigh any more than that, it has to give up neurons. So it's either a large body or a large number of neurons. When you eat like a primate, you can't afford both.
我們發現到 因為神經元消耗太多 這是身軀大小和神經元數量之間的妥協 一個靈長類一天進食八小時 可提供最多 530億個神經元的能量 但同時牠的身軀無法超過 25公斤 如果增加體重 則須捨棄神經元 所以只能單單擁有龐大的身軀 或只能單單擁有大量的神經元 如果你像靈長類一樣進食 你無法同時有這兩項
One way out of this metabolic limitation would be to spend even more hours per day eating, but that gets dangerous, and past a certain point, it's just not possible. Gorillas and orangutans, for instance, afford about 30 billion neurons by spending eight and a half hours per day eating, and that seems to be about as much as they can do. Nine hours of feeding per day seems to be the practical limit for a primate.
跳脫這限制的其中一個方法 是每天花更多時間進食 但這會提高危險 所以這方法行不通 以大猩猩和紅毛猩猩為例 為了養活大約 300億個神經元 每天要花8.5小時進食 這已經是它們所能做的了 每天進食九小時 應是靈長類的極限
What about us? With our 86 billion neurons and 60 to 70 kilos of body mass, we should have to spend over nine hours per day every single day feeding, which is just not feasible. If we ate like a primate, we should not be here.
那我們呢? 我們有 860億個神經元 體重約 60~70公斤 我們每一天應該都要 花超過九小時在進食上 但這不可行 如果我們像靈長類一樣的吃 我們就不會在這裡了
How did we get here, then? Well, if our brain costs just as much energy as it should, and if we can't spend every waking hour of the day feeding, then the only alternative, really, is to somehow get more energy out of the same foods. And remarkably, that matches exactly what our ancestors are believed to have invented one and a half million years ago, when they invented cooking. To cook is to use fire to pre-digest foods outside of your body. Cooked foods are softer, so they're easier to chew and to turn completely into mush in your mouth, so that allows them to be completely digested and absorbed in your gut, which makes them yield much more energy in much less time. So cooking frees time for us to do much more interesting things with our day and with our neurons than just thinking about food, looking for food, and gobbling down food all day long.
那我們是怎麼辦到的? 如果我們的腦真的需要大量的能量應付 而且又不能花每個 清醒的時刻在進食上 另一個替代方式就是 從同樣的食物中 用某種方式得到更多能量 很神奇的 這完全符合我們的假設- 150萬年前我們的祖先 發明了烹煮 烹煮是利用火 在我們身外先消化好食物 煮過的食物較軟 易咀嚼 能在嘴裡完全嚼碎 好讓胃能夠完全消化 供你吸收 這樣就能在較短的時間內獲取更多的能量 所以烹煮讓我們有更多的時間 利用我們的時間和神經元 從事更多有趣的事情 這樣就不用為了 老是要找食物,趕著吃 然後花掉一整天的時間
So because of cooking, what once was a major liability, this large, dangerously expensive brain with a lot of neurons, could now become a major asset, now that we could both afford the energy for a lot of neurons and the time to do interesting things with them. So I think this explains why the human brain grew to become so large so fast in evolution, all of the while remaining just a primate brain. With this large brain now affordable by cooking, we went rapidly from raw foods to culture, agriculture, civilization, grocery stores, electricity, refrigerators, all of those things that nowadays allow us to get all the energy we need for the whole day in a single sitting at your favorite fast food joint. So what once was a solution now became the problem, and ironically, we look for the solution in raw food.
也因為烹煮的關係 讓這一度為了應付巨量神經元 消耗能量過度的腦 變成現在主要的資產 讓我們可以同時提供能量給大量神經元 也能花時間利用它們從事有趣的事 所以我認為這說明了為何人腦 能夠在演化中迅速變大 即便仍使用靈長類的腦 也因為烹煮可養活這大腦的關係 我們很快速地從生食發展到養殖 農、文明、雜貨店 電力、電冰箱 現今的這一切 讓我們可以在 喜歡的速食店內一次消費 支撐一整天所需的能量 而過去的解決之道 卻成了當今的問題 很諷刺地,我們在生食中尋求解答
So what is the human advantage? What is it that we have that no other animal has? My answer is that we have the largest number of neurons in the cerebral cortex, and I think that's the simplest explanation for our remarkable cognitive abilities. And what is it that we do that no other animal does, and which I believe was fundamental to allow us to reach that large, largest number of neurons in the cortex? In two words, we cook. No other animal cooks its food. Only humans do. And I think that's how we got to become human.
那人類的優勢是什麼? 我們擁有什麼 其它動物沒有的呢? 我們的大腦皮質成了答案 它擁有最多的神經元 我想這為我們卓越的認知能力 提供了簡單好懂的解釋 那是什麼讓我們做其它動物做不到的呢? 我認為最基本的方式 讓我們擁有卓越大腦 且含大量神經元的大腦皮質 只有歸於兩個字- 烹煮 其它動物不會煮食物,只有人類會 我認為這就是為何我們身為人類
Studying the human brain changed the way I think about food. I now look at my kitchen, and I bow to it, and I thank my ancestors for coming up with the invention that probably made us humans. Thank you very much. (Applause)
研究人腦改變我對食物的看法 現在我看著我的廚房 都跟它敬禮 我感謝我的祖先 發明這些讓我們得以成為人類 謝謝大家 (鼓掌)