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Why humans have evolved to drink milk?[1]

為何人類會變得能夠喝鮮奶呢？

Humans didn’t start out being able to digest animal milk – but now many populations do. Why has evolution favoured tolerating dairy? 

人類在一開始並無法消化動物奶，但是現在卻有許多族群都可以。為什麼會進化成能夠耐受乳製品呢？

Dairy milk has competition. Alternative “milks” made from plants like soya or almonds are increasingly popular. These alternatives are often vegan-friendly and can be suitable for people who are allergic to milk, or intolerant of it. The runner-up in the 2018 series of The Apprentice (UK) ran a flavoured nut milk business.

乳製品間會產生競爭關係。由大豆或杏仁等植物所製成的「牛奶」替代品越來越受歡迎。這些替代品通常是純素食品，適合對鮮奶過敏或不耐受的人。2018年誰是接班人（英國版）影集中的亞軍給了一個調味堅果奶的公司。

But the rise of alternative milks is just the latest twist in the saga of humanity’s relationship with animal milk. This relationship dates back thousands of years, and it has had a lot of ups and downs.

在人類與動物奶之間的傳奇關係中，牛奶替代品的興起只是個最新的轉折而已。這種關係可以追溯到幾千年前，途中已歷經了許多波折。

When you think about it, milk is a weird thing to drink. It’s a liquid made by a cow or other animal to feed its young; we have to squirt it out of the cow’s udders to obtain it.

若你去想一想的話，其實喝鮮奶是一件怪異的事。它是一種由奶牛或其他動物產來餵幼崽的液體；要取得牛奶，我們得讓它從奶牛的乳房中噴出才行。

In many cultures it is almost unheard of. Back in 2000, China launched a nationwide campaign to encourage people to consume more milk and dairy products for health reasons – a campaign that had to overcome the deep suspicions of many older Chinese people. Cheese, which is essentially milk that has been allowed to go off, can still make many Chinese people feel sick.

這幾乎在許多文明中都是前所未聞的。在2000年時，中國展開了一項全國性的活動，鼓勵大家為了健康，多飲用鮮奶和乳製品。這項活動得克服許多中國老年人深深懷疑的想法。起司本質上其實就是讓鮮奶變質的產品，許多中國人仍然覺得它很噁心。

Set against the 300,000-year history of our species, drinking milk is quite a new habit. Before about 10,000 years ago or so, hardly anybody drank milk, and then only on rare occasions. The first people to drink milk regularly were early farmers and pastoralists in western Europe – some of the first humans to live with domesticated animals, including cows. Today, drinking milk is common practice in northern Europe, North America, and a patchwork of other places.

不同於我們這個物種已長達30萬年的歷史，喝鮮奶卻是一種相當新的習慣。大約在一萬年前左右，幾乎沒有人會喝鮮奶，即使有也只是在極少數的情形下。第一批經常喝鮮奶的人是西歐的早期農民和牧民，他們也是一些最早與馴化動物（包括奶牛）生活在一起的人類。如今，在北歐、北美以及其他零碎的地方，喝鮮奶已經是司空見慣的事了。

Baby food

嬰兒食品

There is a biological reason why drinking animal milk is odd.

有一個生物學原因能夠說明為什麼喝動物奶很奇怪。

Milk contains a type of sugar called lactose, which is distinct from the sugars found in fruit and other sweet foods. When we are babies, our bodies make a special enzyme called lactase that allows us to digest the lactose in our mother’s milk. But after we are weaned in early childhood, for many people this stops. Without lactase, we cannot properly digest the lactose in milk. As a result, if an adult drinks a lot of milk they may experience flatulence, painful cramps and even diarrhoea. (It’s worth noting that in other mammals, there aren’t any lactase-persistent adults – adult cows don’t have active lactase, and neither do cats or dogs, for example).

鮮奶當中含有一種叫做乳糖的糖，它不同於在水果和其他甜食中所發現的糖。當我們是小嬰兒的時候，我們的身體會產生一種叫做乳糖酶的特殊酶，它可以讓我們消化母乳中的乳糖。但是，在我們斷奶之後，許多人就會停止產生乳糖酶。沒有乳糖酶，我們就無法好好地消化鮮奶中的乳糖。因此，若成年人喝下大量的鮮奶，他們可能會出現脹氣、絞痛甚至腹瀉的情形。（值得注意的是，在其他哺乳類動物當中，沒有出現任何能夠耐受乳糖的成年體，例如，成年的牛就沒有具有活性的乳糖酶，貓和狗也是如此）。

So the first Europeans who drank milk probably farted a lot as a result. But then evolution kicked in: some people began to keep their lactase enzymes active into adulthood. This “lactase persistence” allowed them to drink milk without side effects. It is the result of mutations in a section of DNA that controls the activity of the lactase gene.

所以，第一批喝鮮奶的歐洲人可能會因此經常放屁。但是接著，進化就起作用了：有些人開始能夠將乳糖酶的活性保持到成年期。這種「乳糖酶續存性」的情況，使他們可以毫無副作用的喝下鮮奶。這是一段控制乳糖酶基因活性的DNA片段產生突變的結果。

Artwork from the tomb of Methethi in Egypt, dated to around 2350BC, shows an ancient Egyptian milking a cow (Credit: Getty)

這幅出於埃及Methethi墓中的畫作可以追溯至西元前2350年左右，其為一位古代的埃及人在擠牛奶的圖。（圖片來源：Getty）

“The first time that we see the lactase persistence allele in Europe arising is around 5,000 years BP [before present] in southern Europe, and then it starts to kick in in central Europe around 3,000 years ago,” says assistant professor Laure Ségurel at the Museum of Humankind in Paris, who co-authored a 2017 review of the science of lactase persistence.

巴黎人類博物館助理教授Laure Ségurel說：「我們第一次看到歐洲出現乳糖酶續存性等位基因是在大約距今5000年前的歐洲南部，接著它在大約3000年前的中歐才開始出現。」2017年她與人共同撰寫一篇關於乳糖酶續存性學術知識的評論。

The lactase persistence trait was favoured by evolution and today it is extremely common in some populations. In northern Europe, more than 90% of people are lactase persistent. The same is true in a few populations in Africa and the Middle East.

乳糖酶續存性這項特性受到進化的青睞，現今在某些族群中非常普遍。在北歐，超過90％的人有乳糖酶續存性。在非洲和中東的一些族群中也是如此。

But there are also many populations where lactase persistence is much rarer: many Africans do not have the trait and it is uncommon in Asia and South America.

但是乳糖酶續存性在許多族群中也是非常罕見的：許多非洲人都沒有這種特性，而在亞洲和南美洲也不常見。

It is hard to make sense of this pattern because we don’t know precisely why drinking milk, and therefore lactase persistence, was a good thing, says Ségurel: “Why was it so strongly advantageous in itself?”

會難以理解這種模式是因為我們無法確切地知道為什麼喝鮮奶以及為什麼乳糖酶續存性是件好事，Ségurel說：「為什麼它本身有如此強大的優勢呢？」

The obvious answer is that drinking milk gave people a new source of nutrients, reducing the risk of starvation. But on closer inspection this doesn’t hold up.

顯而易見的答案就是喝鮮奶能為人類提供新的營養來源，降低飢餓的風險。但仔細檢查後，會發現這根本站不住腳。

“There’s a lot of different sources of food, so it’s surprising that one source of food is so important, so different from other sorts of food,” says Ségurel.

Ségurel說：「食物有許多不同的來源，所以奇怪的是怎麼會存在著一種與其他類型的食物都不同­，卻又這麼重要的食物來源。」

People who are lactase-non-persistent can still eat a certain amount of lactose without ill effects, so drinking a small amount of milk is fine. There is also the option of processing milk into butter, yoghurt, cream or cheese – all of which reduce the amount of lactose. Hard cheeses like cheddar have less than 10% as much lactose as milk, and butter is similarly low. (Read more about parmigiano, a cheese with so little lactose it can be eaten by the lactose-intolerant). “Heavy cream and butter have the lowest lactose,” says Ségurel.

非乳糖酶續存性的人仍然可以吃下一定量的乳糖而不會產生不良的影響，所以是可以喝少量的鮮奶。還可以選擇將鮮奶加工成奶油、優格、鮮奶油或起司，以上這些食品都可以減少乳糖的含量。切達起司等硬質起司的乳糖含量不到等量鮮奶的10％，奶油也同樣地低。（閱讀更多關於帕瑪森的資訊，這種起司含有的乳糖含量少到即使有乳糖不耐症的人也可以吃）。Ségurel說：「重鮮奶油和奶油含有的乳糖是最低的。」

Accordingly, people seem to have invented cheese rather quickly. In September 2018, archaeologists reporting finding fragments of pottery in what is now Croatia. They carried fatty acids, suggesting that the pottery had been used to separate curds from whey: a crucial step in making cheese. If that is correct (and the interpretation has been questioned), people were making cheese in southern Europe 7,200 years ago. Similar evidence from slightly more recent times, but still more than 6,000 years ago, has been found elsewhere in Europe. This is well before lactase persistence became common in Europeans.

這樣看來，人類似乎很快就發明了起司。2018年9月，考古學家報告說在現今的克羅埃西亞發現了陶器的碎片。它們上面帶有脂肪酸，表示陶器是被用來將凝乳與乳清分離開來：這是製作起司的關鍵步驟。若這是對的（這種解釋已受到質疑），人類早在7200年前就在歐洲南部製作出起司了。而在歐洲其他地方也有發現類似的證據，其時間更為靠近，不過距今仍然超過6000年。這可是早在乳糖酶續存性在歐洲人中變得普遍之前就已經存在了。

That said, there is clearly a pattern behind which populations evolved high levels of lactase persistence and which didn’t, says genetics professor Dallas Swallow of University College London. Those with the trait are pastoralists: people who raise livestock. Hunter-gatherers, who do not keep animals, did not acquire the mutations. Neither did “forest gardeners” who cultivated plants, but not livestock.

倫敦大學學院的遺傳學教授Dallas Swallow說，儘管如此，這顯然有一種模式在背後運作，決定哪些族群會進化出高度的乳糖酶續存性，而哪些則沒有。具有這種特性的人是牧民：飼養牲畜的人。沒有飼養動物的狩獵採集者就沒有產生突變。栽種植物而沒有養牲畜的「森林園丁」也沒有產生。

It makes sense that people who did not have access to animal milk were not under great evolutionary pressure to adapt to drinking it.

這很合理，無法取得動物鮮奶的人就不會出現要去適應飲用它的進化壓力。

The question is, why did some pastoralist people acquire the trait and not others?

問題是，為什麼有些牧民能獲得這種特性，而其他的則無法呢？

Ségurel points to east Asian herding peoples, such as those in Mongolia, who have some of the lowest rates of lactase persistence even though they rely heavily on milk from their animals for food. The mutations were common in nearby populations in Europe and western Asia, so it would have been possible for them to spread into these east Asian groups, but they didn’t. “That’s the big puzzle,” says Ségurel.

Ségurel指向東亞的放牧人，例如蒙古人，儘管他們嚴重依賴動物奶來作為食物，但他們的乳糖酶持續性比率卻是最低的那群人。這些突變在附近的歐洲和西亞族群中很常見，所以它們似乎能夠擴散到這些東亞族群中，但結果卻沒有。Ségurel說：「這是個大謎團。」