為什么人類要演化出忍受牛奶的能力

Why humans have evolved to drink milk
為什么人類要演化出忍受牛奶的能力

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.

乳制品有了競爭對手，由大豆、杏仁等植物制成的替代“牛奶”越來越受歡迎。這些替代品通常適合素食主義者，以及對牛奶過敏或者乳糖不耐受的人。2018年，英國版《學徒》（The Apprentice）節目的亞軍，就經營著調味堅果奶生意。

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.

但替代奶的興起，只是人類與動物奶關系的一個最新發展。這種關系可以追溯到幾千年前，歷經起伏。

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.

想想看，奶水真是一種奇怪的飲品。它是牛或者其他動物喂養幼崽的液體食物，為了獲得牛奶，我們還得把它從母牛的乳房里給擠出來。

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.

在許多文化中，這幾乎是聞所未聞的事情。2000年時，全中國范圍內發起一場運動，鼓勵人們為了健康要多消費牛奶和各種奶制品。這項運動主要針對老年人，幫助他們克服對奶制品的極大疑慮。牛奶變質形成的奶酪，仍然讓很多中國人感到惡心。

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.

與人類30萬年的歷史相比，喝牛奶算是相當新的習慣。大約1萬年前，人類幾乎不喝牛奶，然后是偶爾喝一喝。第一批經常喝牛奶的是西歐早期的農民和牧民，他們是第一批與馴養動物生活在一起的人，其中就包括奶牛。今天，喝牛奶在北歐、北美和其他一些地方，已經很普遍。

Baby food

嬰兒食品

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

喝動物奶為什么奇怪，在生物學上是有原因的。

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).

牛奶所含的乳糖，不同于水果和其他甜食中的糖。當我們還是嬰兒的時候，我們身體會產生一種特殊的酶，這種叫做乳糖酶的東西能幫助我們消化母乳中的乳糖。但在嬰兒期斷奶后，許多人身上就沒有這種酶了，就不能很好地消化牛奶中的乳糖。因此，一個成年人喝了很多牛奶后，可能會感到腸胃脹氣、疼痛痙攣，甚至腹瀉。值得注意的是，其他成年哺乳動物體內也沒有活躍的乳糖酶，成年奶牛沒有，貓狗也沒有；但成年動物并不喝奶，所以也沒有乳糖不耐受的問題。

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.

最早一批喝牛奶的歐洲人可能因此脹氣、多屁。然后演化開始了，一些人身上的乳糖酶開始保持至成年，這種“乳糖耐受能力”使他們喝牛奶不會產生副作用。這是控制乳糖酶基因活性的DNA片段突變的結果。

“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.

“我們所知的歐洲最早的乳糖耐受能力等位基因，出現在大約5000年前的歐洲南部，然后在大約3000年前，出現在中歐，”巴黎人類博物館（Museum of Humankind）的助理教授賽格瑞爾（Laure Ségurel）說。2017年，她與人合著了一篇關于乳糖耐受科學的評論文章。

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.

隨著演化，人類的乳糖耐受能力明顯提高，今天在一些人群中，乳糖酶極為常見。在北歐，超過90%的人具有乳糖耐受能力。在非洲和中東的部分人口中，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.

但在許多地區，乳糖耐受能力要罕見得多：許多非洲人不具備這種特性，在亞洲和南美洲也很少見。

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?”

這種狀況讓人難以理解，我們并不確切知道喝牛奶有什么好處，因此對乳糖耐受的重要性也不太了解，賽格瑞爾說：“為什么有乳糖耐受能力的人有如此大的優勢？”

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.

答案顯而易見，牛奶為人們提供了一個新的營養來源，減少了饑餓的風險。但仔細觀察，這種說法似乎站不住腳。

“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.

“食物來源多種多樣，所以某一種食物如此重要，是其他種類食物不能代替的，這是讓人驚訝的事情，”賽格瑞爾說。

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. “Heavy cream and butter have the lowest lactose,” says Ségurel.

乳糖不耐受的人仍然可以吃一定量的乳糖，不會產生不良后果，所以喝少量的牛奶是沒問題的。還有一個辦法，就是將牛奶加工成黃油、酸奶、奶油或者奶酪，這些食品中的乳糖含量都較少。像切達干酪等硬奶酪，乳糖含量不到牛奶的10%；黃油的乳糖含量也很低。帕爾馬干酪（Parmigiano）是一種硬質的奶酪，乳糖含量極低，乳糖不耐受的人完全可以吃。“高脂肪的奶油和黃油的乳糖含量最低，”賽格瑞爾說。

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.

所以，古人們很快就發明了奶酪。 2018年9月，考古學家在克羅地亞地區發現了一些陶器碎片。這些碎片上帶有脂肪酸，表明這些陶器曾經用于分離凝乳和乳清，這是制作奶酪的關鍵步驟。如果確實無誤的話（這點受到質疑），南歐人在7200年前就開始制作奶酪了。在歐洲其他地方，也發現了類似的證據，雖年代稍晚，但也在大約6000多年前。所以，當乳糖耐受尚未在歐洲普遍的程度，人們就懂得制作奶酪了。

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（University College London）的遺傳學教授斯沃洛（Dallas Swallow）說，即便如此，能否進化出高水平的乳糖耐受能力，顯然受特定因素的影響。最受影響的是牧民，也就是飼養牲畜的人。狩獵者、食物采集者都不養動物，就沒有獲得這種基因突變的條件。所謂“森林園丁”，就是種植農作物的人，身體也沒有出現這種突變。

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

那些不會接觸到動物奶水的人，沒有巨大的演化壓力來消化動物奶，這是可以理解的。

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

問題是，為什么有些牧民獲得了這種特性，有些人則沒有？

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.

賽格瑞爾指出，東亞的游牧民族，比如蒙古人，雖然嚴重依賴動物奶水為食，乳糖耐受能力卻最低。基因突變在歐洲和西亞人群中很常見，所以應該有可能傳播到臨近的東亞人群中，但事實上并沒有。“這是個很大的謎團，”賽格瑞爾說。

Dairy benefits

乳制品的好處

She speculates that drinking milk might have other advantages besides its nutritional value. People who keep livestock are exposed to their diseases, which can include anthrax and cryptosporidiosis. It may be that drinking cow’s milk provides antibodies against some of these infections. Indeed, milk's protective effect is thought to be one of the benefits of breastfeeding children.

她推測，牛奶除了營養價值外，還有其他好處。飼養牲畜的人容易感染疾病，包括炭疽熱和隱孢子蟲病。喝牛奶可能提供抵抗這些病菌感染的抗體。的確，奶水的保護作用同母乳喂養的好處差不多。

But some of the mysterious absences of lactase-persistence could be down to sheer chance: whether anyone in a group of pastoralists happened to get the right mutation. Until fairly recently there were a lot fewer people on Earth and local populations were smaller, so some groups would miss out by plain bad luck.

有些人群乳糖耐受能力不足，有些不可思議，但可能是偶然因素決定，即一群牧民中是否有人碰巧發生了這樣的基因突變。地球人口爆炸是近代才發生的事，此前，地區族群人口非常少，所以有些群體會因為運氣不好而錯過機會。

“I think the most coherent part of the picture is that there’s a correlation with the way of life, with pastoralism,” says Swallow. “But you have to have the mutation first.” Only then could natural selection go to work.

“我認為，演化過程中最有邏輯的部分，是人群乳糖耐受能力與生活方式及畜牧生產方式有關，”斯沃洛說。“但你必須得有突變。”只有這樣，自然選擇才能發揮作用。

In the case of Mongolian herders, Swallow points out that they typically drink fermented milk, which again has a lower lactose content. Arguably, the ease with which milk can be processed to be more edible makes the rise of lactase persistence even more puzzling. “Because we were so good at adapting culturally to processing and fermenting the milk, I’m struggling with why we ever adapted genetically,” says Swallow’s PhD student Catherine Walker.

斯沃洛指出，蒙古牧民通常喝的是發酵奶，這種奶的乳糖含量很低。可以說，為了更易食用而對牛奶進行加工，這并不費力，這也使乳糖耐受的進化變得很難解釋。“我們非常善于從文化上適應牛奶的加工和發酵，我也在試圖搞清楚為什么我們會在基因上適應它，”斯沃洛指導的博士生沃克（Catherine Walker）說。

There may have been several factors promoting lactase persistence, not just one. Swallow suspects that the key may have been milk’s nutritional benefits, such as that it is rich in fat, protein, sugar and micronutrients like calcium and vitamin D.

促成乳糖耐受的因素可能有好多。斯沃洛認為，最關鍵的可能是牛奶的營養價值，比如它富含脂肪、蛋白質、糖，以及鈣、維生素D等微量元素。

It is also a source of clean water. Depending on where your community lived, you may have evolved to tolerate it for one reason over another.

牛奶也是潔凈的水源。特定人群可能由于某種原因，不斷演化，直到可以容忍它，具體哪個原因要看生活的地區。

It’s unclear whether lactase persistence is still being actively favoured by evolution, and thus whether it will become more widespread, says Swallow. In 2018 she co-authored a study of a group of pastoralists in the Coquimbo region of Chile, who acquired the lactase-persistence mutation when their ancestors interbred with newly-arrived Europeans 500 years ago. The trait is now spreading through the population: it is being favoured by evolution, as it was in northern Europeans 5,000 years ago.

斯沃洛說，目前還不清楚，乳糖耐受能力是否仍然在不斷進化提升過程中，是否更多族群會擁有這樣的能力。2018年，她與人合寫了一份研究報告，對象是智利科金博地區（Coquimbo）的牧民。500年前，這個族群與新來的歐洲人出現跨種族生育，混血兒獲得了乳糖酶耐受能力。這種特性現正在族群間傳播，在演化中變得顯著，就像5000年前北歐人一樣。

But this is a special case because the Coquimbo people are heavily reliant on milk. Globally, the picture is very different. “I would think it’s stabilised myself, except in countries where they have milk dependence and there is a shortage [of other food],” says Swallow. “In the West, where we have such good diets, the selective pressures are not really likely to be there.”

但這是一個特例，因為科金博人嚴重依賴牛奶。在全球其他地區，情況則非常不同。“除了那些依賴牛奶、其他食物短缺的國家可能還有一些這類進化，除此以外，我認為已經基本停滯了。”斯沃洛說。“在西方，我們有這么好的飲食來源，基本不存在選擇壓力。”

Dairy decline?

奶制品的衰落

If anything, the news over the last few years offers the opposite impression: that people are abandoning milk. In November 2018, the Guardian published a story headlined “How we fell out of love with milk”, describing the meteoric rise of the companies selling oat and nut milks, and suggesting that traditional milk is facing a major battle.

如果有什么不同的話，那就是過去幾年里，新聞給人們留下了正好相反的印象。2018年11月，《衛報》（Guardian）發表了一篇題為《我們如何失去了對牛奶的愛》的文章，描述了銷售燕麥和堅果牛奶的公司迅速崛起，并暗示傳統牛奶將面臨嚴峻挑戰。

But the statistics tell a different story. According to the 2018 report of the IFCN Dairy Research Network, global milk production has increased every year since 1998 in response to growing demand. In 2017, 864 million tonnes of milk were produced worldwide. This shows no sign of slowing down: the IFCN expects milk demand to rise 35% by 2030 to 1,168 million tonnes.

但統計數據卻提供了不同的說法。根據IFCN乳制品研究網（IFCN Dairy Research Network）2018年的報告，自1998年以來，全球牛奶產量每年都在增長，以滿足不斷增長的需求。2017年，全球牛奶產量為8.64億噸。這一趨勢毫無放緩的跡象，據IFCN預計，到2030年，牛奶產量將增長35%，達到11.68億噸。

Still, this masks some more localised trends. A 2010 study of food consumption found that in the US milk consumption has fallen over the last few decades – although it was replaced with fizzy drinks, not almond milk. This fall was balanced by growing demand in developing countries, especially in Asia – something the IFCN has also noted. Meanwhile, a 2015 study of people’s drinking habits in 187 countries found that milk drinking was more common in older people, which does suggest that it is less popular with the young – although this says nothing about young people’s consumption of milk products like yoghurt.

不過，一些局部的趨勢并非如此。2010年一項有關食品消費的研究發現，在過去幾十年里，美國的牛奶消費量有所下降，取而代之的是碳酸飲料，而不是杏仁奶。全球而言，這種下降被發展中國家（特別是亞洲國家）增長的牛奶消費量所抵消。IFCN也注意到了這一點。2015年，一項研究調查了187個國家的飲用習慣，顯示出老年人喝牛奶的情況更為普遍，年輕人不太喜歡牛奶。這份報告并沒有說明年輕人對酸奶等奶制品的消費情況。

Still, it seems unlikely that alternative milks will make much of a dent in the world’s growing appetite for milk, at least over the next decade.

不過，至少在未來10年內，全球牛奶需求會不斷增長，替代奶對牛奶的影響都不大。

Walker adds that alternative milks are “not a like-for-like substitution” for animal milk. In particular, many don’t have the same micronutrients. She says they are most useful for vegans and for people allergic to milk – the latter being a reaction to milk protein, and nothing to do with lactose.

沃克補充道，替代奶并非是動物奶水的“對等替代品”。尤其是，許多替代奶缺乏牛奶中的微量元素。她說，替代奶對素食者以及對牛奶過敏的人最有用，牛奶過敏是指對牛奶蛋白質過敏，與乳糖無關。

It’s particularly striking that so much of the growth in milk demand is in Asia, where most people are non-lactase-persistent. Whatever advantages the people there see in milk, they outweigh the potential digestive issues or the need to process the milk.

尤其引人注目的是，對牛奶的需求有相當大一部分來自亞洲，那里的大多數人都沒有乳糖耐受能力。無論如何，那里的人們認識到牛奶的好處，因此可能產生的消化不良，或是需對牛奶加工，就都不成問題。

In fact, the United Nations Food and Agriculture Organisation has pushed for people in developing countries to keep more non-traditional dairy animals, such as llamas, so that they can obtain the benefits of milk even if cow’s milk is unavailable or too expensive.

事實上，聯合國糧農組織（United Nations Food and Agriculture Organisation）一直敦促發展中國家飼養更多的非傳統乳畜，比如羊駝，這樣他們就能從這些動物產的奶水中獲益，哪怕牛奶供應不足或價格過高。

What’s more, a major study published in January described a “planetary health diet” that is designed to both maximise health and minimise our impact on the environment. While it entails drastically cutting down on red meat and other animal products, it nevertheless includes the equivalent of one glass of milk a day.

此外，今年1月的一項重要研究報告提出了“全球健康飲食”計劃，希望最大限度增進健康，最大限度減少對環境的影響。該計劃提議大幅減少紅肉和其他動物產品的消費，但它仍然提倡每天一杯奶。

Milk, it seems, is not down and out. If anything it’s still on the up – even if our bodies have mostly stopped evolving in response to it.

人類提高飲奶適應力的進化已經停止。但是，牛奶的生產和消費并沒有走下坡路。它還在上升階段。