Got milk? If you do, take a moment to ponder the true oddness of being able to drink milk after you're a baby.

    No other species but humans can. And most humans can't either.

    The long lists of food allergies some people claim to have can make it seem as if they're just finicky eaters trying to rationalize likes and dislikes. Not so. Eggs, peanuts, tree nuts, fish, shellfish soy and gluten all can wreak havoc on the immune system of allergic individuals, even causing a deadly reaction called anaphylaxis.

    But those allergic reactions are relatively rare, affecting an estimated 4% of adults.

    Milk's different.

    First off, it's not actually an allergy in that there's not an immune response.

    People who are lactose intolerant can't digest the main sugar -lactose- found in milk. In normal humans, the enzyme that does so -lactase- stops being produced when the person is between two and five years old. The undigested sugars end up in the colon, where they begin to ferment, producing gas that can cause cramping, bloating, nausea, flatulence and diarrhea.

    If you're American or European it's hard to realize this, but being able to digest milk as an adult is one weird genetic adaptation.

    It's not normal. Somewhat less than 40% of people in the world retain the ability to digest lactose after childhood. The numbers are often given as close to 0% of Native Americans, 5% of Asians, 25% of African and Caribbean peoples, 50% of Mediterranean peoples and 90% of northern Europeans. Sweden has one of the world's highest percentages of lactase tolerant people.

    Being able to digest milk is so strange that scientists say we shouldn't really call lactose intolerance a disease, because that presumes it's abnormal. Instead, they call it lactase persistence, indicating what's really weird is the ability to continue to drink milk.

    There's been a lot of research over the past decade looking at the genetic mutation that allows this subset of humanity to stay milk drinkers into adulthood.

    A long-held theory was that the mutation showed up first in Northern Europe, where people got less vitamin D from the sun and therefore did better if they could also get the crucial hormone （it's not really a vitamin at all） from milk.

    But now a group at University College London has shown that the mutation actually appeared about 7,500 years ago in dairy farmers who lived in a region between the central Balkans and central Europe, in what was known as the Funnel Beaker culture.

    The researchers used a computer to model the spread of lactase persistence, dairy farming, other food gathering practices and genes in Europe.

    Today, the highest proportion of people with lactase persistence live in Northwest Europe, especially the Netherlands, Ireland and Scandinavia. But the computer model suggests that dairy farmers carrying this gene variant probably originated in central Europe and then spread more widely and rapidly than non-dairying groups.

    The European mutation is different from several lactase persistence genes associated with small populations of African peoples who historically have been cattle herders.

    Researchers at the University of Maryland identified one such mutation among Nilo-Saharan-speaking peoples in Kenya and Tanzania. That mutation seems to have arisen between 2,700 to 6,800 years ago. Two other mutations have been found among the Beja people of northeastern Sudan and tribes of the same language family in northern Kenya.

    喝奶么？喝的話，抽出點兒時間來關注一下你為什么過了嬰兒期還能夠喝牛奶吧。

    只有人類這一物種能夠喝牛奶。同時大多數人又是不能喝的。

    有人說自己對很多食物過敏，就好像他們特別挑剔，給自己喜歡和不喜歡吃的食物找借口。事實并非如此。雞蛋、花生、堅果、魚、海鮮醬和麥麩都能給敏感體質人群的免疫系統造成極大損傷，甚至引起致命的過敏反應。

    但這些過敏反應相對并不常見，大概有4%的人有此反應。

    而牛奶就不同了。

    首先，牛奶并不是那種會引起免疫應答的過敏原。有乳糖抗性的人群不能消化牛奶中的主要糖分--乳糖。正常人在2至5歲之間，體內消化乳糖的酶--乳糖酶就停止分泌了。這些不能消化的糖分最終積聚在盲腸中發酵產氣，引起痙攣、腫脹、嘔吐、腸胃脹氣和痢疾。

    美國人和歐洲人很難想象，成人能夠消化牛奶其實是一個很奇特的基因適應性現象。

    這不是個普通現象。全世界大約不到40%的成年人能夠在童年過后保留消化乳糖的能力。該數字在美洲印第安人中幾乎為0,亞洲人為5%,非洲和加勒比人為25%,地中海人群50%,北歐90%.其中瑞典為乳糖耐受性比例最高的國家。

    能消化牛奶這一功能真的太奇特了。科學家們甚至說我們不應該把乳糖抗性當成一種疾病，那樣的話首先就把它看做不正常了。相反的，科學家們把乳糖抗性稱為乳糖耐受性，以表明成年后能夠繼續喝牛奶才是奇怪的事情。

    過去十年內，有很多研究圍繞這些成年后保留喝牛奶能力的人群基因展開。長期以來的理論是：該突變基因首先出現在北歐，那里的人們從光照中吸收的維生素D少，因此從牛奶中吸收的關鍵激素就多（實際上乳糖不是維生素）.

    但如今倫敦大學學院的一個研究小組表明該突變基因實際上來源于7500年前，也就是漏斗頸陶文化時期，居住在巴爾干半島中部和歐洲中部之間的奶農。

    研究人員對歐洲地區的乳糖耐受性的傳播、乳品業、其它食品聚集過程和基因進行了計算機建模。

    如今，乳糖耐受性比例最高的人群居住在歐洲西北部，尤其是荷蘭、愛爾蘭和斯堪的納維亞。但計算機建模表明，攜帶該基因變種的奶農來自中歐，之后比非乳品業人群分布得廣泛而迅速。

    歐洲的突變基因就與世代游牧的非洲少數人群身上的幾種乳糖耐受性基因不同。馬里蘭大學的研究人員在肯尼亞和坦桑尼亞講尼羅-撒哈拉語的人群中發現了這種突變基因。此基因大概出現于2700至6800年前。另外兩種突變基因也已在蘇丹東北部的貝賈人和肯尼亞北部講貝賈語的部落中發現。