The pace of human evolution has been increasing at a stunning rate since our ancestors began spreading through Europe, Asia and Africa 40,000 years ago, quickening to 100 times historical levels after agriculture became widespread, according to a study published today.

    By examining more than 3 million variants of DNA in 269 people, researchers identified about 1,800 genes that have been widely adopted in relatively recent times because they offer some evolutionary benefit.

    Until recently, anthropologists believed that evolutionary pressure on humans eased after the transition to a more stable agrarian lifestyle. But in the last few years, they realized the opposite was true -- diseases swept through societies in which large groups lived in close quarters for a long time.

    Altogether, the recent genetic changes account for 7% of the human genome, according to the study published in the Proceedings of the National Academy of Sciences.

    The advantage of all but about 100 of the genes remains a mystery, said University of Wisconsin-Madison anthropologist John Hawks, who led the study. But the research team was able to conclude that infectious diseases and the introduction of new foods were the primary reasons that some genes swept through populations with such speed.

    "If there were not a mismatch between the population and the environment, there wouldn't be any selection," Hawks said. "Dietary changes, disease changes -- those create circumstances where selection can happen."

    One of the most famous examples is the spread of a gene that allows adults to digest milk.

    Though children were able to drink milk, they typically developed lactose intolerance as they grew up. But after cattle and goats were domesticated in Europe and yaks and mares were domesticated in Asia, adults with a mutation that allowed them to digest milk had a nutritional advantage over those without.

    As a result, they were more likely to have healthy offspring, prompting the mutation to spread, Hawks said.

    The mechanism also explains why genetic resistance to malaria has spread among Africans -- who live where disease-carrying mosquitoes are prevalent -- but not among Europeans or Asians.

    Most of the genetic changes the researchers identified were found in only one geographic group or another. Races as we know them today didn't exist until fewer than 20,000 years ago, when genes involved in skin pigmentation emerged, Hawks said. Paler skin allowed people in northern latitudes to absorb more sunlight to make vitamin D.

    "As populations expanded into new environments, the pressures faced in those environments would have been different," said Noah Rosenberg, a human geneticist at the University of Michigan, who wasn't involved in the study. "So it stands to reason that in different parts of the world, different genes will appear to have experienced natural selection."

    Hawks and colleagues from UC Irvine, the University of Utah and Santa Clara-based gene chip maker Affymetrix Inc. examined genetic data collected by the International HapMap Consortium, which cataloged single-letter differences among the 3 billion letters of human DNA in people of Nigerian, Japanese, Chinese and European descent.

    The researchers looked for long stretches of DNA that were identical in many people, suggesting that a gene was widely adopted and that it spread relatively recently, before random mutations among individuals had a chance to occur.

    They found that the more the population grew, the faster human genes evolved. That's because more people created more opportunities for a beneficial mutation to arise, Hawks said.

    In the last 5,000 to 10,000 years, as agriculture was able to support increasingly large societies, the rate of evolutionary change rose to more than 100 times historical levels, the study concluded.

    Among the fastest-evolving genes were those related to brain development, but the researchers aren't sure what made them so desirable, Hawks said.

    There are other mysteries too.

    "Nobody 10,000 years ago had blue eyes," Hawks said. "Why is it that blue-eyed people had a 5% advantage in reproducing compared to non-blue-eyed people? I have no idea."

    今天發表的一個研究表明，自從我們的祖先4萬年前開始在歐洲，亞洲，非洲發展，人類的進化步伐在以令人目瞪口呆的加速度加快。

    B研究人員通過對269人的超過3百萬個DNA變化檢驗發現，大概有1800個基因是相對近期被人類廣泛采用，因為這些基因給進化帶來了益處。

    人類學家一直認為在我們進入到相對穩定的農耕社會后，人類的進化壓力減小了。但在最近幾年，大家認識到其實恰恰相反，疾病往往在大量人口長期緊密聚居的社會肆虐。

    國家科學院的研究報告表明，人類7%的基因組變化是近期發生的。

    研究項目負責人，來自威斯康辛-麥迪遜大學的約翰。霍克斯說，除了大約100個基因之外，其余所有基因有什么優勢仍然是個謎。但他的研究團隊可以斷定，傳染病和新的食物是某些基因以如此快的速度成為人類基因的主要原因。

    霍克斯說："如果人群和環境之間沒有不和諧的地方就不會有進化選擇，飲食和疾病的變化創造了產生選擇的環境"

    一個著名的例子就是讓成年人可以消化牛奶的基因的傳播。

    雖然小孩喝牛奶沒有問題，但在長大后他們通常都會產生乳糖不耐受。 但在牛羊在歐洲牦牛和馬在亞洲成為家畜后，發生了基因突變能夠消化牛奶的成年人比那些沒有變化的人有營養上的優勢。

    霍克斯說，結果是他們的后代就可能更健康，這種基因突變就得以擴展。

    這個機制也說明了為什么抗瘧疾的基因在非洲人中普遍存在，他們住在那些帶病原的蚊子肆虐的地方。但在歐洲和亞洲人中就沒有這種基因。

    研究者確認的很多基因變化只在住在同一個地域的人群中出現。我們今天知道的物種在近兩萬年前才開始存在，霍克斯說，那個時候影響皮膚色素的基因開始出現，白皮膚可以讓在北方的人吸收更多的陽光來產生維生素D.

    "當人口擴張進入新的環境，人們在不同的環境感受到的壓力是不同的，"密西根大學人類基因學家諾阿。羅森布格說，他并沒有參加這個研究項目。

    霍克斯和其在加州大學Irvine校區，猶他大學的同事及在圣克拉拉的基因芯片廠商Affymetrix 公司研究了國際HAPMAP聯盟提供的基因數據， HAPMAP聯盟對尼日尼亞人，日本人，中國人，歐洲人的30億個DNA letters中單個letter的差別進行了記錄。

    研究者在很多人中找大量的一模一樣的DNA,這樣就可以表明在單個個體隨機的基因突變有機會發生之前，這類基因大量的被吸收及擴張都是在相對近期發生的。

    他們發現人口增長越多，人類的基因進化得越快。這是因為更多的人口為有益的基因突變創造了更多的機會，霍克斯說。

    .

    這個研究斷定，在過去5000到10000年，當農業可以支撐越來越多的人口時，進化的速度超過歷史最高水平的100倍。

    霍克斯說，進化最快的基因中包括和人類大腦發展有關的基因，但研究者不太確定是什么促使這些基因如此受歡迎。

    還有其它的謎團。

    霍克斯說："一萬年前沒有人是藍眼睛，為什么藍眼睛的人比不是藍眼睛的人在繁殖上有5%的優勢？我不知道。"