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."

    今天发表的一个研究表明，自从我们的祖先4万年前开始在欧洲，亚洲，非洲发展，人类的进化步伐在以令人目瞪口呆的加速度加快。

    B研究人员通过对269人的超过3百万个DNA变化检验发现，大概有1800个基因是相对近期被人类广泛采用，因为这些基因给进化带来了益处。

    人类学家一直认为在我们进入到相对稳定的农耕社会后，人类的进化压力减小了。但在最近几年，大家认识到其实恰恰相反，疾病往往在大量人口长期紧密聚居的社会肆虐。

    国家科学院的研究报告表明，人类7%的基因组变化是近期发生的。

    研究项目负责人，来自威斯康辛-麦迪逊大学的约翰。霍克斯说，除了大约100个基因之外，其余所有基因有什么优势仍然是个谜。但他的研究团队可以断定，传染病和新的食物是某些基因以如此快的速度成为人类基因的主要原因。

    霍克斯说："如果人群和环境之间没有不和谐的地方就不会有进化选择，饮食和疾病的变化创造了产生选择的环境"

    一个着名的例子就是让成年人可以消化牛奶的基因的传播。

    虽然小孩喝牛奶没有问题，但在长大后他们通常都会产生乳糖不耐受。 但在牛羊在欧洲牦牛和马在亚洲成为家畜后，发生了基因突变能够消化牛奶的成年人比那些没有变化的人有营养上的优势。

    霍克斯说，结果是他们的后代就可能更健康，这种基因突变就得以扩展。

    这个机制也说明了为什么抗疟疾的基因在非洲人中普遍存在，他们住在那些带病原的蚊子肆虐的地方。但在欧洲和亚洲人中就没有这种基因。

    研究者确认的很多基因变化只在住在同一个地域的人群中出现。我们今天知道的物种在近两万年前才开始存在，霍克斯说，那个时候影响皮肤色素的基因开始出现，白皮肤可以让在北方的人吸收更多的阳光来产生维生素D.

    "当人口扩张进入新的环境，人们在不同的环境感受到的压力是不同的，"密西根大学人类基因学家诺阿。罗森布格说，他并没有参加这个研究项目。

    霍克斯和其在加州大学Irvine校区，犹他大学的同事及在圣克拉拉的基因芯片厂商Affymetrix 公司研究了国际HAPMAP联盟提供的基因数据， HAPMAP联盟对尼日尼亚人，日本人，中国人，欧洲人的30亿个DNA letters中单个letter的差别进行了记录。

    研究者在很多人中找大量的一模一样的DNA,这样就可以表明在单个个体随机的基因突变有机会发生之前，这类基因大量的被吸收及扩张都是在相对近期发生的。

    他们发现人口增长越多，人类的基因进化得越快。这是因为更多的人口为有益的基因突变创造了更多的机会，霍克斯说。

    .

    这个研究断定，在过去5000到10000年，当农业可以支撑越来越多的人口时，进化的速度超过历史最高水平的100倍。

    霍克斯说，进化最快的基因中包括和人类大脑发展有关的基因，但研究者不太确定是什么促使这些基因如此受欢迎。

    还有其它的谜团。

    霍克斯说："一万年前没有人是蓝眼睛，为什么蓝眼睛的人比不是蓝眼睛的人在繁殖上有5%的优势？我不知道。"