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THE DNA TRAIL Everybody loves a good story, and when it's finished, this may be the greatest one ever told. It begins in Africa with a group of people. There are perhaps just a few hundred, surviving by hunting animals and gathering fruits, vegetables, and nuts. It ends about 200,000 years later, with their seven billion descendants spread across the Earth. In between is an exciting tale of survival, movement, isolation, and conquest, most of it occurring before recorded history. Who were those first modern people in Africa? What routes did they take when they left their home continent to expand into Europe and Asia? When and how did humans reach the Americas? For decades, the only proof was found in a small number of scattered bones and artifacts that our ancestors had left behind. In the past 20 years, however, DNA technologies have allowed scientists to find a record of ancient human migrations in the DNA of living people. Tracing Ancestry in DNA "Every drop of human blood contains a history book written in the language of our genes," says population geneticist Spencer Wells. The human genetic code, or genome, is 99.9 percent identical throughout the world. The bulk of our DNA is the same. However, the remainder is responsible for our individual differences—in eye color or disease risk, for example. On very rare occasions, a small change—called a mutation—can occur. This can then be passed down to all of that person's descendants. Generations later, finding that same mutation in two people's DNA indicates that they share the same ancestor. By comparing mutations in many different populations, scientists can trace their ancestral connections. These ancient mutations are easiest to track in two places. One is in DNA that is passed from mother to child (called mitochondrial DNA, or mtDNA). The other is in DNA that travels from father to son (known as the Y chromosome, the part of DNA that determines a child will be a boy). By comparing the mtDNA and Y chromosomes of people from various populations, geneticists can get a rough idea of where and when those groups separated in the great migrations around the planet.

DNA痕迹 每个人都喜欢一个好故事，当它结束时，这可能是有史以来最伟大的故事。 它始于非洲的一群人。 也许只有几百个， 以狩猎动物和采集水果、蔬菜和坚果为生。 大约20万年后，它结束了，它们的70亿后代遍布地球。 在这两者之间是一个关于生存、运动、孤立和征服的激动人心的故事，其中大部分发生在有记录的历史之前。 非洲最早的现代人是谁？ 当他们离开祖国大陆向欧洲和亚洲扩张时，他们采取了哪些路线？ 人类何时以及如何到达美洲？ 几十年来，唯一的证据是在少数零散的骨头和文物中发现的 我们的祖先留下的。 然而，在过去的20年里，DNA技术使科学家们 在活人的DNA中找到古代人类迁徙的记录。 DNA中的祖先追踪 “每一滴血都包含着一本用我们基因语言写成的历史书。” 人口遗传学家斯宾塞·威尔斯说。 全世界的人类遗传密码或基因组有99.9%是相同的。 我们的DNA大部分是相同的。 然而，其余的是我们个体差异的原因，例如眼睛颜色或疾病风险。 在极少数情况下，可能会发生微小的变化，称为突变。 然后，这可以传给该人的所有后代。 几代人之后，在两个人的DNA中发现相同的突变表明他们有着相同的祖先。 通过比较许多不同人群的突变，科学家可以追踪他们的祖先联系。 这些古老的突变在两个地方最容易追踪。 一种是从母亲传给孩子的DNA（称为线粒体DNA，或mtDNA）。 另一种存在于从父亲传给儿子的DNA中 （被称为Y染色体，决定孩子是男孩的DNA部分）。 通过比较不同人群的mtDNA和Y染色体， 遗传学家可以大致了解这些群体在全球大迁徙中何时何地分离。

Out of Africa In the mid-1980s, a study compared mtDNA from people around the world. It found that people of African descent had twice as many genetic differences from each other than did others. Because mutations seem to occur at a steady rate over time, scientists concluded that modern humans must have lived in Africa at least twice as long as anywhere else. They now calculate that all living humans maternally descend from a single woman who lived roughly 150,000 years ago in Africa, a "mitochondrial Eve." If geneticists are right, all of humanity is linked to Eve through an unbroken chain of mothers. This Eve was soon joined by "Y-chromosome Adam," the possible genetic father of us all, also from Africa. DNA studies have confirmed that all the people on Earth can trace their ancestry to ancient Africans. What seems certain is that at a remarkably recent date—probably between 50,000 and 70,000 years ago— one small group of people, the ancestors of modern humans outside of Africa, left Africa for western Asia. They either migrated around the wider northern end of the Red Sea, or across its narrow southern opening. Once in Asia, genetic evidence suggests, the population split. One group stayed temporarily in the Middle East, while the other began a journey that would last tens of thousands of years. Moving a little farther with each new generation, they followed the coast around the Arabian Peninsula, India, and Southeast Asia, all the way to Australia. "The movement was probably imperceptible," says Spencer Wells. "It was less of a journey and probably more like walking a little farther down the beach to get away from the crowd." Archeological evidence of this 13,000-kilometer migration from Africa to Australia has almost completely vanished. However, genetic traces of the group that made the trip do exist. They have been found in the DNA of indigenous peoples in Malaysia, in Papua New Guinea, and in the DNA of nearly all Australian aborigines. Modern discoveries of 45,000-year-old bodies in Australia, buried at a site called Lake Mungo, provide physical evidence for the theories as well. People in the rest of Asia and Europe share different but equally ancient mtDNA and Y-chromosome mutations. These mutations show that most are descendants of the group that stayed in the Middle East for thousands of years before moving on. Perhaps about 40,000 years ago, modern humans first advanced into Europe. Peopling the Americas About the same time as modern humans pushed into Europe, some of the same group that had paused in the Middle East spread east into Central Asia. They eventually reached as far as Siberia, the Korean peninsula, and Japan. Here begins one of the last chapters in the human story—the peopling of the Americas. Most scientists believe that todays Native Americans descend from ancient Asians who crossed from Siberia to Alaska in the last ice age. At that time, low sea levels would have exposed a land bridge between the continents. Perhaps they only a few hundred people—were traveling along the coast, moving from one piece of land to the next, between a freezing ocean and a wall of ice. "A coastal route would have been the easiest way in," says Wells. "But it still would have been a hell of a trip." Once across, they followed the immense herds of animals into the mainland. They spread to the tip of South America in as little as a thousand years. Genetic researchers can only tell us the basic outlines of a story of human migration that is more complex than any ever written. Many details of the movements of our ancestors and their countless individual lives can only be imagined. But thanks to genetic researchers—themselves descendants of mtDNA Eve and Y-chromosome Adam— we have begun to unlock important secrets about the origins and movements of our ancient ancestors. ALTERNATIVE ROUTES? Scientists have long believed that modern humans originated in Africa, because that's where they've found the oldest bones. Geneticists have come to the same conclusion based on analysis of human DNA. However, there is less consensus about the routes our ancestors took. For example, genetic data suggests that Europe might have been settled by an inland migration from India, rather than directly from the Middle East. "I think the broad human prehistoric framework is in place," says geneticist Peter Forster of the McDonald Institute for Archaeological Research, "and we are now fitting in the details."

走出非洲 20世纪80年代中期，一项研究比较了世界各地人们的线粒体DNA。 研究发现，非洲人后裔之间的遗传差异是其他人的两倍。 因为突变似乎随着时间的推移以稳定的速率发生， 科学家们得出结论，现代人类在非洲的生活时间至少是其他任何地方的两倍。 他们现在计算出，所有活着的人类都是从一个女人身上下来的 大约15万年前生活在非洲的“线粒体夏娃” 如果遗传学家是对的，那么整个人类都是通过一条完整的母亲链与夏娃联系在一起的。 这个夏娃很快就被“Y染色体亚当”加入了 他可能是我们所有人的基因之父，也来自非洲。 DNA研究证实，地球上所有人的祖先都可以追溯到古代非洲人。 似乎可以肯定的是，在最近的一天，可能在5万到7万年前-- 一小群人，非洲以外现代人的祖先，离开非洲前往西亚。 它们要么迁移到红海更广阔的北端， 或者穿过它狭窄的南部开口。 遗传证据表明，一旦进入亚洲，人口就会分裂。 一组人暂时留在中东， 而另一个开始了持续数万年的旅程。 随着每一代人的成长， 他们沿着阿拉伯半岛、印度和东南亚的海岸一直走到澳大利亚。 斯宾塞·威尔斯说：“这场运动可能是难以察觉的。”。 “这不是一次旅行，可能更像是在海滩上走得更远，远离人群。” 从非洲到澳大利亚的13000公里迁徙的考古证据几乎完全消失了。 然而，进行这次旅行的群体的遗传痕迹确实存在。 在马来西亚、巴布亚新几内亚、巴布亚新几内亚和巴布亚新几内亚的土著人民的DNA中发现了它们， 几乎所有澳大利亚原住民的DNA中都有。 澳大利亚现代发现的45000年前的尸体， 埋在蒙戈湖的一个地方，也为这些理论提供了实物证据。 亚洲和欧洲其他地区的人共享不同但同样古老的mtDNA和Y染色体突变。 这些突变表明，大多数是该群体的后代 在中东停留了数千年之后才继续前行。 大约4万年前，现代人类首次进入欧洲。 美洲人民 大约在现代人涌入欧洲的同时， 在中东停留的同一群体中的一些人向东扩散到中亚。 他们最终到达了西伯利亚、朝鲜半岛和日本。 这里开始了人类故事的最后一章之一——美洲人民。 大多数科学家认为，今天的美洲原住民是古代亚洲人的后裔 他们在上一个冰河时代从西伯利亚穿越到阿拉斯加。 那时，低海平面会暴露出大陆之间的陆桥。 也许他们只有几百人——沿着海岸旅行， 在冰冻的海洋和冰墙之间，从一块陆地移动到另一块陆地。 威尔斯说：“沿海路线是最简单的进入方式。”。 “但这仍然会是一次地狱般的旅行。” 一旦穿越，他们便跟随庞大的动物群进入大陆。 它们在短短一千年内就传播到了南美洲的顶端。 基因研究人员只能告诉我们一个故事的基本轮廓 人类迁徙比以往任何时候都要复杂。 我们祖先的运动和无数个人生活的许多细节只能想象。 但多亏了基因研究人员——他们自己是mtDNA夏娃和Y染色体亚当的后代-- 我们已经开始揭开关于我们古代祖先起源和运动的重要秘密。 替代路线？ 长期以来，科学家们一直认为现代人类起源于非洲， 因为他们在那里发现了最古老的骨头。 遗传学家根据对人类DNA的分析得出了同样的结论。 然而，对于我们祖先所走的路线，人们的共识较少。 例如，基因数据表明，欧洲可能是由印度的内陆移民定居的， 而不是直接来自中东。 “我认为广泛的人类史前框架已经到位，” 麦当劳考古研究所的遗传学家彼得·福斯特说， “我们现在正在处理细节。”

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