核心词汇 (Target Vocabulary)

学习 5A 人类遗传学与史前迁徙核心词汇，点击语音按钮可听纯正美音发音，掌握核心释义与真题例句。

bulk [bʌlk] n.
The largest part or majority of something.
大部分；主体
"The bulk of human genetic history took place across the vast savannas of the African continent."
calculate [ˈkælkjuleɪt] v.
To determine an amount, date, or number mathematically.
计算；推测
"Geneticists calculate that all living humans maternally descend from an ancestral woman in East Africa."
descendant [dɪˈsendənt] n.
A person, plant, or animal that is descended from a particular ancestor.
后代；后裔
"Every person alive today is a direct genetic descendant of early Homo sapiens wanderers."
identical [aɪˈdentɪkl] adj.
Exactly the same, or very similar in every detail.
完全相同的；极其相似的
"Despite superficial physical differences, human beings are 99.9 percent genetically identical."
immense [ɪˈmens] adj.
Extremely large, huge, or great in scale.
巨大的；无边无际的
"Crossing the immense expanse of the Pacific Ocean required extraordinary maritime navigation."
rate [reɪt] n.
A measurement of the speed or frequency at which something happens.
比率；速度
"DNA mutations accumulate at a remarkably steady rate over hundreds of generations."
roughly [ˈrʌfli] adv.
Approximately; not exactly but close to a number.
粗略地；大约
"Roughly 60,000 years ago, a tiny band of humans left Africa and crossed the Red Sea."
scattered [ˈskætəd] adj.
Covering a wide area irregularly; dispersed.
分散的；零散的
"Remains of ancient stone tools lie scattered across remote river valleys in Eurasia."
trace [treɪs] v./n.
To find or describe the path or development of something; a tiny sign.
追踪；查出；微量痕迹
"By mapping unique genetic markers, scientists can trace ancestral migration trails."
vanish [ˈvænɪʃ] v.
To disappear suddenly and completely.
消失；绝迹
"Older hominid cousins like the Neanderthals eventually vanished as modern humans expanded."
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The DNA Trail

人类DNA寻踪：基因突变与走出非洲的十万年全球大迁徙

Paragraph A
Reading, 5A, the DNA trail, everybody loves a good story, and when it's finished, this may be the greatest one ever to old. 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 7 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 roots 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.
Paragraph B
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% 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 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.
Paragraph C
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. 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.
Paragraph D
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.
Paragraph E
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. Archaeological 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 Nugini, 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 dichromosome mutations. These mutations show that most are descendants of the group that stayed in the Middle East for thousands of years before moving on.
Paragraph F
Perhaps about 40,000 years ago, modern humans first advanced into Europe. People in 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 people in the Americas. Most scientists believe that today's 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 hurts of animals into the mainland.
Paragraph G
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 M.T. DNA 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 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.

课文配套练习 (Reading Exercises)

严格对应教材 Page 80–87 官方练习：Before You Read、Reading Comprehension 及 Chronology 史前时间线。

1.
Geography (p.80) According to genetic evidence, where did all modern humans originate?
2.
Geography (p.80) Which continent was among the last to be settled by prehistoric humans?
3.
Evidence (p.80) Which evidence helps scientists reconstruct ancient human migrations?
4.
Main Idea (p.86) What is the main idea of Reading 5A?
5.
Detail (Para C, p.86) Why are mitochondrial DNA (mtDNA) and Y-chromosome DNA especially valuable for tracing ancestry?
6.
Inference (Para G, p.86) What does Peter Forster's research suggest about the initial settlement of Europe?
7.
Detail (Para E, p.86) What archeological site provides physical evidence confirming early migration to Australia?
8.
Detail (Para G, p.86) How quickly did early humans reach the southern tip of South America after crossing into the Americas?
9.
Chronology (p.86) Prehistoric Timeline Matching: Modern humans arise in Africa around:
10.
Chronology (p.86) Prehistoric Timeline Matching: A small group of humans journeys out of Africa:
11.
Chronology (p.86) Prehistoric Timeline Matching: Humans arrive in Australia and Lake Mungo:
12.
Chronology (p.86) Prehistoric Timeline Matching: Humans cross the Bering land bridge into the Americas around:

Critical Thinking · Motivations for Migration
Throughout history, what forces drove humans to abandon their homelands and journey into unknown frontiers? (e.g. food scarcity, climate shocks, curiosity, adventure)
What are the positive and negative impacts of human migration on modern host economies?