4、    WASHINGTON(AP)——Human beings may have had a brush with extinction 70，000 years ago，all extensive genetic(基因的)study suggests．

    The human population at that time was reduced to small separate groups in Africa，apparently because of dry climate，according to an analysis．A study by researchers at Stanford University pointed out that the number of early humans may have decreased as low as 2．000 before numbers began to expand again in the early Stone Age．

    “This study shows the extraordinary power of genetics to expose some of the key events in our history。”Spencer Wells，National Geographic Society explorer。said in a statement．“Tiny groups of early humans，forced apart by severe environmental conditions，came back to reunite and populate the world，which was written in our DNA．”

    Previous studies using mitochondrial DNA—which is passed down through mothers—have found the origins of modern humans related to a single“mitochondrial Eve，”who lived in Africa about 200，000 years ago．It seems that humans began to move out of Africa to populate the rest of the world about 60，000 years ago．

    The researchers concluded that humans separated into small populations before the Stone Age，when they came back together and began to increase in numbers and spread to other areas．Eastern Africa experienced a series of severe dry weather between l35，000 and 90，000 years ago and the researchers said this climatological shift may have contributed to the population changes，dividing into small，separate groups which developed independently．As recently as 70，000 years ago，extremes of climate had reduced our population to such small numbers that we were on the very edge of extinction．

1．Scientists found that humans were likely to be extinct by studying     ．

    A．climate changes  B．people’s DNA  C．human’s history        D．old environment

2．The scientists did the research in Africa because          ．

  A．humans began to come and live in the rest of the world 70．000 years ago

  B．DNA in the people of Africa can be passed down through mothers

  C．only humans in Africa faced severe environmental conditions

  D．human beings are considered to have originated from Africa

3．The period when humans kept in small populations probably last        ．

  A．60．000 years    B．10，000 years    C．20，000 years   D．45，000 years

4．What can we learn from the last pagraph?

  A．It seldom rained for a long time，which decreased our population．

  B．People lived together to fight against the climate changes．

  C．People were divided into small groups to hunt more animals．

  D．People returned in the Stone Age and the number increased．

5．Which of the following is the best title of the passage?

  A．Humans may have extinction 70，000 years ago

  B．Stone Age：A period when our population increased

  C．Getting out of our homeland-Africa

  D．Severe droughts will cause extinction of humans