揽瓜阁阅读做题小分队第154天人类生命的起源 - ChaseDream

It is now commonly accepted that human life originated in East Africa. There is less agreement as to whether the humans that left Africa in a final exodus as recently as 100,000 years ago replaced all other hominids (thus becoming ancestors to everyone now alive) or humans evolved independently in geographically separated regions. Recently, the replacement scenario, as it is sometimes called, has been lent support from genetic research.

Genetic investigations into the origins of human life most often focus on mitochondrial DNA. As opposed to nuclear DNA, mitochondrial DNA is transmitted only from the mother. This allows for the tracing of mutations that arise independently of changes that occur because of the combining of the mother’s and father’s DNA. As useful as this is, the high mutation rate of mitochondrial DNA allows scientists a look at only relatively recent prehistory. Nuclear DNA, on the other hand, has a low mutation rate, making it ideal for looking into the more distant past. Studying the nuclear DNA of fossils now shows a substantial decrease in population size in Europe and Asia approximately 50—80 thousand years ago. No such decrease happened in Africa. This supports the idea that migrants from Africa replaced all previous humans, and did not interbreed with earlier migrants.

Other research shows less genetic diversity the farther human populations are located from Africa. This difference in diversity, which continues to the present day, also supports the hypothesis that modern human life came from Africa and gradually spread throughout the world. It is worth noting that there is no discontinuity in the decrease of diversity as one travels from Africa; this points to less distinct ethnic and racial divisions than is popularly thought to be the case.

1. Which of the following is the primary purpose of the passage?
(A) To present statistical evidence that points to a new hypothesis
(B) To illustrate the ways in which new forms of scientific research can bring about surprising results
(C) To offer evidence that supports one of two competing theories
(D) To cast doubt on the reliability of premises that underlie a conclusion
(E) To describe a model that can be applied to a variety of scientific undertakings

2. According to the passage, which of the following is NOT characteristic of mitochondrial DNA?
(A) Transmission comes from the mother only.
(B) It can be used to trace the origins of human life.
(C) It is used more frequently than nuclear DNA to study the origin of human life.
(D) It can be used to investigate distant prehistory.
(E) It has a relatively high mutation rate.

3. Which of the following most strongly argues against the replacement scenario?
(A) Mitochondrial DNA from a human skeleton found in East Africa matches the DNA found in a known route taken by migrants from Africa to Europe.
(B) Remains have been found of humans in Southeast Asia that developed independently of humans in Central America.
(C) The DNA from human specimens taken in China and Spain are nearly identical.
(D) No evidence can be found to support the idea that the first Africans to leave Africa bred with the second group to make the trip.
(E) DNA from a Neanderthal—an early hominid—shows some similarity with modern human DNA.

4. According to the passage, which of the following can be inferred about genetic diversity among humans?
(A) It is easier to detect through the examination of nuclear DNA than through the study of mitochondrial DNA.
(B) It is affected only by an individual’s mother’s DNA.
(C) It has gradually diminished since about 100,000 years ago.
(D) It is not as ambiguous as most people think.
(E) The amount of diversity within an existing population can be predicted by its geographic location.