托福阅读真题+题目+答案：When Did Life on Earth Originate?

托福阅读真题+题目+答案：When Did Life on Earth Originate?

The oldest true fossils are finely layered structures known as stromatolites. These objects come in a variety of shapes and forms, from simple cones to large, branching columns. They are the predominant fossil type in all sedimentary rocks from just over 3.4 billion years ago until about 600 million years ago. Stromatolites are still forming today, although only in a few localities. Close examination of these modern analogs of the ancient fossils has provided crucial insight into how and where they grow. The intricately layered structures are made up of thin mats of microbes—colonies of single-celled bacteria—that act as traps for sediment grains and gradually build up mounds or domes or columns. They grow in warm, shallow ocean water along the edges of continents, either completely submerged throughout their lives or partly emergent during low tides.

1. The word "predominant" in the passage is closest in meaning to

A. only

B. most common

C. oldest

D. most diverse

2.Paragraph 1 suggests that which of the following is assumed about the way ancient stromatolites grew?

A. They grew where sediment collected in deep ocean environments

B. They grew in a manner very similar to modern stromatolites.

C. They grew in fewer types of environments than they do today.

D. They grew wherever bacteria formed.

3. The word "diverse" in the passage is closest in meaning to

A. varied

B. important

C. primitive

D. complex

At least seven different varieties of the 3 4-billion-year-old fossil stromatolites have been identified. The local distribution of these varieties and the types of rocks they are associated with indicate that they constituted a diverse ecosystem along an ancient shoreline that had recently been submerged due to a rising sea level, with different stromatolite types occupying slightly different environments. This paints a picture of an Earth on which life was already abundant by 3.4 billion years ago, and perhaps one on which—if the modern stromatolites are a reliable guide—bacteria were already consuming carbon dioxide and producing oxygen through photosynthesis. The abundance and diversity of the early stromatolites also suggest that life arose long before 3.4 billion years ago, possibly during the Hadean era (4.5 to 3.8 billion years ago).

4. According to paragraph 2, which of the following is NOT true of the oldest existing fossil stromatolites?

A. There were seven or more different types.

B. Different varieties existed in somewhat different environments.

C. They formed along a shoreline that had recently risen out of the ocean.

D. They were plentiful in the locations where they formed.

5. According to paragraph 2, what do the fossil stromatolites suggest about the emergence of life on Earth?

A. The first living organisms were bacteria that consumed oxygen.

B. Life emerged in one specific location on our planet.

C. Life had emerged before the Hadean era.

D. Living organisms had existed much earlier than 3.4 billion years ago.

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