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Difficulty: MediumCentral Ideas

For decades, biologists assumed that the deep subsurface of the Earth was entirely sterile due to extreme heat, pressure, and the absolute absence of sunlight. However, the discovery of diverse microbial communities thriving miles beneath the seafloor has overturned this assumption. These subterranean organisms, often referred to as "intraterrestrials," do not rely on photosynthesis; instead, they harvest energy from inorganic chemical reactions, such as the oxidation of hydrogen or sulfur compounds. This finding has not only expanded our understanding of the environmental limits of life on Earth but has also broadened the search for extraterrestrial life, suggesting that biosphere-sustaining conditions might exist beneath the surfaces of otherwise inhospitable planets.

Which choice best states the main idea of the text?

  1. A
    Geothermal and chemical environments are far more suitable for sustaining complex ecosystems than solar-dependent environments are.
  2. B
    Deep-subsurface microbes have adapted to extreme heat and pressure by developing highly efficient mechanisms for photosynthesis.
  3. The discovery of deep-subsurface microbes that thrive on inorganic chemical energy has expanded the known boundaries of life on Earth and guided the search for life elsewhere.Answer
  4. D
    Extraterrestrial life will likely be found in environments that closely replicate the chemical composition of Earth's seafloor.

Answer

The correct answer is the option stating that the discovery of deep-subsurface microbes thriving on inorganic chemical energy has expanded the known boundaries of life on Earth and guided the search for life elsewhere.
The correct answer accurately summarizes the main idea of the passage. The text outlines how finding microbial life deep underground that relies on chemical energy rather than solar energy has changed our understanding of life's environmental limits on Earth and expanded the possibilities for finding life on other planets.

Step-by-Step Solution

1
Analyze the passage to identify its focus and argument flow.
The passage shifts from a past assumption (the deep subsurface is sterile) to a new discovery (microbes thrive there using chemical energy, not photosynthesis) and details the implications of this discovery (expanded limits of life on Earth, broadened search for extraterrestrial life).
Understanding the structure of the passage helps isolate the central argument from supporting details.
2
Summarize the central idea in your own words.
The discovery of deep-subsurface life relying on chemical energy has redefined where we believe life can exist on Earth and other planets.
Formulating a pre-selected summary prevents falling for common distractor traps.
3
Evaluate the answer choices against your summary and the text, eliminating options that make unsupported claims, misread details, or generalize too broadly.
The option explaining that these microbes expanded the boundaries of life and guided the extraterrestrial search matches the passage, while the others are either too broad, misrepresent details, or speculate excessively.
This step ensures that the selected choice is supported by the entire passage rather than a single detail or external assumption.

Key Concept

Identifying the central idea of a passage requires synthesizing the main point of the entire text and distinguishing it from secondary details or unsupported generalizations.
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Central Ideas

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