PrepTest 136

Correct. Argument or Facts:
Facts

Question Type:
Paradox

Stimulus Summary:
Electrical sparks/lightning can make amino acids (life).
Requirement - An atmosphere high in hydrogen and low in oxygen
Current belief - Early Earth's atmosphere was high in oxygen and low in nitrogen

Answer Anticipation:
First off, we know that amino acids—and thus life—did develop on Earth, so there must have been a "reducing" atmosphere somehow.

Second, the question stem tells us to assume the current belief about Earth's atmosphere when life began didn't have a "reducing"atmosphere—oxygen is low in one, whereas oxygen was high in early Earth's atmosphere.

However, a "reducing" atmosphere is defined also as being high in hydrogen, but the current belief notes that the atmosphere was low in nitrogen. That's a new piece of information that we don't have much context for.

Whatever the science, we know that a "reducing" atmosphere is necessary for life to form from lightning, so the correct answer should highlight that Earth, despite the high oxygen/low nitrogen atmosphere, somehow had a "reducing" atmosphere. It may relate to that nitrogen fact.

Answer Explanation:
Not related to the nitrogen at all, but this answer does establish that, at least at times and in certain places, there was a "reducing" atmosphere on Earth. That explains how the current belief could be correct and Earth's overall atmosphere at the time wasn't "reducing," but there were localized "reduced" atmospheres where life could have formed.

Key Takeaway:
Make sure to read the question stem and clearly know what it's asking you to do. Here, tying the explanation to lightning provided a lot of guidance as to what the correct answer would need to do, and missing that would have made several other answers look tempting.

Incorrect. This answer feels right because the stimulus notes that a "non-reducing" atmosphere has amino acids not forming easily and "tend[ing]" to break apart, but if only one is needed, maybe one could make it through. However, this answer is ultimately too weak to serve as an explanation—it's possible that a single amino acid "could" result in life on Earth . . . only in a "reducing" atmosphere and not in the atmosphere we're told to accept Earth had. By not tying this to that atmosphere, this answer doesn't serve as an explanation.

Incorrect. We're told to accept the current belief about the composition of Earth's atmosphere at the time life formed—that's not based on the current atmosphere of Earth. As such, what happened after life formed is out of scope here.

Incorrect. Even if less common, it still could have been pretty common—after all, lightning isn't exactly unheard of today.

Incorrect. This provides an alternative explanation for how amino acids ended up on Earth, so it doesn't address the question stem which tasks us with explaining how lightning could have resulted in life in a "non-reducing atmosphere."