It is theoretically possible that bacteria developed on Mars early in its history and that some were carried to Earth...

GMAT Critical Reasoning : (CR) Questions

Source: Official Guide

Critical Reasoning

Logically Completes

MEDIUM

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Notes

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It is theoretically possible that bacteria developed on Mars early in its history and that some were carried to Earth by a meteorite. However, strains of bacteria from different planets would probably have substantial differences in protein structure that would persist over time, and no two bacterial strains on Earth are different enough to have arisen on different planets. So ______.

What would be the best option to complete the passage?

Whether or not bacteria actually developed on Mars.

Its likely it is that Martian bacteria were transported from Earth.

Martian bacteria could have been carried to Earth by means other than meteorites.

All bacteria now on Earth could have arisen from transported Martian bacteria.

There could have been strains of bacteria that originated on Earth and later died out.

Solution

Passage Analysis:

Text from Passage	Analysis
It is theoretically possible that bacteria developed on Mars early in its history and that some were carried to Earth by a meteorite.	What it says: Bacteria might have originally formed on Mars and traveled to Earth via meteorites What it does: Sets up a hypothesis about bacteria's possible origin story What it is: Author acknowledging a scientific possibility Visualization: \(\mathrm{Mars} \rightarrow \mathrm{meteorite\ carrying\ bacteria} \rightarrow \mathrm{Earth}\)
However, strains of bacteria from different planets would probably have substantial differences in protein structure that would persist over time	What it says: Bacteria from different planets should have major protein differences that don't disappear over time What it does: Introduces a key scientific principle that challenges the Mars hypothesis What it is: Author's reasoning based on biological expectations Visualization: \(\mathrm{Mars\ bacteria\ proteins} \neq \mathrm{Earth\ bacteria\ proteins}\) (differences remain constant over millions of years)
and no two bacterial strains on Earth are different enough to have arisen on different planets	What it says: All Earth bacteria are too similar to each other to have come from different planets What it does: Provides the crucial evidence that contradicts the Mars origin theory What it is: Author's factual observation about Earth's bacteria Visualization: \(\mathrm{Earth\ Bacteria\ A\ vs\ Earth\ Bacteria\ B} = \mathrm{small\ differences}\) (not Mars-level differences)

Argument Flow:

The argument starts by acknowledging a popular scientific theory (Mars bacteria traveling to Earth), then systematically dismantles it using two key pieces of evidence: what we'd expect to see if the theory were true versus what we actually observe on Earth.

Main Conclusion:

Bacteria on Earth did not originate from Mars (this is what should complete the blank based on the evidence presented).

Logical Structure:

This follows a 'hypothesis elimination' structure: If bacteria came from Mars, then we should see major protein differences between Earth bacteria strains. But we don't see those differences. Therefore, bacteria didn't come from Mars.

Prethinking:

Question type:

Logically Completes - We need to find a conclusion that naturally follows from the evidence presented in the argument

Precision of Claims

The argument makes specific claims about protein structure differences between planetary bacteria (substantial and persistent) and similarity levels of Earth bacteria (not different enough to suggest different planetary origins)

Strategy

Since this is a logically completes question, we need to identify what conclusion the author is building toward. The argument starts with a hypothesis (bacteria came from Mars), then provides a test for this hypothesis (different planets should create substantially different protein structures), and finally gives us the evidence (Earth bacteria aren't different enough). We need to complete the logical chain by stating what this evidence tells us about the original Mars hypothesis.

Answer Choices Explained

Whether or not bacteria actually developed on Mars.

'Whether or not bacteria actually developed on Mars' - This is asking a question rather than providing a conclusion. The argument gives us evidence and reasoning that should lead to a definitive conclusion, not leave us with an open question. This doesn't complete the logical flow of the argument.

Its likely it is that Martian bacteria were transported from Earth.

'Its likely it is that Martian bacteria were transported from Earth' - This suggests the opposite direction of transport (Earth to Mars rather than Mars to Earth), but the argument doesn't provide any evidence or reasoning about bacteria traveling from Earth to Mars. This introduces a completely new idea not supported by the premises.

Martian bacteria could have been carried to Earth by means other than meteorites.

'Martian bacteria could have been carried to Earth by means other than meteorites' - This misses the point entirely. The argument isn't about the method of transport (meteorites vs. other means), but about whether bacteria from Mars came to Earth at all. The evidence about protein similarities contradicts any Mars-to-Earth transfer, regardless of the transport method.

All bacteria now on Earth could have arisen from transported Martian bacteria.

'All bacteria now on Earth could have arisen from transported Martian bacteria' - This perfectly completes the logical chain. If bacteria from different planets should have substantial protein differences, but all Earth bacteria are similar enough to have come from the same planet, then they all could have originated from Mars and been transported together. This reconciles the evidence with a modified version of the original hypothesis.

There could have been strains of bacteria that originated on Earth and later died out.

'There could have been strains of bacteria that originated on Earth and later died out' - This introduces speculation about extinct Earth bacteria that has no connection to the argument's focus on Mars-Earth bacterial transport and protein structure differences. The argument doesn't provide any evidence or reasoning about extinction patterns.

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