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, even if bacteria did arrive on Earth from Mars, they must have died out.

The argument is most vulnerable to which of the following criticisms?

It fails to establish whether bacteria actually developed on Mars.
It fails to establish how likely it is that Martian bacteria were transported to Earth.
It fails to consider whether there were means other than meteorites by which Martian bacteria could have been carried to Earth.
It fails to consider whether all bacteria now on Earth could have arisen from transported Martian bacteria.
It fails to consider whether there could have been strains of bacteria that originated on Earth and later died out.

Please explain in details...i had a bouncer on this one.....

Is the Answer "D"

Is it E?

The argument states that all bacteria found on Earth are from the "same planet"
> two bacterial strains on Earth are different enough to have arisen on different planets

The author then concludes that the "planet" is Earth.
He does not eliminate the possibility that all the bacteria on Earth is that from Mars.

Hence D.

perfect.

to the original poster: what other answer choices were you seriously considering?

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Whats wrong with E ?

(e) is irrelevant to the argument. the argument is only concerned with whether there could be bacteria of martian origin on earth.

the argument claims that, if there were martian bacteria on earth, then those bacteria must be gone by now.

whether there are bacteria originally from earth that have also disappeared is beside the question; it has no effect on the argument at hand.

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Hi Ron;
Sorry for reopening this thread.

For options E, can't we infer that "because the earth's bacteria has died down and so Mars's bacteria is the only one which is surviving"

by writing the words that i put in boldface here, you have actually written a flawless justification of the correct answer choice (d).

choice (e) doesn't accomplish this reasoning, because it just says that some earth-based bacterial forms have died out. there's no reason to take the statement to such an extreme -- especially when choice (d) is already an exact statement of that precise extreme.

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Hi

E is wrong because we need to weaken the argument that the bacteria from mars has died out.

Pointing that bacteria from earth has died out will be out of scope as we still does not know that single strain of bacteria is from earth.
We just know that there are single strains of bacteria and concluding that they are not from mars.

So saying that all bacteria are from mars will most weaken the argument. Hence answer is D

Saying that bacteria from earth has died out will not negate the conclusion.

pretty much, yes.

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Hi Ron,

One thing is not clear to me in the passage.

Is the author assuming that " earth already has strains of bacteria developed in the earth itself" or that "strains of bacteria came from some other planet other than mars" ?

The argument states:
No two bacterial strains on Earth are different enough to have arisen on different planets.

If you think about the consequences of this statement, you'll have the answer to your question.

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what i can infer from the aforementioned italicized statement is that all bacteria strains on the earth are similar and hence these bacteria strains have not arisen on different planets (as we know different planets will have strains with different protein structures).

does that mean bacteria strains on earth have arisen on earth itself?

Correct up to here.
In simpler terms ("explain it to a 10-year-old""”the most valuable thing you can do with any CR passage), we know that they're all from the same planet.

The issue, though, is that we don't know WHICH planet. That's the only reason this argument is subject to "critical reasoning" in the first place"”the only doubt is whether they might all be from a different planet.

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