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?

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.

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.