Unlike the other six continents, Antarctica has no native population.
Antarctica, which is not like the other six continents, has no native population.
Unlike the other six continents, Antarctica has no native population.
Antarctica, which is not like the other six continents, has no native population.
The correct idiom is "Unlike A, B.", in which A and B must be grammatically and logically parallel. The phrasing "which is not like A" is needlessly wordy and awkward, and it doesn't quite capture the desired logical meaning; this latter form would never be correct on the GMAT.