Ok, first think, have we changed our belief of how first Americans entered the U.S continent. If we have then we can say were. However, if we still hold the same belief, that they entered via the bering straight, then we have to use "are" the present tense, and not "were", which is in the past tense. In other words we continue to believe the historical fact that took place millions of years ago.

Thanks for the explanation. Its helpful
However I think the GMAC should avoid this type of question because there is a very little chance for any international student to know what the the historical belief of any American event. Suppose if I ask about the concept of bridge Ram setu which connects India with Gulf area in history that would be very hard for any international student to know whetther Indians still believe it or not.

It is possible that Native Americans originally have migrated to the Western Hemisphere over a bridge of land that once existed between Siberia and Alaska.
(A) have migrated to the Western Hemisphere over a bridge of land that once existed
(B) were migrating to the Western Hemisphere over a bridge of land that existed once
(C) migrated over a bridge of land to the Western Hemisphere that once existed
(D) migrated to the Western Hemisphere over a bridge of land that once existed
(E) were migrating to the Western Hemisphere over a bridge of land existing once

Please help me to know what we should use now in comparison to last sentence