I came across a sentence which has confused me because it is written in simple past instead of past perfect.
The sentence is:
In the centuries after the group separated, they evolved in two separate directions.
Shouldn't 'had separated' be used instead of 'separated' here?
When events are given in chronological order, the perfect aspect is not necessarily needed. Therefore, this would be correct:
After the group separated, they evolved in two separate directions.
That might have been why the author wrote the sentence as he or she did. However, the actual sentence begins with "in the centuries". Those centuries were after the group "separated", so those two "events" (if we can consider centuries to be events) are not in chronological order. Therefore, "had separated" is, indeed, justifiable.
Keep in mind that English-speakers can be fairly loose with verb tenses in casual speech and writing, so some people would not mind the use of the past tense here.
As usual, the choice between a simple past and a past perfect is nothing to do with the objective facts, but to do with the temporal focus.
If you are choosing to focus on the later time when the groups evolve, and to look backwards from them to the split, it is natural to use the past perfect had separated.
If you have no particular temporal focus, and you are presenting the events in the order they happened, you can use the simple past separated.
There's also the point, as Marcin mentioned, that English speakers don't always use the past perfect even when they could, if the temporal relationships are clear.
