When an action happens (and is completed) in the past BEFORE another event in the past, we typically need to use the PAST PERFECT ("had" + participle). HOWEVER, if the If the order of the past events is obvious, then we don't necessarily need to use past perfect tense.
Consider this example: Gary baked a cake and served it to his family.

Here, we have two actions that occurred in the past, and we can tell from the context that the baking occurred before the serving. So, it would seem that we should use the past perfect tense and write: Gary had baked a cake and served it to his family.

Now it isn't grammatically incorrect to write Gary had baked a cake and served it to his family, but it isn't necessary to do so since we can tell from the context that the baking must have occurred before the serving.

In your example, we know that one past event occurred recently, and the other event occurred during the last ice age. So, the order of the actions seems clear, which means we need not use the past perfect tense.

NOTE: there's no perfect rule for determining whether or not the order of certain actions is 100% obvious, so it is highly unlikely that you would ever need to determine this on a test.

What you do need to know is that, if an action is expressed in the past perfect tense form, then that action must have been completed before some other action was completed in the past.

Aside: If you're interested, we have a free video on all 3 perfect tenses: https://www.gmatprepnow.com/module/gmat- ... on?id=1178


Cheers,
Brent