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I found the following sentence on one of videos that I watched. What I cannot understand is the meaning of the "could have been crucial". The main reason for this is I thought the only place where we can use "could have" is to talk about some hypothetical past events that didn't happen. But, I cannot find such circumstances in the following sentence. Furthermore, the evolution is not a past event. I mean, it has been occurring since life began on the Earth. Therefore, please someone tell me why they have such a construction to talk about it.


Scientists believe these tides could have been crucial for the evolution of animal life on Earth by offering a route from under the sea to above it.

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dileepaThe main reason for this is I thought the only place where we can use "could have" is to talk about some hypothetical past events that didn't happen.

No, this isn't right. We can use "could have ~" when we think a past event is possible/likely, but we don't know for sure that it happened. It is similar to "might have ~".

dileepaFurthermore, the evolution is not a past event. I mean, it has been occurring since life began on the Earth.

The evolutionary events that they are concerned with in this sentence happened in the past.

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They meant that the tides "might" have been crucial. People use "could" this way when they think "might" sounds too doubtful. And they might have been crucial for that phase of evolution, when animals started to appear on dry land.

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 GPY's reply was promoted to an answer.

Thank you very much for the answer.