This is a good question! "During" works, but doesn't quite mean the same thing. "Over the course" is a common phrase that means that something happened gradually as the time passed by. To use a different phrase, you might need to change the order of the sentence. For example: "Over the course of millions of years, the mountains eroded." could be rewritten as "It took millions of years for the mountains to erode." or "Millions of years were required for the mountains to erode." It is difficult to find a way to get the same meaning. "Over the course" is the most precise phrase. I hope this helps!

What does "course" mean in the following phrase: "Over the course of millions of years"? It means "period" Can we use "during the millions of years.? No, but: You could say: "...during millions of years..." Both of these expressions are used to describe progressive change/ evolution/ development through time. (The time period could be either short or long.) The difference between them is that "over the course..." emphasises the progressive nature of the change that is being described, more than "during", which is focused more on the period of time as a unit. - During the 19th century, there was a rapid development of industrial processes in the UK. - Over the course of the 20th century, China experienced a number of far-reaching changes to its system of governance and the organisation of its society. - During lunch, she told me all about her upcoming divorce. - Over the course of lunch, she recounted the long sorry story of her many failed relationships. Can you see a slight difference?

Awesome, Thank you very much. Now, I am crystal clear about this phrase.