I believe that specifically the civilization of the ancient giants did not exist. At the moment, there is not enough material evidence to classify giants to the concept of civilization. But a certain group of gigantic people still existed, and where it came from remains questionable.
I have some suggestions about the involvement of the giants in the demigods. Now I will explain a little about this moment.
If in ancient times on the Earth there was at first a highly developed civilization of the gods, then after it came the period of the demigods. The demigods were either the result of merging the gods with ordinary people, or the results of more successful experiments.
Ultimately, demigods differed from ordinary people in one way or another. These parameters include weight and height.
The demigods could have a height of 3-5 meters and have a real heroic strength, proving their belonging to the caste of gods. Since there were few demigods, and gigantism was only one of the parameters, this can explain the small number of the found material base.
I consider it a mistake, the prevailing modern concept of giants. They allegedly pulled huge blocks and built megaliths. The giants were demigods, and they in turn ruled over people. Perhaps at that time they had technology or their remnants from the ancient gods.
Thus, the giants did not work with their hands, but only led people, along the way using the remnants of fantastic technologies.
Although it can be assumed that the ancient gods wanted to make themselves strong workers, and could try to bring out a certain group of giant people.
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With our knowledge about ancient history, it remains only to guess, because if someone knows the truth, then he is in no hurry to share with society.