Quran is a religious book revealed for the guidance of human kind, and has covered various topics like Historical facts and incidents, laws, commandments, stories of prophets and their struggles to implement and spread the message of truth.

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Quran is not a book of science, it’s a book of SIGNS..but has many verses related to science on various topics.

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As mentioned above, Quran was revealed for the guidance of mankind, and it cannot be revealed in the beginning as there was no human population.

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First man on the earth was Prophet Adam (A.S- peace be upon him), and first woman was Hawwa (A.S- peace be upon her)…and there was no population and issues, where guidance and law was required.

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Also they did not have any history or childhood on the earth.

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As and when population started growing, and people started facing issues, God kept on sending Prophets and messengers like Noah, Abraham, Moses, Jesus and Muhammad (peace be upon them all),  revealing his guidance through messages via angel Gabriel.

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God revealed 3 major messages and commandments in the form of books before Quran, which where Zaboor, Toha and Injeel.

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Quran was the last book revealed with completion of laws, and required messages, and commandments for coming generations to follow till world end, and no changes can be done in the book..

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Hence, Quran was not revealed in the beginning of time due to above mentioned reasons related to historical incidents, population,  issues faced by people.

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Allah knows the best.