My atheist friends ask me that if Adam and Eve were the first people on earth, and their sons were Able and Cane, who did they marry and how did their offspring come about?

When Allah Ta?ala sent Aadam and Hawa (Alayhis salaam) to the earth, the progeny of man began.

Haawa (Alayhis salaam) became pregnant twice with twins, one boy and one girl. Therefore, the two boys of both pregnancies married the alternative sisters. (Ma?ariful Qur?aan vol.3 pg.112; Darul Kitaab)

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai