As Muslims we believe our human origin goes back to God’s creation of Adam and Eve, the first the humans on earth.
But how did Adam and Even (upon whom be peace) pro-create?
How did the first generation of mankind start?
Did their children sleep with each other, and that led to the next human generation?
Isn’t that incest? Would it mean we come from a lineage of illegitimate offspring?
Our methodology of how to understand all of this and get satisfying answers to our queries cannot be via logical deductions or rational equations, but only through what has reached us from the holy Quran and authentic narrations.
A fundamental point that needs to be mentioned here is that Divine Legislation changed and evolved throughout time. Something that was Halal during one era might have changed to become Haram in another era, and this evolved until the completion of God’s Shari’ah with the advent of Islam. An example for this is in previous Shari’ahs a man can marry two sisters and have them both at one time, whereas in Islam this is impermissible and invalid.
Marriage is a matter of legislative concern, and not something rational or logical. It is something that could have changed or evolved due to certain circumstances or conditions.
We understand that with Adam and Eve being a couple, they had sons and daughters, which is obvious, but what about the next generation after these children.
We know that Adam and Eve had twins: A male whom they called “Cain” and a female by the name of “Iqlimah.” They then had a second pair of twins: A male “Abel” and a female “Liyudha.”
They had another child called Seth. Our belief as Shi’ah Muslims is that the lineage of mankind returns back to Seth, and not Cain or Abel. However, as far as the children of Adam’s marriage, here are some theories:
It could be that at the beginning of creation for the necessary purpose of bring about an offspring siblings had to pro-create, and with the growth of society the legislation changed. It would not have been dangerous then because there would not be any problems of mutant genes or problems like that.
This view is mainly accepted by Sunni scholars. Allamah Tabataba’i also sees this view as not that far-fetched.
Another view is that Almighty God did not create the offspring of Prophets from haram, but rather He created Houries, and the males from Adam’s children married these Houries. Almighty God also created Houries for Adam’s daughters, and they got married. These marriages produced the generations after them. Another view relevant to this one son [Abel] married a Hourie and another son [Cain] married a Jinn.
As God’s initiation creation of Adam and Eve was miraculous, and as Bilqis bringing Sheba’s throne as quick as the glimpse of an eye is beyond our rational comprehension, bringing down Houries is certainly within the capacity of God.
Imam al-Sadiq (a.s.) has said in reply to the view of siblings marrying one another:
سبحان الله عن ذلك علوّاً كبيراً ، يقول من يقول هذا: إنّ الله تعالى جعل أصل صفوة خلقه وأحبائه وأنبيائه ورسله وحججه والمؤمنين والمؤمنات والمسلمين والمسلمات من حرام !! ولم يكن له من القدرة ما يخلقهم من الحلال ، وقد أخذ ميثاقهم على الحلال والطهر الطاهر الطيب؟!
God Almighty is greater than to say such things about Him. Has He created prophets, apostles, believing men and women and Muslim men and women from haram!! Did He not have enough power to create them through halal means, and He has taken a covenant from man for purity and cleanness from sins.
The first view is problematic because marrying a sibling is something naturally repulsive and detrimentally damaging in reality, not only as far as legislative law.
A Third View:
Imam Ali al-Ridha (a.s.) was asked about how Adam’s children pro-created and his reply was that Eve bore the twins in seperate wombs, and the second twins in seperate wombs, and they married each other, and then after this it became prohibited.
A Fourth View:
Adam’s children married humans from other races, seeing that Adam was not the first human on earth, as pointed out by certain hadiths and scientists have also mentioned how humans could have lived on earth for millions of years.
In any case this is among the issues that hold various views and theories. And God knows best.
 al-Mizan fi Tafsir al-Quran, vol. 4, p. 277
 Surah al-Baqarah, 259
 Bihar al-Anwar, vol. 11, p. 223
 Bihar al-Anwar, vol. 11, p. 226
 Tafsir Nemuneh, vol. 3, p. 247
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