In the name of Allah, the Gracious, the Merciful
All people alive today have descended from Adam and Eve, the first two human beings to exist. This fact is important because it obliterates racism and forges a link between any two human beings, that on the most basic level all people are related and deserve fundamental human rights and dignity.
But the story also presents a theological question about the procreation of the first humans. How did the children of Adam and Eve procreate the species if not by mating with each other? Was this incest as is typically understood today?
We do not know exactly how human procreation began because the only sources to discuss it are Jewish and Christian traditions. Nothing is authentically narrated from the Prophet (s) about it, so we can accept the people of the Book’s explanation or leave the matter to Allah in the Unseen (perhaps the procreation, like their initial creation, was miraculous and extraordinary in ways we do not know).
Nevertheless, the Bible has been interpreted to suggest genetic separation was intended to be a general rule for humanity.
It is written in the Genesis:
כד על כן יעזב איש את אביו ואת אמו ודבק באשתו והיו לבשר אחד
That is why a man leaves his father and mother and is united to his wife, and they become one flesh.
Source: Genesis 2:24
Incestual relationships (that is, sexual relations within immediate family members) was then forbidden by the revelation given to the Prophet Moses (s).
It is written in the Torah:
יז ואיש אשר יקח את אחתו בת אביו או בת אמו וראה את ערותה והיא תראה את ערותו חסד הוא–ונכרתו לעיני בני עמם ערות אחתו גלה עונו ישא
If a man marries his sister, the daughter of either his father or his mother, and they have sexual relations, it is a disgrace. They are to be publicly removed from their people. He has dishonored his sister and will be held responsible.
Source: Leviticus 20:17
The Quran likewise upheld the prohibition of incestual relationships in a detailed manner, although acknowledging Allah’s forgiveness for previous unintended mistakes.
حُرِّمَتْ عَلَيْكُمْ أُمَّهَاتُكُمْ وَبَنَاتُكُمْ وَأَخَوَاتُكُمْ وَعَمَّاتُكُمْ وَخَالَاتُكُمْ وَبَنَاتُ الْأَخِ وَبَنَاتُ الْأُخْتِ وَأُمَّهَاتُكُمُ اللَّاتِي أَرْضَعْنَكُمْ وَأَخَوَاتُكُم مِّنَ الرَّضَاعَةِ وَأُمَّهَاتُ نِسَائِكُمْ وَرَبَائِبُكُمُ اللَّاتِي فِي حُجُورِكُم مِّن نِّسَائِكُمُ اللَّاتِي دَخَلْتُم بِهِنَّ فَإِن لَّمْ تَكُونُوا دَخَلْتُم بِهِنَّ فَلَا جُنَاحَ عَلَيْكُمْ وَحَلَائِلُ أَبْنَائِكُمُ الَّذِينَ مِنْ أَصْلَابِكُمْ وَأَن تَجْمَعُوا بَيْنَ الْأُخْتَيْنِ إِلَّا مَا قَدْ سَلَفَ ۗ إِنَّ اللَّهَ كَانَ غَفُورًا رَّحِيمًا
Prohibited to you (in marriage) are your mothers, your daughters, your sisters, your father’s sisters, your mother’s sisters, your brother’s daughters, your sister’s daughters, your nursing-mothers who nursed you, your sisters through nursing, your wives’ mothers, and your step-daughters under your guardianship born of your wives unto whom you have gone in. If you have not gone in unto them, there is no sin upon you. And (also prohibited are) the wives of your sons who are from your own loins, and that you take in marriage two sisters simultaneously, except for what has already occurred. Verily, Allah is forgiving and merciful.
Surat al-Nisa 4:23
The underlying wisdom for this prohibition, as we now understand, is because genetic mutations and defects are more likely to be passed down through incestual relationships, or ‘inbreeding,’ which then impairs individuals and leads to all kinds of harmful outcomes. For this very good reason, there is also a cross-cultural social taboo on incest.
However, can we really say that the first human beings to exist committed incest in the same way someone does so today? This time period is, after all, before any civilization ever existed and there was no risk of genetic mutations and defects in the first generation.
That being the case, a popular explanation of human procreation, which is sourced in Jewish and Christian traditions, was circulated by the early Muslims as a sufficient answer to the question.
Al-Suddi reports that a number of the Prophet’s (s) companions said:
كَانَ لَا يُولَدُ لِآدَمَ مَوْلُودٌ إِلَّا وُلِدَ مَعَهُ جَارِيَةٌ فَكَانَ يُزَوِّجُ غُلَامَ هَذَا الْبَطْنِ جَارِيَةَ هَذَا الْبَطْنِ الْآخَرِ وَيُزَوِّجُ جَارِيَةَ هَذَا الْبَطْنِ غُلَامَ هَذَا الْبَطْنِ الْآخَرِ
Adam would not beget a boy but that he was accompanied by a twin girl. He would marry the boy from this pregnancy with the girl from another pregnancy, and he would marry the girl from this pregnancy with the boy from another pregnancy.
Source: Tafsīr al-Ṭabarī 33:72
We can see the marriages of Adam’s descendants were kept as far apart from each other as possible, foreshadowing the principle of genetic separation and the eventual prohibition of incest. In this view, the marriage of siblings from different pregnancies was not incest in the formal way it is defined today. Instead it was a temporary solution that did not involve the harms usually associated with incest.
Does that mean Muslims are allowed to marry within their immediate families today? Of course it does not, because as was mentioned, the prohibition of incest was already anticipated in the time of Adam and was later decisively forbidden by multiple divine revelations.
Herein lies the wisdom of divine abrogation, that certain laws specific to a time period can be repealed by a genuine revelation from Allah when they are no longer applicable.
مَا نَنسَخْ مِنْ آيَةٍ أَوْ نُنسِهَا نَأْتِ بِخَيْرٍ مِّنْهَا أَوْ مِثْلِهَا ۗ أَلَمْ تَعْلَمْ أَنَّ اللَّهَ عَلَىٰ كُلِّ شَيْءٍ قَدِيرٌ
We do not abrogate a verse or cause it to be forgotten but that We bring forth one better than it or like it. Do you not know that Allah has power over all things?
Surat al-Baqarah 2:106
As such, the Prophets always carried the same basic message of faith, monotheism, and charity, but the laws they applied differed according to their circumstances.
Qatadah, may Allah have mercy on him, said:
الدين واحد والشريعة مختلفة
The religion is one but the laws of the prophets are different.
Source: Tafsīr al-Ṭabarī 5:48
In sum, according to the prevalent opinion, the siblings of Adam and Eve from different pregnancies married each other as a means to propagate the beginnings of the human race. This view is not necessarily confirmed by any authentic Islamic text, though, and in a sense it is speculation. It is entirely possible that the procreation of the species occurred in a form known only to Allah, and therefore we may leave the matter to the knowledge of Allah in the Unseen.
Success comes from Allah, and Allah knows best.