Praise be to Allah.

Firstly: 

What is stated by the Holy Qur’an about the story of the beginning of mankind’s creation is as follows: 

1. Allah, may He be exalted, initially created man from clay, namely Adam, the father of mankind (peace be upon him).

Allah, may He be exalted, says (interpretation of the meaning):

“Who made everything He has created good, and He began the creation of man from clay”

[as-Sajdah 32:7]. 

2. After Allah, may He be glorified and exalted, created the first man – namely Adam (peace be upon him) – He created from him his wife, then He created all humans from them.

Allah, may He be exalted, says (interpretation of the meaning):

“O mankind! Be dutiful to your Lord, Who created you from a single person (Adam), and from him (Adam) He created his wife (Hawwa (Eve)), and from them both He created many men and women and fear Allah through Whom you demand your mutual (rights), and (do not cut the relations of) the wombs (kinship). Surely, Allah is Ever an AllWatcher over you”

[an-Nisa’ 4:1]

“O mankind! We have created you from a male and a female, and made you into nations and tribes, that you may know one another. Verily, the most honourable of you with Allah is that (believer) who has At-Taqwa (i.e. one of the Muttaqoon (pious). Verily, Allah is All-Knowing, All-Aware”

[al-Hujuraat 49:13]. 

3. The Holy Qur’an states that Adam (peace be upon him) was created from clay, but his progeny are created by means of reproduction through water (semen).

Allah, may He be exalted, says (interpretation of the meaning):

“And indeed We created man (Adam) out of an extract of clay (water and earth).

Thereafter We made him (the offspring of Adam) as a Nutfah (mixed drops of the male and female sexual discharge) (and lodged it) in a safe lodging (womb of the woman).

Then We made the Nutfah into a clot (a piece of thick coagulated blood), then We made the clot into a little lump of flesh, then We made out of that little lump of flesh bones, then We clothed the bones with flesh, and then We brought it forth as another creation. So blessed be Allah, the Best of creators”

[al-Mu’minoon 23:12-14]

“Who made everything He has created good, and He began the creation of man from clay.

Then He made his offspring from semen of worthless water (male and female sexual discharge).

Then He fashioned him in due proportion, and breathed into him the soul (created by Allah for that person), and He gave you hearing (ears), sight (eyes) and hearts. Little is the thanks you give!”

[as-Sajdah 32:7-9]. 

With regard to the way in which the children of Adam (peace be upon him) were married and how they produced offspring: 

There is nothing in the Holy Qur’an or saheeh Prophetic Sunnah to explain that; rather it was attributed to some of the Sahaabah – and it may be that it was taken from the books of the People of the Book – that in every pregnancy, Adam’s wife would bear him one male and one female, and a male born from one pregnancy would marry a female born from another pregnancy, and in this manner the number of his descendants increased. 

Ibn Jareer at-Tabari (may Allah have mercy on him) narrated, with his isnaad from as-Suddi: 

It was narrated from as-Suddi, in his narration from Abu Maalik and from Abu Saalih from Ibn ‘Abbaas, and from Murrah from Ibn Mas‘ood, and from some of the companions of the Prophet (blessings and peace of Allah be upon him): No son was born to Adam but a daughter would be born with him, and the boy from this pregnancy would marry the girl from another pregnancy, and the girl from this pregnancy would marry a boy from another pregnancy… End quote. Tafseer at-Tabari (8/322). 

This view is the one that is well known among the scholars of Islam. Al-Haafiz Ibn Hajar (may Allah have mercy on him) said: 

As-Suddi mentioned in his Tafseer, narrating from his shaykhs with his isnaads, that the reason why Qaabeel killed his brother Haabeel was that Adam used to marry the male from each pregnancy that bore him children to the female of another pregnancy, and that the sister of Qaabeel was more beautiful than the sister of Haabeel. Qaabeel wanted to keep his sister and marry her himself, but Adam did not let him, so when he insisted, he instructed them both to make an offering. Qaabeel offered a sheaf of corn, because he was a grower of crops, and Habeel offered a fat lamb, because he kept livestock. Fire came down (from heaven) and consumed the offering of Habeel, but not that of Qaabeel [i.e., Haabeel’s offering was accepted and Qaabeel’s was not], and that was the cause of the trouble between them. This is the well-known view.

End quote from Fath al-Baari (6/369) 

This view – even though we have suggested that its only source was the books of the People of the Book – is not objectionable to the Muslim mind, and there is no text in our religion to say that it is wrong. There is no valid reason that prompts us to reject it or prevents us from narrating it. It was narrated from ‘Abdullah ibn ‘Amr that the Prophet (blessings and peace of Allah be upon him) said: “Convey from me, even if it is only one verse. Narrate from the Children of Israel and there is no problem. But whoever tells a lie about me deliberately, let him take his place in Hell.” Narrated by al-Bukhaari (3461). 

Al-Haafiz Ibn Hajar (may Allah have mercy on him) said: 

Ash-Shaafa‘i said: It is well-known that the Prophet (blessings and peace of Allah be upon him) did not permit the narration of lies. What is meant by narrating from the Children of Israel is that which you do not know to be a lie. As for what you think sounds reasonable, there is nothing wrong with you narrating it from them.

End quote. Fath al-Baari (6/499) 

Secondly: 

It is well-known in Islam that marriage to one’s sister is haraam, according to the Qur’anic text. 

Allah, may He be exalted, says (interpretation of the meaning):

“Forbidden to you (for marriage) are: your mothers, your daughters, your sisters…”

[an-Nisa’ 4:23]. 

But does this verse contradict what is narrated about the way in which Adam arranged the marriages of his children? 

The answer is that there is no contradiction, and this may be explained by noting two points: 

1.

We are certain that Islam is the only religion of Allah, and that it is the religion of all the Prophets. 

Allah, may He be exalted, says (interpretation of the meaning):

“Truly, the religion with Allah is Islam”

[Aal ‘Imraan 3:19]. 

Despite the fact that the religion, in principle, is one, this does not mean that there cannot be differences between the laws of the Prophets with regard to some practical rulings, on the basis of wisdom that is known to Allah, may He be exalted. 

Allah, may He be exalted, says (interpretation of the meaning):

“To each among you, We have prescribed a law and a clear way”

[al-Maa’idah 5:48]. 

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:

What Allah has sent down is one religion, on which the Books and Messengers are agreed. They are agreed on the fundamentals of religion and the basic principles of sharia, even though they differed in laws and ways, between that which abrogates and that which was abrogated. It is akin to the variety you find in a single book. In the beginning the Muslims were enjoined to pray facing towards Bayt al-Maqdis [Jerusalem], then they were instructed to pray facing towards al-Masjid al-Haraam [Makkah], and in both cases they were only following what Allah , may He be glorified and exalted, had revealed.

End quote. Al-Jawaab as-Saheeh (2/438). 

So there is no reason why it could not have been part of the law of Adam (peace be upon him) that it was permissible for a brother to marry his sister who was not his twin, because that served a significant purpose, whilst this is prohibited in our religion because that reason and purpose is no longer applicable. 

Al-‘Ayni (may Allah have mercy on him) said: 

Rulings were prescribed to serve people’s interests, and were changed at different times. 

Undoubtedly marriage to sisters was permitted in the law of Adam (peace be upon him), and by means of that offspring were produced. This is something that no one denies. Then that was abrogated in the laws of other prophets.

End quote. Sharh Sunan Abi Dawood (4/356) 

2.

The scholars studied the texts and rulings of Islam, and from this examination of the texts it was concluded that the rulings of sharia were only prescribed in order to serve the best interests of people and to ward off harm and mischief from them. Allah, may He be exalted, does not prohibit anything unless it is likely to lead to harm and mischief that outweighs any interest that may be served, and He does not permit anything unless it is free of that harm and mischief. 

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:

Allah sent the Messengers to show people how to attain what is in their best interests, and to abolish and reduce what is detrimental to their interests. With regard to everything that Allah and His Messenger enjoined, its benefits outweigh its harms, even if people dislike it.

End quote. Majmoo‘ al-Fataawa (24/278) 

Ibn al-Qayyim (may Allah have mercy on him) said: 

Sharia is based and founded on wisdom and what is in people’s best interests in this world and the hereafter. It is all justice, all mercy, all benefits and all wisdom. Any matter that drifts away from justice to injustice, from mercy to its opposite, from benefit to harm, and from wisdom to purposelessness is not part of sharia, even if it is inserted into it by means of misinterpretation. End quote. 

I‘laam al-Muwaqqi‘een (4/337) 

In the law of Adam (peace be upon him), marriage of a sister to her brother who was not her twin achieved an important and necessary purpose, which was to preserve the human race and protect it from ceasing to exist and vanishing, because if they did not get married, there would be no perpetuation of the human race. As for negative consequences, they were non-existent or hardly worth mentioning in the light of this great interest. 

Ibn ‘Allaan (may Allah have mercy on him) said: 

The law of Adam (peace be upon him) was that it was the pregnancies of Hawwa’ that determined who was closer and who was not close. Otherwise it would not be possible for marriage to take place. The interest of perpetuating the human race dictated that that should be permissible. End quote. 

Daleel al-Faaliheen (2/448) 

Indeed, even if we assume that these reports are not sound, and that one of them could marry his twin sister, there is no reason why that could not have been permissible for them. Allah gave permission for that on the lips of His Prophet Adam (peace be upon him). Indeed this type of marriage – whether it was to a twin sister or otherwise – was one of the necessities of life that Allah intended for His slaves, and it is not possible to imagine the perpetuation of the human race, in the way that Allah intended for them, without that. No law or reason would disallow that! 

This is in contrast to marriage to a sister in our law; it is prohibited, because it does not serve any purpose or meet any need, and there is no need for it at all. At the same time, it is not free of serious negative consequences, the least of which are what you have referred to in your question of diseases and problems. 

Thirdly: 

With regard to hereditary diseases in the case of marriage between first-degree relatives, yes, doctors and researchers have confirmed this harm. Through research into genetics, researchers have reached the conclusion that many diseases and changes that the parents may be exposed to because of genetic mutations or environmental factors will be passed on to their offspring. 

They also reached the conclusion that if the spouses are closely related, their children – in addition to being vulnerable to the same genetic diseases as other children – will be vulnerable to more genetic diseases because of recessive genes. These are diseases for which one of the conditions of their appearance is that each of the parents should be a carrier of the disease-causing gene. This becomes a greater possibility if both parents are very closely related. Therefore marriage to first-degree relatives poses the greatest risk of genetically transmitted disease. 

But the facts mentioned above do not prove wrong the reports about the way in which the children of Adam (peace be upon him) produced offspring, for the following reasons: 

1.

This research clearly affirms that it is speaking of the probability of the occurrence of these diseases and problems; it is not saying that such things will definitely occur. Therefore there is nothing in the reports about the way in which the children of Adam (peace be upon him) were married that is contrary to scientific facts and theories, because part of the nature of probability is that it does not happen sometimes, even if the probability is high. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said, in his discussion on the issue of hunting with dogs:

What that implies is that it is something to be overlooked, because Allah, may He be glorified, is Almighty, and He is the Creator and the Lawgiver. If something is to be overlooked according to sharia, then the harm will be removed by the decree of Allah. For example, dead meat (that which died without being slaughtered in the prescribed manner) is impure and prohibited, but if a person is compelled to eat it, then it becomes permissible and there will be no harm in it for the one who is compelled to eat it.

End quote from ash-Sharh al-Mumti‘ (1/420). This is clear in our case. 

2.

It is most likely that what you referred to is talking about incest between mahrams in several successive generations. But in the story of the children of Adam, the narrative is speaking of marriage among the members of the first generation only; it does not say that this way of marriage continued in the following generations. 

3.

What is affirmed by the science of genetics is only true with regard to humanity at present, because there is hardly anyone whose parents or grandparents were not subject to problems and disorders. 

But that is not true in the case of Adam (peace be upon him), because he was the first created human being and he was created in the best stature. 

Allah, may He be exalted, says (interpretation of the meaning):

“Verily, We created man of the best stature (mould)”

[at-Teen 95:4]. 

Then Allah caused him and his wife to dwell in Paradise, which is the place of perfect bliss, in which there are no troubles or health problems. 

Allah, may He be exalted, says (interpretation of the meaning):

“And indeed We made a covenant with Adam before, but he forgot, and We found on his part no firm will-power.

And (remember) when We said to the angels: "Prostrate yourselves to Adam." They prostrated (all) except Iblees (Satan), who refused.

Then We said: ‘O Adam! Verily, this is an enemy to you and to your wife. So let him not get you both out of Paradise, so that you be distressed in misery.

‘Verily, you have (a promise from Us) that you will never be hungry therein nor naked.

‘And you (will) suffer not from thirst therein nor from the sun’s heat’”

[Ta-Ha 20:115-119]. 

So it cannot be imagined that he had any kind of diseases or genetic mutations that he would pass on to his children. Rather what may possibly be the case and what makes sense is that all these problems happened with the passage of time and the succession of generations after him. 

Fourthly: 

Your inability to understand that Adam and his wife on their own could have been the source of this huge number of people could be valid, if you knew for certain how long mankind has been on this earth, and the rate of reproduction in each era. But so long as all of that is unknown, it makes no sense to raise such issues. Moreover mathematics leaves no room for finding it strange, if we assume that the human race has been around for a considerable length of time, and the rate of reproduction is reasonable. 

We ask Allah, may He be exalted, to show you the truth and guide you to it. 

And Allah knows best.