He has a problem with the way in which the children of Adam (peace be upon him) got married

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.