Praise be to Allah.
Necessity dictated that the children of Adam (peace be upon him) had to marry one another in order to produce offspring and populate the Earth.
In some reports it says that no son was born to Adam (peace be upon him) but a girl was also born with him, so the boy born from this pregnancy married a girl from another pregnancy, and the girl born from this pregnancy married a boy from another pregnancy; it was forbidden for the boy and girl from the same pregnancy (i.e., fraternal twins) to marry one another.
This is mentioned in reports narrated by Ibn Jareer at-Tabari in his Tafseer (10/205-207) from Ibn ‘Abbaas and Ibn Mas‘ood (may Allah be pleased with them).
However, we do not find in the Holy Qur’an or the Prophet’s Sunnah any mention of the historical development of stages in the rulings on prohibitions on marriage to close relatives (mahrams) or on the degree of blood relationship that makes a person a mahram (and thus prohibited in marriage). But what we do find in the Holy Qur’an is the prohibition on marriage to mahrams, where far-reaching degrees of kinship are mentioned. It includes marriage to one’s brother’s daughter, one’s sister’s daughter, one’s father’s wife and even includes mahrams because of breastfeeding. That is the verse in which Allah, they He be glorified and exalted, says (interpretation of the meaning):
“Forbidden to you (for marriage) are: your mothers, your daughters, your sisters, your fathers sisters, your mothers sisters, your brothers daughters, your sisters daughters, your foster mother who gave you suck, your foster milk suckling sisters, your wives mothers, your step daughters under your guardianship, born of your wives to whom you have gone in - but there is no sin on you if you have not gone in them (to marry their daughters), - the wives of your sons who (spring) from your own loins, and two sisters in wedlock at the same time, except for what has already passed; verily, Allah is OftForgiving, Most Merciful”
Anyone who speculates about the historical development of the rulings on marriage to close relatives, and the details of that, is only speculating on matters of conjecture about events in history and the laws of the Messengers, basing it, for the most part, on reports from the People of the Book. This – as is obvious – is likely to be wrong and based on imagination, and subject to addition and subtraction, as narrated by the historians in the story of Adam (peace be upon him) and by some of the mufassireen in their commentary on the verse in which Allah, may He be exalted, says (interpretation of the meaning):
“And (O Muhammad) recite to them (the Jews) the story of the two sons of Adam (Habil (Abel) and Qabil (Cain)) in truth; when each offered a sacrifice (to Allah), it was accepted from the one but not from the other. The latter said to the former: ‘I will surely kill you.’ The former said: ‘Verily, Allah accepts only from those who are Al-Muttaqoon (the pious)’”
Some contemporary writers have also looked for such information in the Torah and Gospel which have been distorted, and in some of the history books of earlier nations.
There is no benefit in knowing when marriage to close relatives was forbidden in earlier laws; if that were the case, Allah, may He be exalted, would have mentioned it, or His Messenger (blessing and peace of Allah be upon him) would have mentioned it, for he did not omit anything that was beneficial or useful but he told us of it. What should concern us is the fact that Allah, may He be exalted, has definitively forbidden marriage to close relatives (mahrams) in His Book, and this is also indicated by the Prophet’s Sunnah and scholarly consensus.
And Allah knows best.