Dear Jake:

Adam and Eve had many children, who had children, who had children. The other people alive at the time were cousins. Yes, there was inter-breeding, but in at this time, the DNA was relatively pristine, without defect and mutations to cause problems with marrying a close relative.  With each generation, of course, the greater the distance of consanguinity.

Today, the Church permits the marriage of relatives of a certain distance in their relationship, but determining the details is rather complex.

Canon Law States:

Can.  1091 §1. In the direct line of consanguinity marriage is invalid between all ancestors and descendants, both legitimate and natural.

§2. In the collateral line marriage is invalid up to and including the fourth degree.

§3. The impediment of consanguinity is not multiplied.

§4. A marriage is never permitted if doubt exists whether the partners are related by consanguinity in any degree of the direct line or in the second degree of the collateral line.

Can. 108 §1. Consanguinity is computed through lines and degrees.

§2. In the direct line there are as many degrees as there are generations or persons, not counting the common ancestor.

§3. In the collateral line there are as many degrees as there are persons in both the lines together, not counting the common ancestor.

The term “collateral line” refers to all relationships that at least partly involve siblings. Brothers and sisters are the clearest example of relatives in the collateral line, but collateral-line consanguinity refers also to relations between aunt/uncle and niece/nephew and cousins.

Valid marriage to a relative must be of the fifth degree or more.

But, in the beginning the need for this system of consanguinity was not needed. This is where the people came from, as cousins.

God Bless,
Bro. Ignatius Mary