Edited sentence: As with genetic variations of a common ancestor, many of the memes for Christianity and Islam also have common ancestors - the memes for Judaism and for Zoroastrianism.However, taking a typical human generation as forty years (almost certainly an over-estimate in historical terms), Christianity has so far lasted for a mere fifty human generations.One can measure the persistence of the memes for religion in terms of generations of the users (the humans) who pass on these memes.One can postulate that the longevity of the memes within this evolutionary tree of religion comes from the memes' basis in unquestioning faith in the meanings of those memes, and from the regular re-inforcement of their principles, thus minimising the likelihood of them mutating wildly or of dying out completely.Two thousand earth-years for one particular type of religion (let's regard Christianity as equivalent to a biological genus) comprises a long time for humans.Within that time, the Christian memeplex has mutated to spawn variations within the genus — for example, Catholicism and Protestantism, each having spawned further descendants such as Anglicanism,  Old Catholicism, Methodism, and Presbyterianism