Was Beren and Luthien the first man-elf marriage

[Lefty Scaevola]

JRRT notes in HoME says effectively that humans and elves are the same species geneticly, and were indistinguishable as infants, that the difference between them was the difference in their frear and that the spirit adapted the development of the body to better fit it. The biggest differnce in their spirits was that jumens had some place esle to go and elves did not. Thus the human spirit adapted the body for mortality an moving on, and the elvish spirit adapted the body of long life because it was bound to the Earth. He said the differences started become fairly noticeable around age 6 or 7.
Thus, once the being bound to the earth is removed from the spirits of Luthian and Arwen, their spirit will begin to readapt them to mortality and they should gradually become more human like and less efl like. Because they are fullly grown adults, the adaption smay take place more slowly than it does among children. Being genecticly the same species explains there being no problem with interbreeding. The sparseness and nature of this JRRT idea leaves enomours room to speculate about what a mixed human/elf would be or what there death fate was without the intevetion of the Valar to make them one or the other.