An alien, though human like, race escapes from their dying world on a FTL ship and land on Earth in prehistory, settling peaceably and mating with the natives (possibly in Ancient Egypt). The story post script has a Chinese astronomer witnessing the 1054 Crab Nebula supernova and proclaiming it as a portent of miracles - the text says the miracle is already here and lies in seven of his genes.
I thought it might be an Arthur C Clarke story, but I can't find it amongst his output. I read the story in 1976.