Ooh, I really like that idea. It could easily explain the frequent recurrence of the atavism. They harvested the DNA of one of their ancestors, but they couldn't make a pure specimen that survived very long past birth. They needed randomized human DNA to stabilize the genome. Except even using this method, there's still only a one-in-a-thousand chance that a child will be born with the required attributes. Using the advanced spawning technology, each host female is able to give birth to five half-blood spawn per year. The newborn is immediately thrown into a red-hot fire to test to see if it has the flame attributes. Since there are only ten known spawning hosts in the world, it was seen as extremely fortutous that two Ancestral Children, from the same host unit no less, were born in as many years.
It's yucky and evil. I love it.