Where do the Mother Trees came from?

Question

Warning: this question is a significant spoiler to the "Speaker of the dead"!

Lets talk about sex. No, not the ordinary one but that of the Pequeninos (Piggies) from the Enderverse, which have a bloody (ha!) complicated life cycle:

If I understand it right, on the Mother Tree live slug-like "little mothers" feed on tree sap, then when they are big and fat are transported by mature Piggies to a Father Tree that pollinates them. Then the Little Mother is put inside Mother Tree, and after a while a new set of slugs eats its way out of Little Mother. Those slugs live inside for a while and again fertile female ones leave the inside to live on the outside and repeat the cycle, why males and infertile females wait a little longer until they start looking like baby piggies then they join the slugs competing for the sap. After a while an adult males leave to join other "brothers" while infertile "wives" stay to take care of the mother tree. Then at some stage, during one of the bloody wars or during a ritual adult male is killed and from his corpse a new father tree grows.

Question then is how does the mother tree came to grow in the first place?

Answer 1

In Xenocide it is shown that when the old Mothertree died in a fire started by the villagers after Quim was killed, they "planted" the most respected wife to grow a new mothertree.

In the middle of a clearing strewn with ash, Ender, Valentine, Miro, Ela, Quara, Ouanda, and Olhado all stood and watched as the most honored of the wives was flayed alive and planted in the ground, for her to grow into a new mothertree from the corpse of her second life. As she was dying, the surviving wives reached into a gap in the old mothertree and scooped out the bodies of the dead infants and little mothers who had lived there, and laid them on her bleeding body until they formed a mound. Within hours, her sapling would rise through their corpses and reach for sunlight.

Using their substance, she would grow quickly, until she had enough thickness and height to open up an aperture in her trunk. If she grew fast enough, if she opened herself soon enough, the few surviving babies clinging to the inside of the gaping cavity of the old dead mothertree could be transferred to the small new haven the new mothertree would offer them. If any of the surviving babies were little mothers, they would be carried to the surviving fathertrees, Human and Rooter, for mating. If new babies were conceived within their tiny bodies, then the forest that had known all the best and worst that human beings could do would survive.

Answer 2

This is revealed in Xenocide and Children of the Mind:

The Descolada virus was an artificially created virus that modified the ecosystem in unnatural ways. The virus was engineered by a mysterious alien race referred to as the 'Descoladores' as an experiment in terraforming. The Descoladores will presumably appear in the upcoming novel Shadows Alive.