In a world for an RPG setting I’m working on, there’s a species of humanoids known as the Auri. Auri are directly evolved from humans, but have a completely different society.
Over the thousands of generations between the catastrophe that cut them off from other humans and the time they re-emerged, the Auri began to practice polyamory and polygamy, mating in groups of seven (a holy number). All parents contribute genes to any baby, and one parent (of any of the ‘female’ sexes) carries it to term. There are seven distinct sexes (A, B, C, D, E, F, G, with A, B, and C being ‘female’, D being nearly neutral, and E, F, and G being ‘male’), and one individual of each sex is required to reproduce. Auri have 7 copies of each chromosome except the sex chromosomes, which they only have one of (this determines biological sex).
A player is playing an Auri married to six other (mostly NPCs but the player has final say over them) Auri, and has expressed a desire for their character to have a child. I had originally put in all this as a world building detail, but now I realized I don’t know how this could develop over time.
I’m not asking about the specific anatomy, just about the process of evolution.
How can humans evolve from diploid creatures with two sexes to heptaploid creatures with seven sexes? If it helps, the technology went from early Industrial Revolution to about 2010 equivalent from the time they were cut off until their reemergence. I’d appreciate answers that don’t use magic to explain things; this is for a fantasy world, but I want scientific answers. I’d also appreciate it if answerers would keep in mind that the youngest player is only 10 and I might need to show the players this as a group (please, I really don’t want their mom mad at me).