Tardigrades are weird. As in really weird.
They can survive anything. They're eight legged, multicellular water critters who can survive in space. Literally - they can handle the temperature extremes, the vacuum, the radiation, they can go without eating or drinking for ten years. Oh, and they can handle pressure far higher than happens on Earth.
(Maybe they actually are aliens).
It turns out tardigrades have comic book level adaptability. When under extreme stress, their cell walls break down...and they absorb DNA from the critters already there. 17.5% of their DNA comes from external sources (compared to less than 1% of ours). Bacteria do this kind of thing all the time. Complex animals (tardigrades are fairly simple, but they're still not bacteria) don't.
Then the genes that are useful become incorporated into their DNA.
Highly complex animals like humans can only do this by mating with other species, which restricts the external genes which can be included.
I think there might be a story seed in here...