Each sorquine is an intelligent organism that has another organism inside of it, whose body it takes over and controls. In their case, the body inside is a human body.

What this means in practice is that a human can be turned into a sorquine, but will become a different person and won't have access to their old memories. This process can be reversed, but it basically requires killing off the outer organism, which is icky and gross.

Sorquines - and all the animal species on their planet - are only about 750 years old, and started when their parent species basically raided Earth for bodies (human and animal) that had been left for dead, and assimilated them into their own race.