It's far more likely than a transporter accident is two cause two radically different species to reconstitute into one stable hybrid with the union of their memories and skills. Come on, this is Star Trek! In the Star Trek universe, a black hole has the gravity of several solar masses even if the mass of the black hole is only a few ounces.
You could argue that a good deal of the genes they have in common make up the prefrontal cortex and the upright skeletal structure. Maybe reptilian is just the human translation of a Xindi word and the similarities to earth reptiles are superficial. Also maybe the aquatics and insectoids are farther removed genetically than the other three.