User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-4212769-20120302173919/@comment-1011548-20120302192345

Having a gene malfunction that gives you a third pair of useless limbs on your back is a disadvantage. It's something for predators to grab onto, it's more surface that can get cut and infected, it's more body that you have to channel nutrients towards... it'd be selectively bread out of a population rather quickly.

I've not really thought about the long-term evolution of MH world too much, there's not enough info to go on - it's not as though there's a full, accurately dated fossil record provided to tell us what the monster's ancestors looked like.