Frankly, I've never heard of a beneficial mutation. I always thought they were just the random variations in the nucleotide composition of DNA/RNA. I think this idea that evolution means an organism can only become more advanced and complex is misguided. I don't know if that's what you're thinking, but it seems to me you believe evolution is "progress" as in more beneficial traits are gained and the organism gains a competitive advantage over others which in turn boosts its ranking in the hierarchy of biological organization. I'm not saying you're wrong, but I don't believe that's the way it works. There's actually a very interesting show on Netflix called "Your Inner Fish" and I highly recommend it. It shows how anatomical characteristics of humans are derived from aquatic and reptilian ancestors and how we are really just recycled parts and DNA from long ago (with some contemporary changes, of course)