well, to be completely honest, the sport has evolved. the discussion is how much. theres a good case either way. the sport is still in its infancy, so it couldnt have evolved so much.
but look at a guy like werdum, hes old as shit as far as pro athletes go, but hes measurably better than he was back in the day. just the fact that hes still around the top means it hasnt evolved a ton, but his experience and improvements means it has evolved some. if you look at the top of the pride HW division and the UFC hw division right now, the HW division is a lot bette now. neweguys like cain, junior and stipe prove that. they wrestled in college, or they boxed their whole adult life. they started doing this because of the pioneers who showed them there was a future in it. so its not necessarily for any reason other than fedor, cro cop and big nog paved the way.
there will be a heavyweight better than stipe one day, too. just like MJ was better than bill russell, and now it seems basketball is evolving past an MJ. lebron is superhuman. so its the natural order of things. we evolve.