Once upon a time, there was a leak in a riverside laboratory, and rightfully so, that river became polluted. 

All creatures that drink the contaminated water grow two long human-like legs in a short time. They also then grow to human size and develop intelligence, with an average IQ between that of a macaw and a dolphin. Thus, during this stage of change they need a lot of nutrients and energy, and when they are hungry, they become violent and even attack humans!

Different organisms will also mutate differently from one another, but most will still retain some of their signature behaviors and even reinforce them. For example, male birds are good at dancing and showing off, but now the female and male birds will dance together and are at a level where they can take the stage.