In E.V.O., there are five evolutionary stages, each with their own set of EVO points. Once you evolve from one class of species to another, you will lose all accumulated evolution points and start as the most basic creature within the class.
Fish
This is your first form.
Amphibian
This is the second form, and the first one that is designed for use on land. (While fish can go on land, they only do so in the last portion of the first chapter.)
Reptile
The reptile has the most options to evolve. In addition to the previous evolutions, it can experiment with using only two legs instead of four.
Bird
The bird is a special evolution form obtained in Chapter 3: Age of Dinosaurs. While it is the only class capable of flight, it may be difficult to attack some ground enemies at first.
Mammal
The mammal is the final evolutionary stage, obtained in Chapter 4: Ice Age. It is only available if you killed enough creatures, and is lost forever if you enter the final chapter without obtaining this new form. Unlike the reptile, mammals can walk on ice without trouble.
Human
Once you are a mammal you can choose Cat Jaws and Rabbit Body to allow the option Ramothecus Body. It will evolve you into a monkey. After 2 evolves, you turn into a human. Also, when still a mammal you can go to the Final Ocean level and then try to evolve Hands and Feets, you'll turn into a seal, then a dolphin, then a strange creature and then, finally, a beautiful mermaid. However, after defeating the boss in this level, you'll back to have your old mammal body.