You can evolve into a bird in the Reptile Era (with the dinosaurs). Go to Mt. Brave and climb up to the top. Keep away from the enemies, which will throw you all the way back down. A voice at the top will tell you to jump. Jump off the left side to evolve into a bird. At this time the bird is very weak, but may be upgraded fast. Wait until the cloud floating around in the overworld stops at a level. Try to enter that level while the cloud is above it. The bird will fly up to the cloud. The cloud is similar to a maze, but is not to hard to navigate. Get the red crystal that will change the bird into a dragon for a short time. It is easier to fly around this way since you are faster. You should also save this form for later use. The exit in the cloud is somewhere near the top. Use it to reach space. Fly all of the way to the left, which may take some time. After moving all the way to the left, go all of the way up to find an entrance to a large object that may be an asteroid at the top of the screen. Enter it to find an alien doing something to a pterodactyl. He will tell you that he is too busy and leave. Take the object he drops to get +9999 E.V.O. points. Upgrade your teeth, and then your body, fully. Use the remaining points for any purpose. Move to the right to find more openings. Most of them are informational, but one contains a red orb. You can also kill the pterodactyls for +100 E.V.O. points each, allowing even more upgrades.
Human Body
After becoming a mammal, evolve the cat jaws and rabbit body. Then, select the Ramopithecus body. It will become human after two more evolutions.
Mermaid Body
Accumulate 10,000 evolution points by the final ocean stage of Chapter 5. Select "Hand and Feet" when evolving your character, then select the "Try To Evolve option". Continue to evolve three times to become a Mermaid.