I guess that would depend on the difficulty you played on and the kind of approach you take with your creatures when it comes time to either A) allying with other creatures or B) wiping them out via combat. One will be easier or harder for you to do dependnig on your early development; herbivore vs carnivore vs omivore.
You also need to find parts to evolve, they're hidden all over the game map and you also recieve them when allying or wiping out other creatures. Supposively the real fun of the game comes in when you're able to make it into space and roam the galaxy.
You also get to choose when you evolve to the next level; so when you reach the requirements you're not forced to evolve if you don't want to.




