So in C1 we have aliens who fly and so on, 3 years later in New York, we have aliens who are completely evolved and have heavy armor.
In Crysis 3, which is more than 20 years since C2 we have only 2, 3 new types of aliens and I just want to emphasize that we are at the end of C2 killed millions of them and there it is 2, 3 new types of aliens. It is logical to re-evolve again!