History
Humans in the Animorphs and Neomorphs universe were first introduced to aliens during the events of Animorphs. Between the two series, the humans had been working extensively with the Andalites to get human technology "up to date". Although now a player on the galactic stage, humans were largely irrelevant, even after the defeat of the Yeerks on Earth. However, with the onset of the new galactic war, humans are quickly taking center stage.
The One sent the renegade Yeerks to Earth as a test for humanity. According to the One, humans stand in a unique position. They have the potential to change the entire galaxy, to bring it into a great new age or to lead it into chaos and ruin. The current war, then, is a test of human ability. If humans, represented by the Animorphs pass the test, it is told that they will lead the galaxy forward. However, if they fail, mankind will be completely eradicated.
Biology
Look in a mirror.