The Humans are a bipedal species and the main species of planet Earth. Their existance - particularly, the fact that they were intelligent life - is the reason the Martians needed to conquer Earth instead of colonising it. The Martians desired humans when they were at the peak of their evolution. Why pick an apple when it has not fully ripened? The Martians picked now to invade becasue we still did not possess defenses that could put up a resistance.
The Humans tried to defeat the Martians during the invasion, but failed (only managing to destroy a few tripods by themsleves in the entire war), winning only due to their natural immunity to the bacteria of Earth.