Write a correct answer to the given question based on its associated fact. Make sure that your answer is contained in the associated fact. Don't be creative and introduce any new word that is not mentioned in the associated fact! Remember that the associated fact has been rearranged to form the question. So, the correct answer words must lie within the associated fact. The correct answer can be a word, phrase, or even a sentence.

Fact: roundworms may be free-living or live off a host. 
Question: aside from free-living, where else can roundworms exist?
live off a host.

Fact: The continents drifting causes earthquakes. 
Question: What causes earthquakes?
Continents drifting.

Fact: Descendants of pelycosaurs invaded Earth. 
Question: Where did pelycosaur descendants invade?

Humanity's home planet.