Actually...
If you REALLY loved THHGTTG then you would know that humans came from a different planet because it was becoming so over-populated, and they decided to rid themselves of an entire useless third or thier popultaion, by telling them that the planet was about to be destroyed. This useless third was all the middle-class people like hairdressers and telephone sanitsers, and became the human race. Meanwhile, on their original planet, the rest of the population were wiped out from a desease originating from a dirty telephone.