Humans are not the intended native inhabitants of Earth. We're descended from the Golgafrincham B Ark colonists, having wiped out the native ape-like inhabitants. Therefore, the program running on the planetary computer we called Earth, designed to work out the question to the answer of life the universe and everything, had a major input error introduced into it.Serious question for H2G2 nerds. Famously, Deep Thought was built to provide The Answer to the Ultimate Question of Life, the Universe and Everything. It gave the answer "42". Which then brought up the conundrum as to what the question was.
So they built an even bigger computer, called "Earth", which was just about to reveal the question when it was destroyed to make way for a hyperspace by-pass (IIRC). In "So Long and Thanks for All the Fish" (probably some errors here) Ford Prefect was trying to use the remnants of earth amongst the telephone handset sanitisers, middle management and other members of the human race that were not either the great thinkers and movers, or the grafters. Realising that Arthur Dent was also a part of the great computer "Earth", Ford Prefect got him to pick random letters out of a scrabble bag to see what he came up with. And the answer was "What do you get when you multiply six by nine ". (thanks, Wikipedia).
Does this mean that the answer from Deep Thought was wrong, or that despite Arthur Dent generating a cohesive sentance from random Scrabble letters, he failed to provide the question that they were looking for?
My take on Dentarthurdent being able to produce a cohesive but incorrect question is a suggestion that the Golgafrinchans cross-bred with the natives.