Why do humans have cannabis receptors?

Why do flies exist? Could sound propel a spaceship? Why do humans have receptors for tetrahydrocannabinol in their brains? How do cars calculate their fuel consumption? And is all life descended from bacteria? Plus, we report on the recent discovery of a plume of methane on Mars: what does this mean for the likelihood of finding life on the red planet? Like this podcast? Please help us by supporting the Naked Scientists