2) Jacobson’s Organ

This tiny little nubbin in the centre of your brain, lying at the end of your nasal passages is useless to you. But it hasn’t always been. Jacobson’s organ — or the vomeronasal organ — is essential in other mammals, fishes and reptiles.

The organ detects pheromones which trigger sexual responses, alarm and feeding responses. If you get rid of the organ in a hamster they literally can’t find members of the opposite sex and will never mate again.

You know when animals pull this weird face:

That’s called the flehmen response and it isn’t just for fun as it appears. The ridiculous upper lip jazz helps get scent molecules up into the Jacobson’s organ.

If you remove Jacobson’s organ from a human nothing happens at all.