The Rare Earth hypothesis was put forward in 2000, by P. D. Ward and D. Brownlee, and claims that simple single-celled life is likely very prevalent in the galaxy and universe, but advanced animals (including intelligent civilizations) life is extremely rare, and the Earth has provided a unique environment for advanced animal life. Although controversial, it has seeped into many physical sciences like geology, paleontology, and astrobiology, and many now believe it is likely true. We will review the reasoning and science behind it and complaints about it.