Why Do We Have Belly Buttons?

Everyone has one, but how did it get there? Learn all about your belly button!
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SOURCES:

http://www.livescience.com/32471-why-do-we-have-belly-buttons.html

http://www.huffingtonpost.com/2013/09/12/what-your-belly-button-says-about-you_n_3901527.html

http://mentalfloss.com/article/64416/do-other-animals-have-belly-buttons

http://www.dogster.com/lifestyle/do-dogs-have-belly-buttons