According to the newly published paper http://www.sciencemag.org/content/347/6229/1452 Ab initio calculation of the neutron-proton mass difference Science 27 March 2015: Vol. 347 no. 6229 pp. 1452-1455 DOI: 10.1126/science.1257050 we owe our existence to an interplay between electromagnetic and mass isospin breaking effects. The preprint can be found at http://arxiv.org/abs/1406.4088 This is one of the problems one of my favourite lecturers as an undergrad said he wanted an answer to before he dies (he is still alive).