DEATH AND GENERATION OF COMPLEXITY. “Programmed cell death plays a central role in development, participating in particular in morphogenesis (the sculpting of the form of embryos), in sexual differentiation, and in the epigenetic self-organization processes that allow the emergence of the two most complex regulatory organs of our body, the immune system and the nervous system.”
http://www.nature.com/cdd/journal/v9/n4/full/4400950a.html