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Abstract.

In evolving to its great size the human brain has retained the distinctive features and chemistry of three kinds of brains that reflect an ancestral relationship to reptiles, early mammals, and late mammals. It constitutes, so to speak, a psychencephalon comprised of three-brains-in-one, a triune brain. In the evolution from reptiles to mammals two key changes were the development of nursing and maternal care. Through the agency of “newer” parts of the brain a parental concern for family eventually generalizes not only to other members of the species but to the entire biosphere, a psychological development that amounts to the evolution of responsibility and what we call conscience. Given our freedom to decide “yes” or “no” on various issues, we need not look beyond the evolving family to find a reason for being, an ethic to live by.