are Human Biology and Descriptive Statistics prerequisites for some MSW programs (e.g. Pittsburgh)? (Biology is especially puzzling.)

No, but they were pre-requisites for my undergraduate program in psychology. Stats are important to understand research and design. Biology is extremely important too, especially if you believe that the mind and body must work together. One must understand the brain and nervous system especially in the social work field. MSWs must have a basic knowledge of drugs used for disorders and how they work, side effects etc. Basic biology helps.