Human immunodeficiency virus entered human populations after evolving from a simian immune no deficiency virus. Nikolaas Tinbergen (1963) proposed explaining shifts in traits from two perspectives: dynamic versus static, and proximate versus ultimate. This framework can be used to understand the evolution of a trait in four ways: 

(i) causation (proximate/static): the mechanism of the trait as it works in the present; 

(ii) survival value (ultimate/static): how function of the trait enhances survival or reproduction; 

(ii) ontogeny (proximate/dynamic): the development of the trait in an individual; and 

(iv) evolution (ultimate/dynamic): the phylogenetic history of the trait. Use these categories to discuss the causes for the virus shifting to humans from other primates.