The existence of multiple groups of HIV-1 and HIV-2 suggests that zoonotic transmissions of SIV have occurred at a low rate for centuries. Hence, an increase in the rate of human-to-human transmission may be necessary and sufficient to explain the emergence of HIV as an epidemic in the twentieth century. Three common hypotheses to explain accelerated transmission are (1) social changes accelerated sexual transmission, (2) health care changes accelerated parenteral transmission, and (3) serial passaging adapted HIV for persistent infection and sexual transmission. These hypotheses can be compared to a range of evidence. Temporal and geographic discontinuities in HIV epidemic growth are not easily explained by supposed increases in sexual transmission over time. Large historic changes in sexual transmission are hard to explain based on weak evidence associating HIV prevalence in African communities with differences in sexual behavior. On the other hand, documented iatrogenic outbreaks show high rates of parenteral transmission. The distribution of hepatitis C virus infections and the history of multiinjection treatment for trypanosomiasis in Central Africa suggest widespread parenteral transmission of blood-borne viruses during 1920-1940, coinciding in time and place with the early HIV epidemic. This suggests an important role for parenteral transmission in the early spread of HIV. Further research could improve our understanding of the early HIV epidemic.