Final answer:
HIV-1 was transmitted to humans from chimpanzees, likely through hunting and blood contact, leading to the global pandemic.
Step-by-step explanation:
HIV-1 is believed to have been transmitted to humans primarily from chimpanzees. This interspecies transmission, also known as zoonosis, likely occurred when humans hunted chimpanzees for meat and came into contact with their infected blood. Over time, the virus adapted to the human body, leading to the global HIV pandemic we are familiar with today. As the virus was first identified in 1981, it has since then caused immense health, social, and economic challenges worldwide, especially in sub-Saharan Africa where it is most prevalent and the resources to fight it are least available.