How did aids start?

Monkey blood: Most likely through contact with the blood of infected monkeys during the hunting and butchering process in africa. In the latter half of the 20th century, improved transportation systems in africa allowed more travel within and outside of the continent, allowing it to spread beyond the small villages where it probably existed for decades before the actual aids pandemic began.

Non-human primates: Molecular biologists devote their lives and scientific reputations to studying questions like this. Hiv-1 is a chimp virus that jumped to humankind in the congo around 1900, hiv-2 from a sooty mangabey in western africa. Social changes, increased mobility, changing mores and population displacements allowed these to become epidemic. No reasonable, truthful source doubts this nowadays.