Look, you’re not going to get a straight answer, because while this topic has been theorized about extensively, no one knows for sure.
We have no idea at what stage in the human lineage we lost our body hair, and it is very likely that we’ll never know, because hair doesn’t fossilize. For all we know, only anatomically modern humans were hairless, and neandertals were as hairy as chimps. Or hairlessness could go all the way back to Lucy and the first australopithecines.
My own pet theory is that hairlessness is somehow connected to bipedalism. On the african savanna hair doesn’t protect you from cold, it protects you from the sun. But if you’re bipedal, the midday sun only hits your head and shoulders…so long head hair could protect you from 70% of the sun, and then hairlessness on the rest of the body could allow more effective sweating, humans are one of the sweatiest mammals. We sweat and drink much more than other apes. So maybe increased brainpower allowed our early ancestors to track down and/or store water that would have been unavailable to chimpanzee-level apes.
But this is all theorizing, and we’ll likely never know for sure.