This may sound very "hindsight is always 20-20" but why was evolution not always obvious? Traits that help reproduction will increase. Traits that hinder reproduction will decrease. Why was it controversial, and what did people believe before that?
Different theories of evolution existed before Charles Darwin, but the mechanism driving the evolution was not understood.
Lucrete (Roman writer) had ideas close to Darwins natural selection - he believed the species had been initially created by all kinds of organs and body parts combinimg at random, but only the “fit” ones surviving. But I think Lucrete thought this was a historcal event, not an ongoing process.
It is amazing how ancients had great insights like also about atoms but it took close to two millennia to have these ideas refined.
That theory wasn’t directly related to the origin of man, but it gives an idea of where scientific theories were at the time.
In fact, the Oscar nominated movie Arrival is based entirely on this idea.
The strongest argument the creationists use when trying to push back against evolution, which actually works, is basically “do you really think your great grandfather was a monkey?”.
It takes incredible leaps of imagination to accept that different species had common ancestors and evolved from very different looking species.
Even if you do buy evolution it takes many leaps of imagination to propose and accept that not only do reptiles have a common ancestor with each other, and birds have a common ancestor and fish, etc. but all of them also evolved from a common ancestor.
And finally even after you accept that, while it wouldn’t take as much of a leap of imagination, it would definitely require fighting millennia of cultural and religious indoctrination to accept the idea that humans were also subject to the same processes as other animals.