If we've had large brains and language for hundreds of thousands of years, why did it take us so long as a species to figure out our true origin? What constraints were operating on us that meant it was only 150 or so years ago that someone figured it out?

I wonder if one factor could be, for example, the necessity of travel technology, such that a critical mass of travellers were able to report on the patterns of biological diversity around the world? Or another factor the rise of agrarian societies, liberating enough free time to indulge in such speculations?

Interested in your thoughts..