Complex sentence: Until the 1700s, it was a general category for any animal of the "monkey tribe" but by the early 20th century, it referred specifically to the tailless (and therefore exceptionally human-like) nonhuman primate.
Split into simpler sentences: Until the 1700s, it was a general category for any animal of what was then called the "monkey tribe". By the early 20th century, it referred specifically to the tailless (and therefore exceptionally human-like) nonhuman primate.