For some reason agriculture does not arise. Let's say that there's some mild ecological disaster worldwide. At the same time, viruses kill off all the major plants that will become food crops. Implausible for that to happen at the same time, but that's not the point.
Let's say that the only crops that are domesticated are weird nutrient-poor tubers and other similar plants.
With agriculture stifled, urban civilization, which arguably slows down evolution, is impossible.
How do humans evolve after this point?