The fossil record shows long stretches where species barely change, then rapid shifts. The interesting question is not why change happens in bursts — it is what holds things still for so long.
The same technology gets overestimated early and underestimated late. Both errors come from drawing a straight line through something that is slow first and rapid afterwards.
Every account is assembled from what survived, by someone who already knows the ending. That single fact makes events look more inevitable than they were — and explains why careful history sounds less certain.