Studies on fossils from Turkana basin at the time of the conjectured dramatic shift 2.5 million years ago found that the rate of adaptation was significantly less than had been predicted by the turnover-pulse hypothesis, with a 50% to 60% species turnover spread over 1 million years instead of a 90% turnover in a few hundred thousand. Suggested explanations for the discrepancy include that "variations in fossil abundance through time skewed Vrba's data, creating a false peak" or that the "Turkana rift valley—which held a river bounded by woodland at this time—was buffered from the dramatic climatic shifts".