Scientists generally attribute the transformation of the Sahara to changes in the Earth's orbit, which deprived the tropics of sunlight, leading to a drop-off in summer rainfall. Wright says that human migration pushed the region to a tipping point. The landscape did not transform slowly and uniformly, as would be expected if variations in the Earth's orbit were the only factor. Rather, it changed in fits and starts, following the spread of livestock. Wherever humans went, they left scrubland in their wake.