1. The origin and history of aridlands and their vegetational cover is closely related to geological history, especially in relation to plate tectonics, mountain building, land-and sea-level changes and ice ages, and the arrival of modern humans and their subsequent development.
2. A close relationship exists between world temperatures and precipitation. Therefore, the development of present-day zonal, aridland vegetation and climate cannot be divorced from the history of the latest ice age.
3. The combined geological and palaeobotanical evidence demonstrate that whilst the origin of modern openland and aridland vegetation went back to the time of the origin of angiosperms during the Cretaceous, its subsequent expansion went hand in hand with the lowering of world temperatures during the Palaeogene and Neogene, when a series of angiospermous families having dominantly herbaceous and openland taxa, as members, appeared successively in the stratigraphic record.
4. The successive lowering of world temperatures had the overall effect of reducing precipitation levels all over the globe. Consequently, high rainfall areas, bearing closed forests, became progressively smaller and smaller and the decreased rainfall promoted the evolution and expansion of low biomass, openland and aridland vegetation.
5. The break-up of regional closed forests had started from the middle Miocene but the main expansion of zonal, aridland vegetation, in low and middle latitudes, appears to have taken place, together with the expansion of tundra vegetation, at high latitudes, from the late Pliocene. Glaciations of a magnitude of at least two-thirds that of the late Pleistocene glacial maxima started to occur about that time, 25 Ma ago.
6. The alternations between cold, glacial and warm, interglacial periods, during the late Neogene, especially with increased amplitude during the last 0.4 Ma, allowed less and less time for forest vegetation to expand and stabilize during the warm intervals, with the result that openland and aridland vegetation was able to expand to unprecedented levels.
7. Further, as man increased fire frequencies during the late Pleistocene, the relatively fire-sensitive and mesophytic taxa were selectively eliminated and more and more forests were opened to invasion by openland taxa in low and middle latitudes. Later on, with the clearance of forests for agriculture, the overall effect on vegetation was to create open landscapes which favoured the expansion of openland taxa at low, middle and high latitudes, during an interglacial, that is, the Holocene, a feature that is unprecedented in the entire earlier geological record.