Evolution of Cactus ... 2/4

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In the beginning there was one land mass - Pangaea - which in turn split into Laurisa (north - North America, Europe, and Asia) and Gondwanaland (south - Africa, South America, India, Madagascar, Antarctica, and Australia) about 200-165 MYA (million years ago). About 130 MYA Gondwanaland separated into Africa, Madagascar and South America and these started moving apart. Caryophyllales were in an area of Gondwanaland that split into Africa and South America.

map

By 65 MYA there was about 800km between the two continents of Africa and South America, which is sufficient to ensure there be no interchange of organisms between the two land masses e.g. migratory birds or floating rafts of flotsam, so evolution of the species on each land mass occurred in different and independent directions.

Because cacti are only found in the new world it is assumed that they only started to appear after the separation of South America from the other continents in Gondwanaland, and after the 65 MYA mark and then moved into North America when these two land masses connected, then evolved separately when the two landmasses of North and South America became separate again when the land bridge drifted away, only to be rejoined with the central America land bridge in later times. In fact from studies it is thought cacti first appeared/evolved 30 MYA.

Caryopyllales were now split into 2 continents (really the Cactaceae-Portulacaceae-Didiereaceae complex were stranded and then diverged on five separate land masses from the original Gondwanaland as mentioned in "relatives of cacti" above) and the south American ones evolved into cacti about 30 MYA.
The "exact" starting point in terms of area for the origin of the first cacti has been disputed for many years, possibles are the Caribbean, southern Mexico, northern South America or along the Andes rain-shadow. When South American connected with North America (twice), cacti moved northwards (and south) so that today cacti extend from central Canada to near the bottom of South America and every place in between.

map Between 30 MYA and today the climate changed, glaciers retreated, ocean currents changed - all factors leaving behind arid deserts especially at a belt 30 degrees latitude worldwide. During this time Cactaceae had evolved in a similar way to the African succulents with CAM/C4 metabolism, water storage cells, reduction of leaves and transfer of photosynthesis to the stem, and decreased surface area etc., but with a few extra adaptations like areoles. They expanded into this increasingly arid territory, evolving into new species. Isolation e.g. the Baja California peninsula (which was at one stage a separate landmass) and its offshore islands led to new species and varieties evolving in their "own little world".