Evolution of Cactus ... 3/4

One Theory - Maihuenia

Some consider the genus Maihuenia (within the Pereskia subfamily) as the most primitive Cactus, or at least the closest living thing to the ancestor that is growing today. There are similarities between Maihuenia and Portulaca, also a lot of the features in Maihuenia are primitive in the Cactaceae family.
The three subfamilies of today evolved from (something similar to) Maihuenia.
The fleshy South American "Chollas" [Austrocylindropuntia and Maihueniopsis] would seem to be close to what the Primitive Opuntioids may have looked like. They are not that distinctive in form from Maihuenia, plus the addition of the glochids and bony aril found in the opuntia tribe that is not found in Maihuenia.
In the Cactoids, Copiapoa is probably the most primitive, having the most primitive seed, as well as other primitive features.

Another Theory - Pereskia

Pereskia genus is usually considered the most primitive because of it's lack of succulence and the presence of leaves linking it to an "ordinary" plant, but with the addition of aeroles and spines linking it to the cactacae. So the first Cacti were leafy plants that got fatter in responce to enviromental conditions and turned into climbing Cereoids then barrels, or Opuntias.
Playing devil's advocate, the first Cactus plant would have evolved in a harsh environment adapting to survive the dry seasons, not in the easy going (high rainfull/forest) habitats that Pereskia prefer.
Pereskia may have diverged in the early evolution of the cactus family, chosing the easier (wetter) climate to live in, and becoming a more conventional leafy plant i.e. a devolution or 'taking the easy way out'.

Update on Pereskia - Novemeber 2007

Michael Donoghue and Erika Edwards, plant evolution researchers at Yale University (Connecticut, US) studied 7 members of the genus Pereskia, as they sequenced selected DNA regions of 38 cactus species. They have found that modern Pereskia evolved as two evolutionary groups, one of which probably split off from the rest of the cacti, before they had undergone their dramatic anatomical changes familar to us as the thick-stemmed leafless cacti of today, and very different from other leafy plants living in the same localities of South and Central America. This Pereskia group is more closely related to other cacti even though it does not resemble them anatomically

As the leaves of both Pereskia groups can store much more water, and they seem to use an alternative, water-conserving photosynthetic pathway typical of the leafless cacti when conditions are particularly dry, then this must have evolved first before their ancient evolutionary split.

Therefore the first cacti were "shrubs or small trees with photosynthetic leaves".

The Rhipsalis Puzzle - Yet another scenerio

Rhipsalis is the only cacti found naturally outside of the Americas, others e.g. Opuntia have been artificially introduced by man into Africa, Mediterranean, Australia etc in fairly modern times. Rhipsalis grows on the Africa continent (in an area 3500km across in the tropics) as well as Madagascar, Seychelles and Sri Lanka plus neighbouring islands.
It was not introduced there by man, unless some ancient African peoples sailed to America and back, or conversely some ancient American people sailed to Africa AND went to the trouble of bringing some obscure cacti that grows in the trees that neither has a nice flower or any ethnobotanical uses, with them (and didn't bother to record it in their history).
The distance between America and Africa also means it unlikely that it was carried there by migratory birds, also birds tend to migrate North/South according to seasons not in the East/West direction, also introduction by rafting or floating on flotsam in ocean currents from America to Africa is an unlikely scenario.

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