In the 1960s and 70s, many scientists believed the cactus family originated on Pangea about 100 million years ago. The belief was as the continents broke apart; cacti became segregated in the Americas and began to diversify. Recent research and advancements in technology have challenged this hypothesis.
“It is much more likely that cacti originated about 30 to 35 million years ago in the Americas. And it wasn’t until 15 to 20 million years ago that the cactus family really began to diversify,” says Dr. Lucas Majure, biologist of New World Succulents. “We know from fossil records that during this time there was a lot of drying out and a reduction in CO2 available in the Americas, making the conditions ideal for cacti to start diverging.”
To understand the evolutionary history of a plant family, the first step is to determine the closest related ancestor. Pereskias exhibit ancestral traits of cacti and are thought to be one of the earliest diverging lineages of the cactus family.
The original process to determine an ancestral relationship was through morphology – studying the physical appearance of plants. By analyzing the characteristics of a plant and understanding the anatomy, relationships can be determined. As technology improved, this process has evolved.
“We gained traction about determining where cactus evolved with the advent of DNA sequencing technologies. Being able to analyze the DNA of a plant and coupling it with morphological data has given scientists the ability to look at patterns and figure out ancestry. In many situations it backed up what we thought, and in others it really threw us for a loop,” says Majure.
The new hypothesis is cacti originated in the Andes and in southern South America. From there species of cacti migrated throughout the rest of the Americas. As fruit/seeds were dispersed into different climates and habitats, cacti continued to evolve and diversify into the cactus family we know today.
“The idea that the cactus family originated in these two places is still being tested. To truly know where something originated you have to know its closest relative, and that is still in a state of flux for the cactus family. Since there are no cactus fossils, we have to use current distribution of cacti to try and predict the past. Sometimes it works perfectly and sometimes it is ambiguous,” says Majure.
The process for tracking evolutionary history:
- Select a group of species of cacti to study.
- Determine the distribution of that group and collect samples in each region.
- Extract the DNA, by hand, from each sample.
- Create herbarium samples of all plants tested.
- Send samples to a lab to sequence the DNA.
- When data is returned, it is pieced together and the sequences are aligned so that each base pair of DNA is in the exact location for each sample and those are then comparable across all samples.
- Sequence alignments are analyzed using a diversity of algorithms and a phylogenetic tree is created and interpreted.
- Morphological and biogeographic information is incorporated into the phylogenetic tree to track movement of plants and the evolution of characteristics through time.
It took Majure four years to create the first nearly complete evolutionary history of the prickly pear genus Opuntia, and he is currently working on the chollas (Cylindropuntia and relatives).
“We have a lot of work to do, and it is going to be a while before we have a clear picture. We have to keep reconstructing the evolutionary history of different groups of cactus species and relatives, and eventually that will lead us to a clearer picture of the origin of the cactus family,” says Majure.