The team then sifted through the literature on the timing of diversification in other succulents from regions around the globe. Succulents include aloes, the agaves of North America, the ice plants of South Africa and other lineages. Their comfort zone is in water-limited climates, and they have adapted physical characteristics to cope in those locales, such as shallow root systems, specialized water-storing tissue and exchanging gas at night, when it is cooler and less humid and so less water is lost. What struck the researchers was that all the succulent lineages, across habitats and continents, underwent major speciation between 5 and 10 million years ago, during roughly the same time period as the cacti.
C4 grasses, the tropical grasses that are now up to 20 percent of our planet's vegetative covering, burst onto the scene as well during this same window of time.
This must be more than a coincidence, the researchers thought. "It isn't overly surprising that most of the standing cactus diversity is relatively young. But when you put these species radiations in the context of all the other changes in plant communities that were happening at that very moment, all over the world, it begs some sort of global environmental driver," Edwards said.
The most plausible causes, the scientists thought, were a drying out of the planet and lowering of atmospheric carbon-dioxide levels. A wealth of research involving oxygen isotopes from a deep-sea organism showed the Earth underwent a drop in temperature, which the researchers believe led to reduced rainfall and increased aridity worldwide.
The carbon-dioxide link is more nuanced and controversial. The authors highlight one study that inferred atmospheric CO2 levels spiraled downward beginning roughly 15 million years ago. Combined with global cooling, "a drop in CO2 concentr
|Contact: Richard Lewis|