by Gar Smith Earth Island JournalWinter 1998, p. 13
For 3.5 billion years, the Earth was dominated by so-called C3 plants,which photosynthesize molecules containing three carbon atoms. Between six and eight million years ago, Earth's vegetation changed radically. So-called C4 plants, which produce four-carbon molecules and include numerous grasses such as sugarcane, began to appear in many areas.
As C4 plants came onto the scene, many of the planet's Miocene woodlands were replaced by savannas and millions of mammals faded into evolutionary oblivion. According to the "savanna hypothesis," this plant revolution spurred our human ancestors to leave the safety of the forests and learn to forage on open grasslands.
"The planet is now a different planet," University of Utah geochemist Thure E. Cerling explained in "Science News." Cerling and others now believe that changing levels of atmospheric carbon dioxide (CO2) are linked to the transition from C3 to C4 plants.
During the era of the dinosaurs, CO2 levels were as high as 1,000 parts per million (ppm), but some eight million years ago, atmospheric CO2 levels fell below 500 ppm. The result, "Science News" reports, was a "massive turnover in the type of mammals populating the continents-an upheaval that set the stage for the evolution of our ancestors."
While C4 vegetation now dominates the globe, C3 grasses still thrive in colder, wetter climates. But the world could be on the brink of another plant revolution. CO2 levels before the Industrial Revolution were 280 ppm. Today CO2 levels stand at 360 ppm and rising.
"By increasing atmospheric [CO2] concentrations, humans may be changing the Earth's atmosphere to conditions not favorable to a 'C4 world,' the world in which we originally evolved," Cerling states. "Science News" shares Cerling's concerns.
"Without massive cuts in greenhouse gas pollution --- ones that far exceed the limits adopted in Kyoto, Japan -- the concentration of CO2 is expected to climb above 500 ppm sometime in the latter half of the next century." When this happens, the world may shift back into a C3 world, "a regime [the Earth] has not seen in the last 8 million years."