Many evolutionary paths
Fortunately, studying the evolution of C4 plants from their C3
ancestors suggests that making the necessary changes may
not be as difficult as it seems.
Probably because of shift in the balance of CO2 released by
volcanism and carbon being buried in sediments, atmospheric
CO2 concentrations fell dramatically from about 1200 ppm 35
million years ago, to a low of around 200 ppm 20,000 years ago.3 As
CO2 concentrations fell, the problem of photorespiration
increased and so did the advantage of C4 photosynthesis. This
drove the evolution of C4 species, which now dominate several
ecosystems, for example, many types of grassland.