text: plants have a competitive advantage over plants possessing the more common  carbon fixation pathway under conditions of drought, high temperatures, and nitrogen or  limitation. When grown in the same environment, at 30 °C,  grasses lose approximately 833 molecules of water per  molecule that is fixed, whereas  grasses lose only 277. This increased water use efficiency of  grasses means that soil moisture is conserved, allowing them to grow for longer in arid environments.
   carbon fixation has evolved in at least 62 independent occasions in 19 different families of plants, making it a prime example of convergent evolution. This convergence may have been facilitated by the fact that many potential evolutionary pathways to a  phenotype exist, many of which involve initial evolutionary steps not directly related to photosynthesis.  plants arose around  during the Oligocene (precisely when is difficult to determine) and were becoming ecologically significant in the early Miocene around .  metabolism in grasses originated when their habitat migrated from the shady forest undercanopy to more open environments, where the high sunlight gave it an advantage over the  pathway. Drought was not necessary for its innovation; rather, the increased parsimony in water use was a byproduct of the pathway and allowed  plants to more readily colonize arid environments.
  Today,  plants represent about 5% of Earth's plant biomass and 3% of its known plant species. Despite this scarcity, they account for about 23% of terrestrial carbon fixation. Increasing the proportion of  plants on earth could assist biosequestration of  and represent an important climate change avoidance strategy. Present-day  plants are concentrated in the tropics and subtropics (below latitudes of 45 degrees) where the high air temperature increases rates of photorespiration in  plants.