c. 30–20 Million BCE

Advanced C₄ Photosynthesis

Photosynthesis, the conversion of sunlight into energy for cellular metabolism, was one of life’s earliest evolutionary innovations, around 3.4 billion years ago. In essence, atmospheric carbon dioxide (CO₂) and water vapor (H₂O) react in the presence of sunlight to produce glucose (C₆H₁₂O₆) and free oxygen (O₂) as byproducts. Cells then process the glucose as fuel. This process is referred to as C₃ carbon fixation. A thick, O₂-poor early atmosphere with ample volcanically provided CO₂ made for optimal conditions for early life forms to exploit plentiful sunlight as a fuel source.

The appearance and rapid proliferation of the first land plants around 470 million years ago began to dramatically decrease the amount of CO₂ in the atmosphere, making C₃ carbon fixation less efficient over time. Simple, single-celled photosynthetic organisms didn’t really care much (and still don’t) because their energy needs are modest, and more than 95 percent of modern plants that rely on C₃ carbon fixation grow in regions where sunlight intensity and temperatures are moderate and groundwater is plentiful. However, for plants to grow in sunnier, hotter, and dryer environments requires higher photosynthetic efficiency, because their energy needs still must be met regardless of decreasing atmospheric CO₂ levels.

Steadily decreasing atmospheric CO₂ levels between about 470 and 300 million years ago thus provided significant pressures on some plants to become more efficient photosynthesizers. A variety of plants did just that, starting around 30 to 20 million years ago, by developing a new scheme to harness the Sun’s energy called advanced C₄ photosynthesis. Plants using this process have evolved specialized photosynthetic cells within their leaves that more effectively trap and concentrate CO₂ and water for eventual delivery to chloroplast cells, where higher rates of “normal” C₃ photosynthesis then get carried out using more CO₂, even though the atmosphere outside the leaf has less CO₂ over time.

C₄ plants have advantages over C₃ plants during droughts, heat waves, or low CO₂ conditions. While they represent less than 5 percent of the plant species today, they account for almost 25 percent of the fixation of atmospheric CO₂, making them a critical sink for the biosequestration of CO₂, which would otherwise further warm our current climate.

SEE ALSO Photosynthesis (c. 3.4 Billion BCE), The Great Oxidation (c. 2.5 Billion BCE), First Land Plants (c. 470 Million BCE), Flowers (c. 130 Million BCE)

Close-up of a leaf, the primary site of photosynthesis in plants.