C4 photosynthesis in grasses
Some grasses evolve the C4 pathway, a more efficient way to fix carbon in hot, dry, low-CO2 conditions. C4 grasses later spread into vast tropical grasslands.
The story
Most plants capture carbon dioxide with an enzyme called Rubisco, a method known as C3 photosynthesis. Rubisco has a flaw: it sometimes grabs oxygen instead of carbon dioxide, wasting energy, and it does so more as temperatures rise. Around 30 million years ago, some grasses evolved a workaround called C4 photosynthesis. They first capture carbon dioxide in outer leaf cells with a different enzyme, then pass it to inner cells where it is concentrated around Rubisco. The extra step costs energy, but it pays off in heat, bright light and low carbon dioxide.
C4 photosynthesis arose independently more than 60 times across flowering plants, including about two dozen times in grasses. It stayed uncommon until the Late Miocene, about 8 to 6 million years ago, when C4 grasses spread across tropical and subtropical regions as carbon dioxide levels were low and many regions were growing drier. Today C4 plants make up only a few percent of plant species but account for roughly a quarter of photosynthesis on land. The group includes maize, sugarcane, sorghum and millet, major crops that feed billions of people.
Why it mattered
- Savannas spread in Africa and elsewhere as C4 grasses took hold in hot, dry places.
- Human ancestors in East Africa lived amid these expanding grasslands, and some species fed on C4 plants.
- Crops such as maize, sugarcane and sorghum rely on C4 photosynthesis, which makes them productive in warm climates.
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