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C4 grasses
spread c. 8–6 million years ago· Evolutionary innovation· Plants & fungi
Strong evidenceGood evidence; dates or details still being refined.
Grasses with a turbocharged form of photosynthesis that took over tropical grasslands about 8–6 million years ago.
See it on the Life map →What happened
Most plants use C3 photosynthesis, named for the three-carbon molecule it first makes. In heat, drought and low carbon dioxide it wastes energy. C4 plants first pump carbon dioxide into special cells around the leaf veins, concentrating it so the key enzyme works efficiently. The trick evolved independently more than 60 times; in grasses it appeared about 30 million years ago. Between about 8 and 6 million years ago, C4 grasses spread across tropical and subtropical grasslands worldwide.
Why it matters
With only about 3% of plant species, C4 plants supply roughly a quarter of plant growth on land; they include maize, sugarcane, sorghum and millet.
Think of it like
Like a turbocharger: C4 plants compress carbon dioxide before it reaches the engine, so they keep running in hot, thin air.
The evidence (fossils, genes, rocks)
Carbon isotopes in fossil soils and the tooth enamel of grazers in Pakistan, East Africa and the Americas shift sharply about 8–6 million years ago (key study 1997).
Still debated
Why C4 grasses expanded so late, long after they evolved, is debated: falling carbon dioxide, drier seasons, more fire or grazing animals.
Connections (2)
▸Grasslands spread were later dominated by C4 grassesStrong evidence
The connection
The first grasslands, from about 20 million years ago, were mostly C3 grasses, like today's wheat and many lawn grasses. Between about 8 and 6 million years ago, C4 grasses, better at handling heat, drought and low carbon dioxide, took over tropical and subtropical grasslands, building the savannas of Africa, South Asia and the Americas.
When and how
Tropical and subtropical regions, including Pakistan's Siwalik Hills, East Africa and the Americas, about 8–6 million years ago.
How sure are we?
Carbon isotope ratios in fossil soils and grazing-animal tooth enamel shift toward C4 plant values at that time (Thure Cerling and colleagues, 1997).
Debates and open questions
Whether falling carbon dioxide, drier seasons, more frequent fire or grazers drove the takeover is debated; older carbon dioxide records showed no clear drop then, but newer ones suggest a decline.
▸C4 grasses include maize, a staple of Agriculture beginsWell established
The connection
Maize descends from teosinte, a wild C4 grass of southern Mexico, domesticated about 9,000 years ago. Sorghum, sugarcane and the foxtail and broomcorn millets, early staples of northern China and Korea, are C4 grasses too. Their efficient photosynthesis in heat makes them highly productive; maize is now the world's largest grain crop by tonnage.
When and how
Balsas River valley, southwestern Mexico, about 9,000 years ago for maize; millets in northern China about 10,000–8,000 years ago; sorghum in Sudan about 5,000 years ago.
The evidence
Maize genetics, notably John Doebley's work, traces it to Balsas teosinte; maize starch on 8,700-year-old grinding stones from Xihuatoxtla rock shelter confirms early use.
Debates and open questions
How teosinte, with a few hard-cased kernels, became large-eared maize, and how much later crossing with a highland teosinte in Mexico mattered.