LIFE × CLIMATE

The Spread of Grasslands

~10 to 3 million years ago

Phanerozoic · Cenozoic · Neogene · Miocene

Tropical grasses adopted a different way of running photosynthesis, one that works better when carbon dioxide is scarce and the air is hot and dry. Botanists call it C4. Between roughly ten and three million years ago these grasses took over the warm continents, woodland giving way to open savanna, with longer fire seasons helping them along.

Why it matters

Open country is the stage on which hominins became walkers and later hunters, so the date matters to us. It has just moved: eastern African sites show C4 grasses locally abundant as early as twenty-one million years ago, more than ten million years before anyone expected them. When the grass arrived and when it took over turn out to be two different questions.

Dating & uncertainty

Grasses using C4 photosynthesis became ecologically dominant between roughly 10 and 3 Ma, but eastern African evidence published in 2023 shows locally abundant C4 grasses as early as 21 Ma, so origin and dominance are separate questions.

Sources

1. Edwards et al. (2010), Science · The origins of C4 grasslands: integrating evolutionary and ecosystem science
2. Cerling et al. (1997), Nature · Global vegetation change through the Miocene/Pliocene boundary
3. Uno et al. (2016), PNAS · Neogene biomarker record of vegetation change in eastern Africa
4. Peppe et al. (2023), Science · Oldest evidence of abundant C4 grasses and habitat heterogeneity in eastern Africa

Earth history as one day

23:56:493 min 10 s before midnight

if Earth’s 4.54 billion years were compressed into a single day