Grassland and savanna plants — The World, with the Evidence

Old painting of a landscape in which Aboriginal people use fire to hunt kangaroos. — Aboriginal people using fire to hunt kangaroos, painting by Joseph Lycett (1817). Joseph Lycett, Public domain, source

Some of the world’s great grasslands and savannas have been maintained by humans, using fire, for a very long time.

They are therefore not exactly “natural” environments in the usual sense of the term — which changes the way we should regard them.

Fire as a tool

Our page on food and agriculture establishes a fact that is worth recalling here: hunter-gatherers shaped their environment with controlled fire.¹

The process is simple and effective:

An area is burnt at the right season to clear away scrub and old dry grass.
Tender shoots appear soon afterwards, far more nourishing for wildlife.
The herbivores come, drawn by the freshness of the plant cover.

This is not luck but a technique passed on, refined and practised over generations.²

What this does to words

We commonly use two verbs to describe the life of hunter-gatherers: to wander and to gather.

Both suggest the absence of any plan: one wanders for lack of a destination, one gathers whatever one finds. Yet burning an area at the right season to attract game requires knowing where one is, what will grow, when to return and why.

These verbs deny them an obvious intentionality. It is a typical case of linguistic bias: a word that decides while believing it merely describes.

How old are grasslands?

The question seems simple, but it calls for three distinct answers, or else one goes wrong:

When did grasses appear? At least 66 million years ago, and perhaps more than 100 million according to genetic analyses. We know this in particular from phytoliths (grains of silica made by plants) found in fossilised dinosaur droppings in India.
When did they dominate the landscape? Much later. In North America, during the Eocene and Oligocene, open-habitat grasses were still only rare or marginal members of forest communities; they did not dominate there until the Late Oligocene or Early Miocene.
When were today’s great grasslands born? Only between 8 and 3 million years ago.³

Existing, dominating and forming a landscape are three stages separated by tens of millions of years.

A revised timeline

It was thought that tropical grasses — those described as C4 — had become dominant in Africa only after ten million years ago. The problem: older plant records were sparse, so the hypothesis remained impossible to test.⁴

In 2023, a team filled this gap by combining several independent methods at nine sites in East Africa.⁵

The results show that between twenty-one and sixteen million years ago, these grasses were already locally abundant. The oldest known date is thus pushed back by more than ten million years — in Africa, and worldwide.⁶

But the most interesting result is not the date. These environments were not uniform: they contained forests as well as wooded grasslands, side by side. A mosaic landscape, not a switch from one setting to another.⁷

The authors draw a conclusion that goes beyond botany: we need to revise the interpretations previously given of mammalian evolution — including our own. Many accounts rested on the idea that our ancestors left a forest to enter a savanna. If the two already coexisted, the question has to be framed differently.⁸

A methodological bias

The fossil record of grasses is very poor for most of the Cenozoic. All the dates obtained are therefore minimum dates: a date in palaeontology indicates how far back a fossil is attested, not the exact moment of its appearance.⁹

The self-sustaining loop

The spread of grasslands is explained by the fall in atmospheric CO₂, marked seasonality and the role of fire:

Fire kills young trees but spares the base of the grasses. This favours the spread of grasses, which produces more fuel, which in turn feeds future fires.¹⁰

⚠ Note: some 20 million years elapse between the fall in CO₂ and the actual spread of grasslands. This lag remains partly unexplained.¹¹

How does grass survive damage?

After fire or herbivores have passed through, how does grass grow back so quickly?

Unlike a tree, whose growth zone (the apical meristem) is high up, the meristem of grasses lies at or near ground level. Grazing the top of the plant does not destroy its ability to grow.¹²

The role of large herbivores

Grasslands do not depend on fire alone. Large herbivores graze, trample, open up paths and carry seeds. Climate alone does not explain these environments: where forests could grow, fire, herbivores or soils hostile to trees can keep them open.¹³

Consequences of fire-suppression policies and conversions

The systematic banning of traditional bush fires in the twentieth century often had the opposite effect to the one intended: the vegetation grew back denser, and fires that used to stay on the ground climbed up into the tree crowns. In addition, the large-scale conversion of grasslands into farmland has fragmented these ecosystems, reducing their species diversity.¹⁴

Note 1

Aboriginal burning in Australia has long been assumed to be a "resource management" strategy. […] Anthropogenic landscapes contain a greater diversity of successional stages than landscapes under a lightning fire regime, and differences are of scale, not of kind.

Bliege Bird, Rebecca et al., “The "Fire Stick Farming" Hypothesis: Australian Aboriginal Foraging Strategies, Biodiversity, and Anthropogenic Fire Mosaics”, Proceedings of the National Academy of Sciences, vol. 105, 2008, p. 14796-14801

See the full note ↓

Note 2

Fire has influenced the way Australian Indigenous people live on, with and through their land for millennia. Over this time, Indigenous Australians have skilfully used fire to adaptively manage their local environments. […] For Indigenous people this material significance is complemented by a cultural and symbolic significance that is passed on from generation to generation.

Cranney, Kate, “Three Practical Ways to Support Indigenous Landscape Burning in Australia”, CSIRO, 25 February 2020

See the full note ↓

Note 3

Silicified plant tissues (phytoliths) preserved in Late Cretaceous coprolites from India show that at least five taxa from extant grass (Poaceae) subclades were present on the Indian subcontinent during the latest Cretaceous.

Prasad, Vandana, Strömberg, Caroline A. E., Alimohammadian, Habib and Sahni, Ashok, “Dinosaur Coprolites and the Early Evolution of Grasses and Grazers”, Science, vol. 310, 2005, p. 1177-1180

Analyses of these assemblages show that open-habitat grasses had undergone considerable taxonomic diversification by the earliest Oligocene (34 million years ago) but that they did not become ecologically dominant in North America until 7–11 million years later (Late Oligocene or Early Miocene). […] The Eocene and Oligocene phytolith record, which suggests that open-habitat grasses occurred as rare or marginal members of forest communities, contrasts strongly with this scenario.

Strömberg, Caroline A. E., “Decoupled Taxonomic Radiation and Ecological Expansion of Open-Habitat Grasses in the Cenozoic of North America”, Proceedings of the National Academy of Sciences, vol. 102, no. 34, 2005, p. 11980-11984

The evolution of grasses using C4 photosynthesis and their sudden rise to ecological dominance 3 to 8 million years ago is among the most dramatic examples of biome assembly in the geological record.

Edwards, Erika J. et al., “The Origins of C4 Grasslands: Integrating Evolutionary and Ecosystem Science”, Science, vol. 328, 2010, p. 587-591

Silicified plant tissues (phytoliths) preserved in Late Cretaceous coprolites from India show that at least five taxa from extant grass (Poaceae) subclades were present on the Indian subcontinent during the latest Cretaceous.

Prasad, Vandana, Strömberg, Caroline A. E., Alimohammadian, Habib and Sahni, Ashok, “Dinosaur Coprolites and the Early Evolution of Grasses and Grazers”, Science, vol. 310, 2005, p. 1177-1180, paywalled

The estimated age of the Poaceae inferred herein range from 107Ma (new fossils excluded) to 129Ma (preferred placement of fossils), compared with published ages of ~70–95Ma (refs 16–18,40).

Prasad, Vandana et al., “Late Cretaceous Origin of the Rice Tribe Provides Evidence for Early Diversification in Poaceae”, Nature Communications, vol. 2, 480, 2011

See the full note ↓

Note 4

C4 grasses are thought to have become ecologically dominant in Africa only after 10 million years ago (Ma). However, paleobotanical records older than 10 Ma are sparse, limiting assessment of the timing and nature of C4 biomass expansion.