Life › The age of mammals
Grasslands spread
c. 20–6 million years ago· Event· Plants & fungi
Strong evidenceGood evidence; dates or details still being refined.
From about 20 million years ago, grass-dominated landscapes spread across the continents as the climate cooled and dried.
See it on the Life map →What happened
Grasses appeared in the late Cretaceous; fossil dung shows some dinosaurs ate them. But wide grass-covered landscapes spread only from about 20 million years ago, as the planet cooled and dried. Grasses grow from the base rather than the tip, so they regrow after grazing, drought and fire, which helped them displace forests in drier regions. Prairies, steppes and savannas formed across the Americas, Asia and Africa, and today cover roughly 30–40% of ice-free land, depending on definition.
Why it matters
Open grasslands drove the evolution of fast, long-legged grazers with tough teeth, and set the scene for human ancestors living outside dense forest.
The evidence (fossils, genes, rocks)
Microscopic silica particles from grass (phytoliths) in ancient soils, fossil pollen, carbon isotopes in soils and teeth, and the spread of high-crowned teeth in grazing mammals.
Still debated
Grasslands spread at different times on different continents; whether drying climate, falling carbon dioxide, fire or grazing animals mattered most is debated.
Inside this (2)
Connections (5)
▸Flowering plants include the grasses of Grasslands spreadWell established
The connection
Grasses are flowering plants, though their flowers are small and wind-pollinated. The grass family evolved in the Cretaceous, but grass-dominated landscapes spread only from about 20 million years ago, when cooler, drier climates favored plants that regrow from the base after grazing, drought and fire.
When and how
Grass origins in the late Cretaceous (phytoliths in Indian dinosaur dung, about 67–65 million years old); grasslands spread from the Miocene, about 20 million years ago.
The evidence
DNA places grasses among the flowering plants, close to sedges and rushes; fossil phytoliths and pollen trace their rise from scattered plants to dominant cover.
Debates and open questions
Why grasses took tens of millions of years to dominate, and how widespread they were in dinosaur times, are still open questions.
▸Antarctica freezes over helped dry the world for Grasslands spreadStrong evidence
The connection
Growing ice locked up water and cooled the oceans, which then evaporated less and supplied less rain. Over the following tens of millions of years, cooler, drier and more seasonal climates spread across the continents, especially after further cooling about 14 million years ago. Forests thinned, and grasses took over the drier ground.
When and how
Global, from about 34 million years ago, with a further cooling step about 14 million years ago; grasslands spread across the Americas, Africa and Asia.
How sure are we?
Ocean-core records of cooling line up broadly with fossil soils, pollen and phytoliths showing open habitats spreading, though not in lockstep.
Debates and open questions
Grasslands spread at different times on different continents, sometimes long after cooling steps, so the link is indirect and its details are debated.
▸Grasslands spread shaped the teeth and legs of HorsesWell established
The connection
Grass is tough and full of silica, and open ground is gritty, both of which wear teeth down. As grasslands spread, horses such as Merychippus, about 17–11 million years ago, evolved tall-crowned teeth that last a lifetime of grinding. Open country also favored speed: longer legs and a single hoof with springy ligaments for outrunning predators.
When and how
North America's Great Plains, about 20–5 million years ago, with later horse spreads into Asia, Europe and Africa across land bridges.
The evidence
Fossil horse teeth change from low to high crowns as grass phytoliths increase in the same rocks; tooth-wear studies show diets shifting to grass.
Debates and open questions
Timing does not always match: in South America, high-crowned teeth appeared before grasslands, so grit and volcanic ash may matter as much as grass itself.
▸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.
▸Grasslands spread may have favored Walking on two legsDebated
The connection
The classic 'savanna hypothesis' holds that as African forests broke up into open grassland, ancestors who walked upright could see farther, travel efficiently between patches of trees and stay cooler under the sun. On this view, upright walking began as an adaptation to open country.
When and how
East and Central Africa, about 7–4 million years ago; the idea dates to the early 1900s and was revived by Yves Coppens' 'East Side Story' (1994).
How sure are we?
Uncertain. Fossil soils, plants and animals suggest early hominins such as Ardi, 4.4 million years ago, lived in wooded settings, not open savanna, though some researchers dispute this.
Debates and open questions
Many researchers now think bipedalism began in mixed woodland, with open grasslands mattering more later, for Homo. How open early hominin habitats were is still argued.