Grass is the most ubiquitous and ecologically significant plant group on Earth, covering roughly 40% of the planet’s land area. This vegetation is the basis of vast natural ecosystems like prairies and savannas, and it also forms the foundation of human civilization. Tracing where grass comes from requires examining its ancestry across geological epochs, from its emergence among dinosaurs to its co-option by human agriculture. The journey reveals a plant family whose success is rooted in unique anatomical defenses and evolutionary adaptation.
Defining the Poaceae Family
Grass belongs to the plant family Poaceae, which encompasses approximately 12,000 species across nearly 800 genera. True grasses are distinguished by unique structural features that set them apart from similar-looking plants like rushes and sedges.
Their stems, called culms, are typically hollow between solid joints known as nodes, providing flexibility and strength. Grass leaves are long and narrow with parallel venation, and the lower part forms a sheath that wraps tightly around the stem.
A unique adaptation is the location of the growth tissue, or meristem, at the base of the leaf blade rather than the tip. This basal growth point allows the plant to quickly recover and regrow after being grazed or mowed, which contributed greatly to their survival alongside grazing animals. Unlike most flowering plants, grass flowers are inconspicuous, lacking bright petals because they rely on efficient wind pollination.
The Deep Evolutionary Timeline
The earliest evidence for the Poaceae family pushes its origin deep into the Mesozoic Era, coinciding with the Age of Dinosaurs. Scientific findings indicate that the first grasses evolved during the late Early Cretaceous period, roughly 113 to 101 million years ago. The oldest direct evidence comes from microfossils called phytoliths, which are tiny silica bodies that form within grass cells and preserve well in the fossil record.
These ancient microfossils have been found within the fossilized dung, or coprolites, of herbivorous dinosaurs in India dating back to the Late Cretaceous period. Phytoliths extracted from the teeth of a hadrosauroid dinosaur in China also confirmed the presence of basal grass lineages around 113 million years ago.
This early diversification suggests that by the time of the dinosaur extinction event, grasses already included relatives of modern groups like rice and bamboo. However, these early grasses were likely small, shade-tolerant plants restricted to the understory of forests or along waterways. They had not yet formed the wide-open, continuous grasslands that characterize the modern world.
Grasslands and Ecological Dominance
The transformation of grass from a minor forest plant to an ecological powerhouse occurred during the Cenozoic Era, millions of years after the dinosaurs vanished. The widespread expansion of open grasslands began during the Late Paleogene and exploded in the Neogene period (approximately 23 to 2.6 million years ago). This dominance was driven by a global shift toward drier, cooler climates and a significant drop in atmospheric carbon dioxide levels.
A major evolutionary innovation was the development of C4 photosynthesis, a highly efficient metabolic pathway that evolved multiple times in grasses starting as early as 32 million years ago. C4 plants are far more efficient at utilizing carbon dioxide and water than their C3 relatives, granting them an advantage in warm, arid environments. The global expansion of C4 grasses, particularly during the Late Miocene (8 to 3 million years ago), led to the formation of the world’s vast savannas, steppes, and prairies.
This ecological shift was reinforced by the co-evolution of grass with grazing mammals, such as early horses, bison, and antelope. Grazing animals helped create the modern grassland biome by consuming and trampling woody shrubs and trees, preventing them from outcompeting the grasses. This continuous disturbance selected for grass species that could tolerate heavy grazing due to their basal growth points. This dynamic explains the geographical distribution of many modern natural grasslands.
Grasses in the Human Era
In the last 10,000 years, humans became the primary force in determining the global distribution of grass species. The domestication of cereal grasses during the Neolithic Revolution fundamentally changed the course of human history and agriculture. This period saw the transformation of wild grasses into the “Big Three” crops that now provide over half of the world’s calories.
Wheat and barley were first domesticated in the Fertile Crescent of the Near East approximately 11,000 years ago, providing the staple grains that fueled the earliest Western civilizations. Rice cultivation emerged independently in Asia, primarily in China, around 9,000 years ago. Corn, or maize, was domesticated from its wild relative, teosinte, in Mesoamerica, roughly 6,000 to 8,000 years ago.
The spread of these domesticated cereals meant that specific grass species were carried far beyond their original homelands and cultivated across every habitable continent. Modern distribution is further influenced by the global use of turfgrasses for lawns and sports fields, representing another human-driven migration of grass varieties into non-native urban environments. Ultimately, the contemporary “where” of grass is a map of human settlement and agricultural necessity.