Grass is far older than most people assume, with its evolutionary timeline extending back into the age of the dinosaurs. This ancient history profoundly shaped Earth’s ecology and the evolution of life. Tracing this history reveals how a small, unassuming plant lineage evolved into the planet’s dominant vegetation type, ultimately setting the stage for human civilization.
Defining the Grass Family
What the average person calls “grass” is scientifically known as the Poaceae family, a massive group of monocotyledonous flowering plants. This family is one of the largest on Earth, encompassing approximately 12,000 species, including familiar forms like bamboo and cereal crops. Grasses thrive in diverse environments, from wetlands to deserts, and are found on every continent except for the most isolated parts of Antarctica. Plant communities dominated by the Poaceae family, such as prairies and savannas, cover over 40% of the Earth’s land surface.
The Earliest Fossil Evidence
The oldest direct evidence of grass places its origins firmly within the Late Cretaceous Period, between 66 and 70 million years ago. This dating was established through the discovery of microscopic silica bodies known as phytoliths, which are highly durable structures formed within the plant’s cells. Scientists found these grass phytoliths preserved within fossilized dung, or coprolites, belonging to large herbivorous titanosaurs in India. This finding demonstrated that grasses were already being consumed by dinosaurs just before the end-Cretaceous extinction event.
These early forms were not the vast, open fields seen today, but rather small, scattered plants growing beneath the forest canopy. The fossilized evidence shows the presence of at least five distinct grass taxa, including relatives of modern rice and bamboo. The existence of multiple grass lineages suggests the Poaceae family had already begun to diversify much earlier than previously thought. Grass spent its first millions of years as a minor component of the global flora.
The Rise of Global Grasslands
The massive expansion of grasslands into sprawling savannas and prairies began much later, during the Cenozoic Era, particularly in the Miocene Epoch around 20 million years ago. A global cooling and drying trend caused the vast tropical forests to shrink, opening up continental interiors to new types of vegetation. This environmental shift favored the Poaceae family, leading to the formation of open landscapes where grass was the dominant plant life.
A significant evolutionary event that drove this global dominance was the development of C4 photosynthesis, a more efficient process than the older C3 pathway. The C4 mechanism allows grasses to concentrate carbon dioxide, enabling them to thrive in hot, arid conditions with lower atmospheric carbon dioxide levels. This caused a rapid ecological shift, with C4-dominated grasslands expanding across four continents between roughly 8 and 4 million years ago. Fire also played a role, as grasses are more tolerant of burning than trees, encouraging the spread of these new, open ecosystems.
How Grass Shaped Mammal Evolution
The expansion of abrasive grass-dominated ecosystems created selective pressure on the mammals that fed on them, leading to profound biological changes. Grass contains tiny, jagged phytoliths that severely wear down the teeth of grazing animals. This gritty diet, often compounded by abrasive dust and soil ingested along with the grass, required a radical dental adaptation.
In response, many grazing mammals, including ancestors of modern horses, rhinos, and bovids, evolved a condition called hypsodonty. This trait features high-crowned teeth that extend deep into the jawbone, providing a larger surface area of enamel for the animal to wear down over its lifetime. The fossil record shows that the appearance of these specialized teeth correlates with the global spread of grasslands in the Miocene. This demonstrates a direct evolutionary feedback loop where the plant’s defense mechanism spurred a counter-adaptation in its primary consumers.
Modern Grasses and Human History
The evolutionary journey of grass culminated in an event that fundamentally altered the trajectory of human history: the Neolithic Revolution. Starting about 10,000 years ago, humans began domesticating wild grasses, transforming them into foundational cereal crops. These domesticated forms included wheat and barley in the Near East, rice in Asia, and maize in the Americas. The ability to cultivate these high-yield, storable grains created a reliable food surplus. This surplus allowed human populations to settle in one place, leading directly to the development of permanent villages, specialized labor, and complex, stratified societies.