Grass is a familiar part of everyday life, carpeting our lawns and filling vast fields, savannas, and prairies. This plant group plays a fundamental role in Earth’s ecosystems, supporting a wide array of life forms and contributing to the planet’s ecological balance. Understanding the deep history of grass reveals its enduring influence on the natural world.
What Exactly Is Grass?
From a botanical perspective, grass refers to a specific type of flowering plant belonging to the Poaceae family, also known as Gramineae. These plants are monocots, characterized by a single cotyledon in their embryonic stage. Grasses feature cylindrical stems, known as culms, which are hollow except at solid points called nodes where leaves attach. Their leaves are long and narrow with parallel veins, and the base of each leaf forms a sheath that wraps around the stem.
A distinctive characteristic of grasses is their ability to grow from the base of the blade, rather than from the tips, due to specialized growth tissues called basal meristems. This adaptation allows grasses to recover quickly from grazing or mowing, contributing to their resilience. Grasses also possess fibrous root systems that help anchor them firmly in the soil. Their flowers are small and inconspicuous, often arranged in structures called spikelets, and their fruit is a caryopsis, commonly known as a grain.
The Earliest Evidence of Grass
Scientific consensus indicates that grasses first appeared during the Late Cretaceous period, around 66 million years ago, coinciding with or just before the extinction event that ended the age of dinosaurs. Some research, involving microfossils, suggests an even earlier origin, pushing the timeline back to approximately 100 to 113 million years ago in the Early Cretaceous. These early forms of grass likely grew in shaded, moist environments, such as forest understories or marshy areas, rather than forming vast open grasslands.
Paleontologists rely on various forms of evidence to trace the ancient history of grass. Microscopic silica bodies called phytoliths are valuable, as these durable structures form within plant cells and retain distinctive shapes specific to different grass species, preserving well in the fossil record. Phytoliths have been found in fossilized dinosaur dung, known as coprolites, dating back 72 to 66 million years ago, indicating that even some large herbivorous dinosaurs consumed grass. Additionally, rare macrofossils, such as a 100-million-year-old grass spikelet encased in amber from Myanmar, provide direct visual evidence of ancient grass forms.
How Grass Spread Across the Continents
Following their initial appearance, grasses underwent significant diversification and geographical expansion, particularly after the extinction of the dinosaurs at the start of the Cenozoic Era. Grasses demonstrated an ability to adapt to diverse climates, thriving in open, sunny, and often harsh conditions where many other plant types struggled.
The spread of grasslands across continents was closely linked to global climate changes during the Cenozoic Era, as the Earth experienced a trend of cooling and drying. These changing conditions led to the shrinking of dense forests and the expansion of open, grass-dominated habitats such as savannas, prairies, and steppes. This ecological shift became prominent in North America around 26 million years ago, with similar transformations occurring later in Africa and Australia. The co-evolution between grasses and large grazing mammals further accelerated this expansion, as grazing pressure favored grasses, while the abundant grass provided a new food source that drove the evolution of specialized herbivores.
Grass’s Transformative Role in Earth’s History
The widespread emergence of grasslands profoundly reshaped global ecosystems. Grass-dominated habitats now cover over 40% of the planet’s land surface, excluding ice-covered regions. This expansion created new ecological niches, leading to the co-evolution and diversification of large herbivorous mammals, including animals like horses, zebras, and antelopes, which adapted to graze on this abundant food source. Grasslands also provide extensive ecological services, supporting rich biodiversity, offering habitats for countless species, and playing an important role in carbon storage within their root systems, water filtration, and soil erosion prevention.
The rise of grasslands also had a substantial, albeit indirect, influence on human evolution. The shift from forested environments to more open grasslands may have encouraged early hominins to adapt, potentially contributing to the development of bipedalism as a means to survey their surroundings for predators in tall grass. Much later, the impact of grass became direct with the advent of agriculture. Around 8,000 to 10,000 years ago during the Neolithic period, humans began domesticating various wild grasses, transforming them into staple cereal grains such as wheat, barley, rice, and maize. These domesticated grasses became the foundation of human civilization, providing more than half of the world’s dietary energy and enabling the development of settled communities and complex societies.