Pollen Study Reveals Grasses' Hidden History

There is a particular kind of history that is written in dust—not the dust of archives or old books, but the dust of pollen, the tiny grains that plants release into the air and that settle, layer by layer, into the sediments of lakes and bogs. For scientists who study ancient ecosystems, pollen is a kind of time capsule, preserving a record of what grew where and when, and how the landscape changed over thousands of years. A new study of grass pollen, published in the journal Science, has used that record to reveal something surprising about the evolutionary history of one of the most important plant families on Earth.

Grasses—the family Poaceae—are everywhere. They cover approximately 40 percent of the planet's land surface, they include the major cereal crops that feed the world, and they dominate ecosystems from prairies to savannas to lawns. But their evolutionary history has been difficult to reconstruct, in part because grass pollen is fragile and does not preserve well in the fossil record. The new study used a different approach: it examined the DNA of modern grass species to reconstruct the evolutionary relationships among them, and then used that tree to estimate when different lineages diverged.

The results suggest that grasses originated much earlier than previously thought. The study's authors estimate that the grass family arose approximately 100 million years ago, during the Cretaceous period—the same era when dinosaurs roamed the Earth. This is significantly earlier than the 60 to 80 million years that earlier estimates had suggested. The finding implies that grasses were present during the age of dinosaurs, and that they may have been a component of the vegetation that dinosaurs ate. The study also found that the major subfamilies of grasses diverged from one another relatively quickly, in a burst of diversification that occurred around the time of the Cretaceous-Paleogene extinction event, 66 million years ago—the same event that killed the dinosaurs.

What makes the study particularly significant is its methodology. The researchers used a technique called "phylogenomics," which compares the entire genomes of different species rather than just a few genes. This approach provides a more robust estimate of evolutionary relationships and divergence times. The study also benefited from the increasing availability of genomic data for grasses, including the complete genomes of major crops like rice, wheat, and maize.

The implications of the study extend beyond evolutionary biology. Understanding when grasses originated and how they diversified helps explain the ecology of ancient landscapes and the evolution of the animals that depended on them. It also has practical implications for agriculture: knowing the genetic relationships among grasses can help plant breeders develop new varieties that are more resilient to drought, disease, and climate change. The pollen record, it turns out, is not just a record of what grew. It is a record of what survived, and how.

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Sources: Science, National Science Foundation, Botanical Society of America