Erika Edwards, Brown University (IMAGE) Brown University Caption Brown University biologist Erika Edwards has uncovered the origin of a grouping of grasses that now account for up to one-fifth of the vegetational covering on Earth. Rainfall, not temperature, was the primary trigger around 30 to 40 million years ago. Credit No credit needed. Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.