Microbe Evolution (IMAGE) Princeton University Caption Princeton University researchers report that the distribution of forest types worldwide is based on the belowground relationships plant species forged with soil-dwelling microbes such as fungi and bacteria to enhance their uptake of nutrients, particularly nitrogen and phosphorus. Fungi and bacteria release nutrients through soil decomposition. In return, the microbes thrive on the carbohydrates that plants provide from photosynthesis. Above is a timeline of the plant-symbiont relationship on land from the Devonian geologic period (left) to the Cenozoic (right), Earth's current era. Roots appeared in the fossil record 413 million years ago (a). Plant-fungal symbiosis (b) developed roughly 407 million years ago. Around 60 million years ago, nitrogen "fixing" bacteria known as rhizobia (d) began infecting plant root cells, converting nitrogen in the air to fertilizer in exchange for carbohydrates. (Image courtesy of Mingzhen Lu, Department of Ecology and Evolutionary Biology) Credit Mingzhen Lu, Princeton Ecology and Evolutionary Biology 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.