One-celled algae colored the surface waters of the oceans shades of green. Some algae joined together into multicellular organisms and formed what scientists think were the ancestors of modern plants. Plants are essential to life on earth. Either directly or indirectly, they are the primary food source for humans and other animals. Whether people eat plants or eat animals that feed on plants, plant life is vital as a food source.
Plants play many other essential roles.
They are the major producers of oxygen on this planet. All plant life, from the tallest redwood tree to the smallest plankton works to produce oxygen. The largest living things on earth are the giant sequoia trees of California while other plants living on the forest floor are so tiny they can barely be seen. Plants are also the oldest living things. One bristlecone pine tree in California started growing 4,000 to 5,000 years ago.