Lichen is something we commonly see growing on rocks or tree branches, on old wood fences and rotting stumps. But how often do you stop to really ponder lichens? Probably not often. And yet lichens are surprisingly fascinating ... and weird ... and beautiful!

Despite their looks, lichens aren't plants. Nor are they in the fungus family. They're a unique composite organism, the result of a symbiotic relationship of organisms from as many as three kingdoms, with the main partner being fungus. As Lichens of North America puts it, "The lichen fungi (kingdom Fungi) cultivate partners that manufacture food by photosynthesis. Sometimes the partners are algae (kingdom Protista), other times cyanobacteria (kingdom Monera), formerly called blue-green algae. Some enterprising fungi exploit both at once." In 2016, a new study published in Science revealed that in addition to fungus and algae that lichens also include yeast. This yeast appears in the lichen cortex and contains two unrelated fungi. Lichens are their own kind of being.

They are also incredibly abundant, found everywhere from temperate forests to icy cold tundra, from the tropics to the deserts. They are the dominant vegetation on as much as 8 percent of the land on Earth, able to survive where many other plant species don't stand a chance.

Already lichen seems far more complex than you probably realized. And this is only the beginning of the story.

Lichens grow across the surface of granite rocks Lichens grow across the surface of granite rocks. (Photo: Gregory A. Pozhvanov/Shutterstock)

Lichen species are able to survive in some amazingly extreme environments. "Lichens grow in the leftover spots of the natural world that are too harsh or limited for most other organisms," according to the Lichens of North America website. "They are pioneers on bare rock, desert sand, cleared soil, dead wood, animal bones, rusty metal, and living bark. Able to shut down metabolically during periods of unfavorable conditions, they can survive extremes of heat, cold, and drought."

It's interesting to think of lichen as "pioneers," but they are in a way. They exist by bringing together two or more life forms that need each other to thrive. In doing so, they create more abundant life where it wouldn't normally be found — essentially colonizing new frontiers and inviting other species to grow in otherwise barren areas. They are also self-sufficient. They do not feed off the surface over which they grow, as parasites do, but instead create their own food through photosynthesis using the algae from which they are partially made.

Rust-colored lichen Jewel lichen, also called rock jewel lichen, is a type of crusty lichen able to grow in harsh areas, even on the surface of rocks. (Photo: gubernat/Shutterstock)

If lichen is part fungus and part algae, what exactly is lichen? The main body of a lichen is called a thallus. Based on that, lichen species are categories in three main categories: crusty, leafy and shrubby. Some other forms, including squamulose, filamentous and gelatinous types are recognized, but mostly they fall under the three umbrella categories. So even if you don't know what species you're looking at, you'll at least be able to tell if it is crusty, leafy or shrubby in appearance.

Cladonia lichens Cladonia lichens, also known as cup lichens, along with miniature moss grow on vertical rock in Karelian forest, Russia. (Photo: Gregory A. Pozhvanov/Shutterstock)

British Soldier lichen British Soldier lichen is a species of cup lichen. The red part of the thallus is the part that makes spores, allowing reproduction. (Photo: Kim Foote/Shutterstock)

Dog lichen Dog lichen grows among mosses and other lichens in Scotland. (Photo: Jan Holm/Shutterstock)

Reindeer moss Reindeer moss is a type of shrubby lichen that grows in many places, especially across the Arctic tundra. (Photo: /Shutterstock)

It's not terribly surprising when lichen is confused for moss, especially when the name is the first thing to mislead. Reindeer moss or caribou moss is a lichen, one that is a major food source for caribou. It is also sometimes used as food or natural remedies for humans for kidney stones or even antibacterial ointments. In fact, there's a long history of lichens being used by humans for various purposes, and they are still being studied for their medicinal value. According to Ohio State University, "Research with lichens around the world is suggesting these organisms hold promise in the fight against certain cancers and viral infections, including HIV."

Meanwhile, lichens have long been used as a natural pigment for dying cloth and wool. They're also dried and used in art, particularly in the construction scale models by architects to railroad enthusiasts. You may very well have used lichen in your own school homework when crafting scale models of farms, missions or towns for class projects.

Cup lichens Cup lichens look like a tiny forest of trees. (Photo: PHOTO FUN/Shutterstock)

Yellow lichen Lichen has been used as a pigment for dyes for centuries. It's not hard to see why when you see this kind of vivid colors. (Photo: LFRabanedo/Shutterstock)

Lichens are incredibly slow-growing — we're talking millimeters or less per year for many species. But with slow growth comes longevity, and as is usually the case with slow-growing organisms, they are some of the oldest living things on the planet. In her book "The Oldest Living Things," Rachel Sussman documents map lichens in Greenland that are between 3,000 and 5,000 years old.

To ward off the dangers of being a stationary organism in a moving world, lichens have developed an incredible array of defenses, including "an arsenal of more than 500 unique biochemical compounds that serve to control light exposure, repel herbivores, kill attacking microbes, and discourage competition from plants," according to the Lichens of North America site. "Among these are many pigments and antibiotics that have made lichens very useful to people in traditional societies."

That longevity is threatened, however. According to UC Berkeley, "The most serious threat to the continued health of lichens is not predation, but the increased pollution of this century. Several studies have shown serious impacts on the growth and health of lichens resulting from factory and urban air pollution. Because some lichens are so sensitive, they are now being used to quickly and cheaply assess levels of air toxins in Europe and North America."

Crusty lichens Crusty lichens can look like little more than a hard film growing across a surface (Photo: Worraket/Shutterstock)

Multi-textured and multi-colored lichen Taking on many textures and colors, lichens can be an exceptionally beautiful part of an ecosystem. (Photo: Sarah2/Shutterstock)

Yellow lichen on a tree. Lichens have many uses, not the least of which is teaching us more about the health of our environment and the potential role they play in human medicine (Photo: Kurkul/Shutterstock)

Editor's note: This story was originally published in March 2015 and has since been updated with new information.

Jaymi Heimbuch ( @jaymiheimbuch ) focuses on wildlife conservation and animal news from her home base in San Francisco.