Lichens first appeared about 400 million years ago so they have been around for a long time. Some individual species such as Hypogymnia physodes have been around for at least 25 million years and maybe for as long as 70 million years. Obviously the lichen symbiosis is a successful strategy. Lichens are scientifically fascinating, the symbiosis mentioned above is intriguing to many people and has been much studied though it is not yet fully understood. Lichens are also of great use to ecologists.
Many lichens show a remarkable sensitivity to air purity.
Some forms growing almost anywhere, others living in only the purist of atmospheres. For many ecologists lichens are an accurate monitor of quality of the air in any given place. You can do simple experiments on this on your own once you know how to identify a few species. Compare the diversity of lichens on a similarly aged members of a given tree species, at home, at school, in the local park and in the nearby countryside. Once you have the basic equipment there is much that they amateur lichenologist can do.