Setting the scene

Fungi are a keystone component of all ecosystems on earth and have shaped the structure and functioning of nature for eons. Their body is made up of an interwoven mass of threadlike filaments, individually called hyphae and altogether known as mycelium. When fungi form spore-forming structures, the so-called mushrooms, these are also built up of hyphae. The three known trophic groups of fungi have had a fundamental functional diversity in the development of life as we know it. The so-called saprotrophs use the complex dead materials of plants, animals, and microorganisms, including other fungi, as their source of energy—playing a crucial role in nutrient recycling in nature. Mycorrhizal fungi, which establish symbiotic relationships with the roots of plants, have gone hand in hand with plants in the colonization of life on land environments since more than 400 million years. Ectomycorrhizal fungi, in particular, are currently essential in maintaining forest masses worldwide since they establish mutualistic relationships with trees and shrubs mainly. According to the recently named Read’s rule, they dominate ecosystems with low mineralization rates in high-latitude regions with cold and dry climates. Furthermore, they have also been considered the earth’s natural internet, or ‘wood wide web’, because they connect plants and enable them to share nutrients, water, and signal compounds among individuals and species. These networks also help stocking carbon in organic forms from the atmosphere, contributing to climate regulation. Some fungi also establish parasitic relationships with plants and animals, working as an evolutionary force and a selection pressure factor of paramount importance in these groups of living organisms. On top of this ecological and evolutionary relevance, the reproductive structures of fungi, the mushrooms, have shapes and colours that have always fascinated humans. Since early human history, mushrooms have also been an important source of food, medicine, and ceremonial use, all around the world. They also cause death or disease, since deadly and poisonous species exist. Nowadays, they are an important source for the search of new antibiotics, enzymes with industrial use, bioremediation, biofuels, cosmetics, inks, and dyes. In this introductory chapter, we will first describe some remarkable ecological facts related to mushrooms, including members of the three trophic groups previously mentioned. Then, we will provide evidences of the ancient relationships between mushrooms and humans; and, finally, we will analyse the relationships between mushrooms, humans, and nature in different parts of the world, describing and illustrating different realities in five continents