In this brief poetic composition, Emily Dickinson celebrates the happiness of a simple, independent life, represented by a small stone wandering freely. Far from human worries and ambitions, the stone finds contentment in its elemental existence. Written in the 19th century, the poem reflects the growing disillusionment with industrialisation and urbanisation, which led many to aspire to a more modest and meaningful life.

How happy is the little stone
That rambles in the road alone,
And doesn’t care about careers,
And exigencies never fears;
Whose coat of elemental brown
A passing universe put on;
And independent as the sun,
Associates or glows alone,
Fulfilling absolute decree
In casual simplicity

Image: Altalena.

Covering nearly 10% of the Earth’s surface and weighing 130.000.000.000.000 tons—more than the entire ocean biomass—they revolutionised how we understand life and evolution. Few would probably bet on this unique yet discrete species: lichens.

Four hundred and ten million years ago, lichens were already there and seem to have contributed, through their erosive capacity, to the formation of the Earth’s soil. The earliest traces of lichens were found in the Rhynie fossil deposit in Scotland, dating back to the Lower Devonian period—that of the earliest stage of landmass colonisation by living beings. Their resilience has been tested in various experiments: they can survive space travel without harm; withstand a dose of radiation twelve thousand times greater than what would be lethal to a human being; survive immersion in liquid nitrogen at -195°C; and live in extremely hot or cold desert areas. Lichens are so resistant they can even live for millennia: an Arctic specimen of “map lichen” has been dated 8,600 years, the world’s oldest discovered living organism.

Lichens have long been considered plants, and even today many interpret them as a sort of moss, but thanks to the technical evolution of microscopes in the 19th century, a new discovery emerged. Lichen was not a single organism, but instead consisted of a system composed of two different living things, a fungus and an alga, united to the point of remaining essentially indistinguishable. Few know that the now familiar word symbiosis was coined precisely to refer to this strange structure of lichen. Today we understand that lichens are not simply formed by a fungus and an alga. There is, in fact, an internal variability of beings involved in the symbiotic mechanism, frequently including other fungi, bacteria and yeasts. We are not dealing with a single living organism but an entire biome.

Symbiosis’ theory was long opposed, as it undermined the taxonomic structure of the entire kingdom of the living as Charles Darwin had described it in On the Origin of Species: a “tree-like” system consisting of progressive branches. The idea that two “branches” (and, moreover, belonging to different kingdoms) could intersect called everything into question. Significantly, the fact that symbiosis functioned as a mutually beneficial cooperation overturned the idea of the evolutionary process as based on competition and conflict.

Symbiosis is far from being a minority condition on our planet: 90% of plants, for example, are characterised by mycorrhiza, a particular type of symbiotic association between a fungus and the roots of a plant. Of these, 80% would not survive if deprived of the association with the fungus. Many mammalian species, including humans, live in symbiosis with their microbiome: a collection of microorganisms that live in the digestive tract and enable the assimilation of nutrients. This is a very ancient and specific symbiotic relationship: in humans, the genetic difference in the microbiome between one person and another is greater even than their cellular genetic difference. Yet the evolutionary success of symbiotic relationships is not limited to these incredible data: it is the basis for the emergence of life as we know it, in a process described by biologist Lynn Margulis as symbiogenesis.

Symbiogenesis posits that the first cells on Earth resulted from symbiotic relationships between bacteria, which developed into the organelles responsible for cellular functioning. Specifically, chloroplasts—the organelles capable of performing photosynthesis—originated from cyanobacteria, while mitochondria—the organelles responsible for cellular metabolism—originated from bacteria capable of metabolising oxygen. Life, it seems, evolved from a series of symbiotic encounters, and despite numerous catastrophic changes in the planet’s geology, atmosphere and ecosystems across deep time, has been flowing uninterruptedly for almost four billion years.

Several scientists tend to interpret symbiosis in lichens as a form of parasitism on the part of the fungus because it would gain more from the relationship than the other participants. To which naturalist David George Haskell, in his book The Forest Unseen, replies, “Like a farmer tending her apple trees and her field of corn, a lichen is a melding of lives. Once individuality dissolves, the scorecard of victors and victims makes little sense. Is corn oppressed? Does the farmer’s dependence on corn make her a victim? These questions are premised on a separation that does not exist.” Multi-species cooperation is the basis of life on our planet. From lichens to single-celled organisms to our daily lives, biology tells of a living world for which solitude is not a viable option. Lynn Margulis described symbiosis as a form of “intimacy among strangers”: what lies at the core of life, evolution and adaptation.