I Contain Multitudes: The Microbes Within Us and a Grander View of Life

by Ed Yong

All zoology is really ecology. We cannot fully understand the lives of animals without understanding our microbes and our symbioses with them. And we cannot fully appreciate our own microbiome without appreciating how those of our fellow species enrich and influence their lives. We need to zoom out to the entire animal kingdom, while zooming in to see the hidden ecosystems that exist in every creature.

Most microbes are not pathogens. They do not make us sick. There are fewer than 100 species of bacteria that cause infectious diseases in humans;8 by contrast, the thousands of species in our guts are mostly harmless. At worst, they are passengers or hitchhikers. At best, they are invaluable parts of our bodies: not takers of life but its guardians. They behave like a hidden organ, as important as a stomach or an eye but made of trillions of swarming individual cells rather than a single unified mass.

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Endnote missing.

Here is a strange but critical sentiment to introduce in a book about the benefits of living with microbes: there is no such thing as a “good microbe” or a “bad microbe”. These terms belong in children’s stories. They are ill-suited for describing the messy, fractious, contextual relationships of the natural world.

Rohwer puts it plainly: “Even though coral reefs are incredibly complex, microbes are the main determinants of [their] health and decline.”

Recall that every individual animal, whether human or coral, is an ecosystem in itself.

In 1976, a paediatrician named John Gerrard noticed a peculiar pattern of diseases among the people of Saskatoon, the Canadian city that he had called home for twenty years. The city’s white population was more likely to get allergic diseases like asthma, eczema, and hives than the indigenous Metis communities, while the latter were more often infected by tapeworms, bacteria, and viruses. Gerrard wondered if those trends were connected, if allergic disease “is the price paid by some members of the white community for their relative freedom from diseases due to viruses, bacteria and [worms]”.

The Extended Phenotype, Richard Dawkins

Mitochondria certainly count: as we’ve seen, these cellular batteries were once free-living bacteria that became permanently enclosed within a larger cell. This process, known as endosymbiosis, was first proposed in the early twentieth century, but it only became accepted several decades later, largely thanks to the outspoken American biologist Lynn Margulis. She turned endosymbiosis into a coherent theory, which she expounded in a genre-hopping paper that contained an impressive mix of evidence from cell biology, microbiology, genetics, geology, paelaeontology, and ecology. It was a bravura piece of scholarship. It was also rejected around 15 times before seeing print in 1967.21

Margulis was dismissed and ridiculed by her peers, but she gave as good as she got. Rebellious, and contemptuous of dogma, she was the consummate scientific iconoclast. “I don’t consider my ideas controversial,” she once said. “I consider them right.” She was certainly right about mitochondria and chloroplasts, but thanks to other oversold claims, she is often viewed with both utmost respect and cautious scepticism.

In 1927, the American Ivan Wallin described symbiosis as an “engine of novelty”. He argued that symbiotic bacteria transformed existing species into new ones, which was the fundamental means through which new species arose. Lynn Margulis echoed his views in 2002, claiming that the creation of new symbioses between distinct organisms – which she called symbiogenesis – has been the main force behind the origin of new species. To her, the kinds of relationships you’ve seen so far in this book were not just pillars of evolution, but its very foundations. She failed to make her case, though. She listed plenty of examples of symbiotic microbes that led to important evolutionary adaptations, but, crucially, presented almost no evidence that they actually gave rise to new species, much less that they are the principal force behind those origins.

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David Quammen

Song of the Dodo,

Helen Macdonald’s H is for Hawk,

David George Haskell’s The Fo...

Kathryn Schulz’s Bei...

two biographies: Antony Van Leeuwenhoek and His “Little Animals” (Dobell, 1932), The Cleere Observer. (Payne, 1970).

Margulis, L., and Fester, R. (1991) Symbiosis as a Source of Evolutionary Innovation: Speciation and Morphogenesis (Cambridge, Mass: The MIT Press).

Margulis, L. and Sagan, D. (2002) Acquiring Genomes: A Theory of the Origin of Species (New York: Perseus Books Group).