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Kindle Notes & Highlights
by
Ed Yong
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September 2 - September 5, 2020
Before October, almost every living thing on the planet consisted of single cells. They would have been invisible to the naked eye, had eyes existed. They had been that way ever since life first emerged, some time in March.
the photosynthetic bacteria in the oceans produce the oxygen in half the breaths you take, and they lock away an equal amount of carbon dioxide.
there are more bacteria in your gut than there are stars in our galaxy.
For roughly the first 2.5 billion years of life on Earth, bacteria and archaea charted largely separate evolutionary courses. Then, on one fateful occasion, a bacterium somehow merged with an archaeon, losing its free-living existence and becoming entrapped forever within its new host. That is how many scientists believe eukaryotes came to be.
There’s a huge void between the simpler cells of bacteria and archaea and the more complex ones of eukaryotes, and life has managed to cross that void exactly once in four billion years.
Other complex structures, from eyes to armour to many-celled bodies, have evolved on many independent occasions but the eukaryotic cell is a one-off innovation.
The latest estimates suggest that we have around 30 trillion human cells and 39 trillion microbial ones – a roughly even split. Even these numbers are inexact, but that does not really matter: by any reckoning, we contain multitudes.
They are so small that, despite their numbers, they collectively weigh just a few pounds in total. A dozen or more would line up cosily in the width of a human hair. A million could dance on the head of a pin.
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.
Your cells carry between 20,000 and 25,000 genes, but it is estimated that the microbes inside you wield around 500 times more.9 This genetic wealth, combined with their rapid evolution, makes them virtuosos of biochemistry, able to adapt to any possible challenge.
Speaking of palms, your right hand shares just a sixth of its microbial species with your left hand.
Many conditions, including obesity, asthma, colon cancer, diabetes, and autism, are accompanied by changes in the microbiome, suggesting that these microbes are at the very least a sign of illness, and at most a cause of it.
mammalian milk contains special sugars that infants cannot digest, but that certain microbes can. When a human mother breastfeeds her child, she isn’t just feeding it; she is also feeding the child its first microbes, and ensuring that the right pioneers settle inside its gut.
the squid and other animals tell us that development is more than this. It progresses using instructions in an animal’s genes, but also in the genes of its microbes. It is the result of an ongoing negotiation – a conversation between several species, only one of which is doing the actual developing. It is the unfolding of an entire ecosystem.
They affect the storage of fat. They help to replenish the linings of the gut and skin, replacing damaged and dying cells with new ones. They ensure the sanctity of the blood–brain barrier – a web of tightly packed cells that lets nutrients and small molecules pass from blood to brain, but bars the way to larger substances and living cells. They even influence the relentless remodelling of skeletons, in which fresh bone is deposited and old stuff is reabsorbed.
The traditional view of the immune system is full of military metaphors and antagonistic lingo. We see it as a defence force that discriminates self (our own cells) from non-self (microbes and everything else), and eradicates the latter. But now we see that microbes craft and tune our immune system in the first place!
Drinking lots of alcohol makes the gut leakier, allowing microbes to more readily influence the brain – could that help to explain why alcoholics often experience depression or anxiety?
The brain parasite Toxoplasma gondii is another puppetmaster. It can only sexually reproduce in a cat; if it gets into a rat, it suppresses the rodent’s natural fear of cat odours and replaces it with something more like sexual attraction. The rodent scurries towards nearby cats, with fatal results, and T. gondii gets to complete its life cycle.
Our gut microbes are different. They are natural parts of our lives. They help to construct our bodies – our gut, our immune system, our nervous system. They benefit us. But we shouldn’t let that lure us into a false sense of security. Symbiotic microbes are still their own entities, with their own interests to further and their own evolutionary battles to wage. They can be our partners, but they are not our friends.
The very term symbiosis has been twisted so that its original neutral meaning – “living together” – has been infused with positive spin, and almost flaky connotations of cooperation and harmony. But evolution doesn’t work that way. It doesn’t necessarily favour cooperation, even if that’s in everyone’s interests. And it saddles even the most harmonious relationships with conflict.
The immune system’s main function is to manage our relationships with our resident microbes. It’s more about balance and good management than defence and destruction.
The average human swallows around a million microbes in every gram of food they eat.
Today, the majority of living mammal species eat plants, and most orders have at least some herbivorous members. Even the Carnivora – the order that includes cats, dogs, bears, and hyenas – count the bamboo-eating pandas among their number.
This is what medicine looks like when you understand that microbes are not the enemies of animals, but the foundations upon which our kingdom is built. Say goodbye to dated and dangerous war metaphors, in which we are soldiers hell-bent on eradicating germs at whatever cost. Say hello to a gentler and more nuanced gardening metaphor. Yes, we still have to pull out the weeds, but we also seed and feed the species that bind the soil, freshen the air, and please the eye.
Add a supposedly beneficial microbe, and it might displace competitors that we also rely on. Lose a supposedly harmful microbe, and an even worse opportunist might rise to take its place.
There’s a popular saying among doctors: There’s no such thing as alternative medicine; if it works, it’s just called medicine.
Even paracetamol (acetaminophen), one of the most familiar drugs in the world, is more effective in some people than others because of the microbes they carry.
Every person aerosolises around 37 million bacteria per hour. This means that our microbiome isn’t confined to our bodies.
In the developing world, around 5 to 10 per cent of people who check into hospitals and other healthcare institutions pick up some kind of infection during their stay,
