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Never Home Alone

From Microbes to Millipedes, Camel Crickets, and Honeybees, the Natural History of Where We Live

By Rob Dunn

More books on Fauna

In wealthy countries, cases of chronic inflammation double every 20 years since 1950. In last 20 years in US, allergies have incr 50% and asthma by a third. Suggestion that not bc of a pathogen, but bc people's immune systems hadn't been exposed to them, and so primed.

The larva of the bearded lacewing live in termite nests, where it stuns termites with "a vaporphase toxicant from it's anus" (and then eats them.

Camel crickets (common inhabitant of American basements) can digest lignin in alkaline baths (the 'black water' waste product of paper making).

Vineyard yeasts over-winter in the guts of wasps, then in spring the wasps carry the yeast from grape to grape. Long before humans started making beer and wine, the wasps were the original yeast habitat.

Drug companies spent millions searching tropical rain forests for miracle drugs. But an expensive mistake. They had the process back-to-front. Instead of random searches, they shd have started with what they knew about existing species and used that to predict which ones most likely to have particular uses.

King Tut was buried with a fly swatter, so his subjects obviously believed that, whatever else the afterlife held, there wd also be houseflies.

When a German cockroach finds itself out in the wild, it is weak. Either gets eaten or it starves to death. Asa result, there are no wild populations of G cockroaches anywhere. They much prefer the same conditions we like - warm, not too dry, not too wet.

Insecticides kill most of a cockroach's competitors, leaving just the resistant cockroaches to prosper. This pattern also applies to bed bugs, head lice, house flies and mosquitoes. Basically, spraying creates an enemy-free zone for them.

Antibiotic-resistant bacteria are poor competitors. They survive only in environments that are so chronically difficult that no other life-forms do well. The compounds produced by their genes that confer resistance are expensive - they use up resources that wd otherwise be used to grow and divide. But if their are no competitors, this slower, dearer lifestyle doesn't matter.

At least 50% French people infected with toxiplasma gondii. 40% Germans, 20% US.

First breads were made with barley, which has little gluten and so cannot trap bubbles of CO2. Egyptians then learned to use emmet wheat to make leavened bread, and we see the chnge in pyramid decorations - flat bread becomes round loaves.

You can make your own starter for sourdough bread. Just put a bowl of flour and water outside and microbes will colonise it. Keep feeding it flour and water and you'll end up with a bubbly, sticky, acidic mix. The more acidic (vinegary) it becomes, the fewer pathogens will survive.

Washing hands doesn't remove microbes. They form a glove-like living carpet that is still there when sampled before and after hand-washing. Rather, what washing does is remove any new microbes before they get a chance to establish.

Baker's hands completely different to microbe popn in most people. Skewed with yeast from the bread they shape.


The good news is that I will never be home alone again. The bad news - well, it's not in fact bad news, but it is slightly unsettling - is that I share my home with at least 5,000 other species: wasps, flies, spiders, silverfish and an exotic bunch of wild bacteria.

All that information is apparently contained in a patch of grey dust I have just swabbed with my right index finger from a door frame in my living room. It's like a DNA test of my house, says Rob Dunn, a 43-year-old American biologist who has come to my house in Copenhagen to hunt microbial life. He carries no lab gear and his blue crewneck jumper and striped Oxford shirt are hardly the combat suit of an exterminator. But with every discovery we make, with every spider we find lurking in the corner or each swab of dust, he displays an almost childlike sense of excitement. He swears and smiles, even whoops with delight: "This dust sample contains bacteria, your body microbes, your wife's body microbes, your child's body microbes. If you smoke weed we would find marijuana DNA in there. Everything is visible, but it's also present in every breath. Every time you inhale, you inhale that story of your home."

Dunn is to house insects and indoor bacteria what Marie Kondo is to neatly folded shirts. He wants us to study the wildlife in our homes and realise that what we discover should spark much more joy than fear. When he began working as a biologist he went to the jungle to study wild beasts, but now his research is dedicated to species much closer to home: to the flies, spiders and bacteria hidden in every nook and cranny of our kitchens, bathrooms and basements. To the 'jungle of everyday life', as he describes it in his new book.

Never Home Alone tracks how we have been disconnected from the ecosystems of our homes. It's a book of hard truths - I now know that I shed 50m flakes of skin every day, providing food for thousands of bacteria, and that cockroaches are basically our perfect interspecies Tinder-match. It also confronts our irrational relationship with cleanliness. Our modern instinct might be to swat a spider on the kitchen worktop or blitz creepy crawlies into oblivion with antimicrobial sprays, but we could be killing useful allies, according to Dunn: The key thing is that your life is going to be full of life. And your only choice is which life. Our default is that we try to kill everything and fill our houses with stuff that’s totally terrible for us. We might kill 99%, but that leaves 1% = and that 1% is never the good stuff.

Dunn is in Copenhagen for work and has agreed to come to my home to go through dusty corners and spider webs to point out where I might find some of the 5,000 species I bunk with. I suspect our 110-year-old house, shared with another family, is a fertile hunting ground, and Dunn seems optimistic. Using a screwdriver and tweezers, he pokes at light fittings and sifts through the basement, which can be a mould-friendly hangout during sticky, wet summers.

While I'm making coffee, Dunn lets out a yelp: "Oh yes, this is good!" He has spotted a globe-shaped lamp hanging in the basement hallway, and in the bottom of the hazy glass cover is a Pollockesque pattern of dead wildlife. He tips the contents of the lamp on to a fold-out table and uses the tweezers to organise the harvest: two types of fly, a wasp, a meal moth, some aphids and various planthoppers. He takes a closer look at two of the dead flies: "I can tell they are likely to be the same species, but I don't know for sure until I look at their genitals."

He turns to the silver-shiny meal moth. "Isn't it just beautiful? It moved in with humans in ancient Egypt and has moved with humans again and again."

But how would it have ended up in our home?

"This meal moth could have come in with grains."

Like from a box of cereal?


Dunn wants us to see our homes the way we see our gardens. There are pests and pathogens we need to control - those that make us sick - but we also want to preserve diversity. He says fewer than 100 species of bacteria, protists and viruses cause nearly all infectious diseases today. We try to keep these in check with vaccinations and antibiotics, and by washing our hands. But that leaves us with a jungle of tens of thousands of other species, many of which we know little about, but which Dunn's work suggests can often be more beneficial than harmful. For example, studies have shown that a higher diversity of bacteria on people's skin - bacteria linked to soil and plants - can reduce the risk of allergies. He also mentions the discovery of a bug that could be a new source of useful enzymes: by testing the microbes in the gut of the camel cricket, found in many American homes, Dunn's research team discovered bacteria that were able to break down industrial waste and turn it into energy. Making 'cool discoveries' like this and enlisting the public to sieve through their homes for wildlife has spurred interest in a field he says has been neglected for too long.

"The first drawings of microscopic life are all of household species," says Dunn, "but once we figured out the germ theory of disease it shifted, and both scientists and the public started to think that what was indoors belonged to pest control. Ecologists went to the Galapagos, we went to the rainforest, but it pushed us away from the home and it left us with this huge blind spot."

Dunn grew up in rural Michigan, hunting snakes and turtles, building tree forts and being more curious about nature than your average child. Today, he is a professor in the department of biological sciences at North Carolina State University, where he has spent the past decade involving his colleagues and the public in studying the microbial landscape of our bodies and our homes. Part of the thrill of his new book is the sense of discovery and engagement with the world outside the lab. We head deeper into the basement, to the water-heating system, where one of the pipes has a leak; water drips into a chalk-crusted yellow bowl. Dunn grabs the bowl and tips it on to its side so we can see the puddle: "You could start a sourdough loaf with this." The water heater is an example, he says, of the extreme conditions we create in our houses. These specific ones are similar to what you might find in an Icelandic geyser, which means that our basement heating system now attracts bacteria that would normally thrive in volcanic hot springs. He moves on to a sprawling mess on the wall next to the boiler. "That's a lovely spider web," he says excitedly. I feel a strange sense of pride and relief that my lack of cleaning is potentially boosting the ecosystem I inhabit. "This kind of spider is super-common in houses and can live for many, many years. This can grow up with your kid."

I'm not sure I'm ready for an in-house pet spider, but while our natural inclination might be to shriek or stamp them out, Dunn wants us to protect them. "The natural enemies of the pests in our homes are very often, whether you like it or not, spiders," he writes in Never Home Alone: "If you kill the spiders in your home (and this is precisely what we do with many kinds of pesticide applications), you do so at your own expense."

The curse of pesticides brings us to the bathroom and another key area of his research: shower heads. Dunn and his team asked people from across the US and Europe to send in swabs from their shower heads. And within the biofilm - 'a fancy word for the gunk' that builds up inside them - they discovered a pathogen, nontuberculous mycobacteria (NTM), that is linked to lung disease in people with weak immune systems. What they didn't know was why this pathogen seemed more prevalent in some regions than others. "There was more of the NTM in the US and in particular in chlorinated water," says Dunn. They came to the conclusion that "residual chlorine kills all the competing bacteria and just leaves the NTM, which are chlorine-intolerant." And when the competition is eliminated, the pathogens thrive.

He unscrews the shower head from the hose and runs his finger along the rubber fitting. There is very little biofilm-gunk, which he credits to the quality of the local water supply. "Most of Copenhagen still has untreated groundwater that relies on the wild biodiversity of crustaceans and bacteria to clean it. But if you break that - like we do in the US - it's super-expensive to fix it and it sucks. And it makes you sick."

We continue through the living room and kitchen. He pulls a book from the shelf to test for book lice, and picks up a log of firewood that has a spidery, grey pattern of reindeer lichen growing on the bark. In the kitchen he checks the dishwasher soaptray for a particular form of bacteria that is otherwise only found in the faeces of tropical fruit bats. Even our salt jar is apparently full of life: Almost every crystal of salt has bacteria inside, and when you brine something in salt, those bacteria contribute to the flavours of the brine. The longer it sits in the brine, the more the microbes in the brine are going to contribute to it. Isn't that cool?

Food is an area where Dunn has found the influence of microbes and bacteria to be more palatable to people he meets. Food is alive - whether it's sourdough bread, kimchi or beer - and we are cool with that. Much more so than with the spider crawling across our windowsill. Dunn describes a study of how specific microbes found on bakers' skin influence sourdough starters and the flavour of bread. His team gathered 15 bakers at a facility in Belgium and tasked each of them with making sourdough starters from identical ingredients. When they tested the bakers' hands afterwards, it showed a close relationship between the microbial makeup of each starter and the bacteria found on the skin of the baker who made it. In his book Dunn describes how the bakers and biologists, drunk on spontaneously fermented Belgian beer, tucked into the breads and broke into an impromptu toast: 'To bread, and to microbes!' And to a house in which both are delicious. 'To bread and to microbes!' And houses in which we are all healthy. 'To bread and to microbes!' And to lives filled with wild species we have yet to study or understand, species that float like mysteries all around us and offer services we are only beginning to measure.

Our house tour and inspection has come to an end. I'm curious about the professor's verdict. Is this a happy, fungi-rich, insect-thriving home? "You have a lot of species that I think of as being part of a healthy house," Dunn says. "To me, your house is rich in biodiversity and seems dominated by those things that you either shouldn't worry about, or that are beneficial. Also, I don't see antimicrobial products all over the place."

He points to the leftovers of a coffee kombucha-glazed Danish pastry we were eating earlier. "And then you have things that spark joy for you: fermented pastries, salt that's alive and healthy water from Denmark that we know is alive."

Whether we like it or not, the species living in our homes are a measure of our lives, says Dunn, and his biggest fear is that we risk killing off that biodiversity. If spiders scare the hell out of you, but you are willing to have one living in the corner, and maybe approach it sometimes to keep an eye on it, then that's great. That's much better than if your response is to jump away from it and spray pesticides all over your home, because that will only favour species that cause us harm and are resistant to pesticides. But it's not easy. This book wont solve [the problem], but maybe it will open some new conversations.

As he says goodbye and walks down the garden path, I spot a tiny moth in our living room, flapping frantically in the corner. I don't freak out or necessarily feel any great joy, but I leave the moth be. After all, there are 5,000 of us here, and you need to give your housemates some privacy.


Picture Rob Dunn approaching his publisher: "I have an idea for a book about all the tiny insects, bacteria, and other creepy-crawly things that live in our homes." Then picture agents, editors, publishers and the general reading public, for that matter, shuddering and saying, "Nope."

But we should be grateful that someone saw that 'Never Home Alone: From Microbes to Millipedes, Camel Crickets, and Honeybees, the Natural History of Where We Live' would not only be utterly fascinating, but also turn out to be an important voice in the science-based argument in favor of more biodiversity.

That said, if you can read through the catalogue of things that invade our homes without wanting to get a hose full of bleach and power-wash every surface you see, you're a stronger person than I. And if you can read about ancient humans picking lice off one another, or a cholera epidemic caused by an old diaper in a well, and then eat lunch, you have a stronger stomach than I.

But if you put aside the queasiness, you'll find that Never Home Alone is a spirited romp through the vast diversity that inhabits our daily lives and how we've changed our ecosystems, often for the worse. Dunn and his colleagues estimate that they've found about 200,000 species inside our homes. A 'ferocious diversity' of fungi, for instance, lives in our air conditioning systems, on the hands of bakers, on clothing, in drywall, on wallpaper and in our water systems.

Don't be put off by the idea of wading through the microbes, viruses and microscopic living creatures that populate this book and our world. It's not hard to love a scientist who partially justifies pursuing this line of research with the quip, "What the heck, I had tenure." This is a guy who never talks about excretion. He talks about poop.

Dunn's main theme, which he unrolls enthusiastically and colorfully, is this: "If ecologists have learned anything in the last hundred years, it is that when you kill species but leave the resources upon which they feed, the tough species not only survive but thrive in the vacuum created by the death of their competition." What he means by "killing the species" is our aggressive overuse of anti-bacterials, antibiotics and other poisons, which has led to the growth of organisms that can survive these poisons and lead to worse infections.

This book will upend much of what we assume about the world around us. A few examples:

* Treated tap water has more bacteria than well water.

* The showerhead is a nasty place: Water that is both warm and pooled in the showerhead creates a swamp of potentially dangerous bacteria falling on our heads.

* Some bacteria in our systems might be able to reduce stress.

* Fungi will probably colonize Mars before humans do because they've been known to survive on 'sterile' space stations.

* That funny smell when you turn on your AC in your house or car is the odor of fungi exhaling.

* Fungi hidden inside perfectly new and dry drywall might be connected to causes of Parkinson's disease.

* Having high levels of fungi in homes doesn't necessarily cause more allergies or asthma.

* Parasites in our systems, such as toxoplasma gondii transmitted by cat feces, can change our personalities, possibly even causing mental illness like schizophrenia.

* All adults have face mites. (An immediate effect of that factoid, in my case, was to cause me to Google 'how to get rid of face mites' and rush to the medicine cabinet to see if our household had any tea tree oil.)

* One of the main causes of tapeworms in humans is letting dogs lick our faces.

"With bacteria, we haven't even scratched the surface," Dunn writes. But he is certainly scratching like crazy.

Dunn, a professor of applied ecology at North Carolina State University and at the Natural History Museum of Denmark, realized early in his career just how many unanswered questions there were in our understanding of the biological world. "I started to wonder," he writes, "whether this phenomenon - of assuming someone else knows - is likely to be more common in homes than in other habitats, more common because homes are the place we are most likely to assume that someone else knows, most likely to assume that everything is more or less under control."

The answer is yes, we do assume. And no, everything is not under control. "An entire world of animal life in plain view had been missed," Dunn says, and is living on our windowsills, in the fur of our pets, inside our kitchen drains. "We know so little even about the animals around us," he writes, "that we can't preclude the possibility that some of the biggest discoveries yet to be made are those right where we wake up every morning."

The story of cockroaches is only one of the compelling narratives. German cockroaches, Dunn writes, evolved to be repulsed by glucose because so many traps attracted cockroaches with glucose and then poisoned them. In fact, he writes, "resistance to our pesticides has evolved among bed bugs, head lice, house flies, mosquitoes, and other common insects in houses."

Dunn's advice: We need to let biodiversity back into our world and homes. In his case, he says, he plans to leave his windows open more often, get out in nature as much as he can and continue to study the kaleidoscope of natural life that enhances our world.

(NY Times)

A young man I know used to say that humans have no need to bathe, since our hair and bodies are designed to self-cleanse. I would fight him on it, being of the opinion that washing up occasionally was good for us - and for the people with whom we lived. But now, after reading the entomologist Rob Dunn's description of the myriad microbial life-forms that take up residence in a typical American showerhead, I'm starting to think maybe that young man was onto something.

With an army of collaborators, Dunn, a professor of applied ecology at North Carolina State University, took samples of the gunk inside hundreds of showerheads, and found a profusion of microbial fauna. Tap water itself, he writes in the chatty, informative 'Never Home Alone,' teems with amoebas, bacteria, nematodes and crustaceans. As the water passes through the showerhead, these microbes lay down a kind of scaffolding known as biofilm to protect themselves from getting washed away with every ablution. They make the biofilm out of their own excretions, Dunn writes bluntly. "In essence, by working together, the bacteria poop a little indestructible condominium in your pipes."

It gets worse. Filtered through that poop-biofilm, the water that washes over you, as you supposedly scrub yourself clean, might contain not only all those harmless amoebas and nematodes but a few bacteria that can be dangerous - in particular some species of Mycobacterium, cousins of the Mycobacterium that causes tuberculosis. And the pathogens are there because we provided the perfect breeding ground for them, when we tried to purify our tap water in the first place. Municipal water treatment plants use chlorine and other chemicals that kill off the bacterium's natural predators, allowing Mycobacterium to thrive. Tap water that comes from a well, in contrast, has never gone through a treatment plant and has a rich microbial life. It might look more dangerous, but it's actually safer, Dunn explains. All those organisms in well water are themselves harmless, and they tend to fight off the potentially dangerous ones like Mycobacterium - that's how biodiversity works.

News from the showerhead biome is just one part of this fact-filled, occasionally disgusting, slightly alarming book. Dunn has been involved in an obsessive quest to document the tiny inhabitants of indoor environments, a project that involves teams of professional and amateur bug-watchers to take samples not only from showerheads but from door frames, refrigerators, hot water heaters, cellars, toilets, pillowcases, all sorts of surfaces from the places we call home. These workers swab and seal, swab and seal, and send their thousands of samples to Dunn's lab in Raleigh, or to his other lab at the Natural History Museum of Denmark, for an ongoing microbial census.

"We expected to find a few hundred species," Dunn writes of his first foray into indoor microbe-hunting, which involved 1,000 homes from around the world. Instead, he and his colleagues found a "floating, leaping, crawling circus of thousands of species," perhaps as many as 200,000, many of them previously unknown to science.

These denizens of interior spaces are our most frequent companions. In the industrialized world, we spend upward of 90 percent of our time indoors. Luckily, most of our co-habitators are either benign or actually beneficial in some way, like the house spiders that keep down indoor populations of flies or mosquitoes that can carry disease. But because we've become so hyper about making our surroundings as pristine as possible - sealing off our homes from the outdoors and using pesticides and antimicrobials with a vengeance - we've tipped the scales away from those harmless or helpful bugs, in favor of some of the bad guys.

According to Dunn, indoor microbes are among the fastest-evolving species on the planet; they have an uncanny ability to live in ecological niches you could hardly imagine existing, like the dead skin cells we slough off every day (which is all that's needed to survive for a class of bacteria known as detritivores). They manage to evade our assaults, and evolve their way out of just about every biocide we throw at them. We're left to contend with the consequences of our own warfare, such as pesticide-resistant German cockroaches and bedbugs, and antibiotic-resistant MRSA bacteria. We have turned a relatively harmless indoor biome into something that can make us sick.

That's the take-home message of Never Home Alone, that the richer the biodiversity in our indoor environment, the better. "The biodiversity of plants and soil can help our immune systems function properly," Dunn writes. "The biodiversity in our water systems can help keep pathogens in the water in check. The biodiversity of spiders, parasitoid wasps and centipedes can help control pests. The biodiversity in our houses provides the opportunity, too, for the discovery of enzymes, genes and species useful to all of us, whether to make new kinds of beers or to transform waste into energy."

I'm not quite as enamored of our microbial roommates as the author is. (I'm sorry, Professor Dunn, but I'm just being honest here: Those photos of the camel cricket and the American grass spider clinging to basement pipes and door thresholds - well, yuck.) It probably takes the soul of an entomologist, or maybe of a 9-year-old child, to love these bugs as much as Dunn does. Still, it's hard not to be occasionally charmed by his prose, as when he catalogs the arthropods with whom we share our homes: "biting midges, mosquitoes, lesser house flies, phantom midges, freeloader flies and shore flies. This is not to mention fungus gnats, moth flies and flesh flies. Or crane flies, winter crane flies and minute black scavenger flies." And it's hard not to share, at least a little, his awe at their diversity, even in a single household. "If you see two flies in your home," he writes, "the odds are that they are two different species. Heck, if you see 10 flies in your house, they are likely to be five different species."

And don't even get him started on the aphids. He's amazed by them, as he is by the wasps that lay their eggs in the bodies of aphids as well as the wasps that lay their eggs in the bodies of the wasps that lay their eggs in the bodies of aphids.

There's a real sense of gee-whiz in this book, but it's mostly in service of Dunn's overarching goal: to preach the preservation of biodiversity, not only in the lush forests and streams that fit our traditional image of nature's abundance, but in the most humble places, too, where the vast majority of us will have most of our cross-species encounters - our basements, mattresses, refrigerator drawers and showerheads.

About those showerheads: Dunn isn't suggesting that we give up showers. But he does say we might want to change the showerhead a little more often - and consider switching from metal to plastic, where biofilm is less likely to accumulate. Nonetheless, his bottom line for showerheads is like his bottom line for other aspects of the roiling microbial mix we live in: Don't be afraid of letting life inside. "The water that is healthiest for bathing is that which comes from aquifers rich with underground biodiversity including crustaceans," he writes. "The crustaceans in these aquifers are an indication not of the dirtiness of the water but of its health."

(Irish Times)

In 1969, the microbiologist Thomas Brock reported a surprising discovery. He'd been studying a strange rust-coloured encrustation around the hot springs of Yellowstone National Park that turned out to be a species of bacteria. Thermus aquaticus, as he named it, was thriving at temperatures of up to 80 degrees, much higher than scientists had thought possible. It was to become a celebrated discovery, but what is less well known is that Brock went on to find Thermus aquaticus in rather less exotic locations. Specifically, he found it coming out of the hot tap at his faculty building, and related species have since been identified in domestic water heaters all over the United States.

Such stories, the ecologist Rob Dunn believes, have a clear moral. In Never Home Alone, he assembles an amiable and often fascinating miscellany of similar curiosities, forming a freewheeling survey of what might be termed domestic ecology. His aim, it seems, is not just to uncover the biological richness of the typical household, but to persuade us of its importance as a site of investigation.

For many of us, biodiversity is a vaguely nice thing that happens outdoors. It may come as a surprise, then, to learn that our houses are not just homes but also habitats, and habitats that may well be teeming with life. To begin with, there are almost certainly hundreds of species of invertebrates living in your house, and possibly several thousand. Some of these (like spiders and flies) will be familiar presences, but most (including countless unseen mites and lice) are probably going unnoticed. And the diversity of these populations is remarkable. As Dunn notes: 'If you see two flies in your home, the odds are that they are two different species.'

If you think that's impressive (or distressing), just wait until you hear about our live-in microbes. In a survey of a thousand homes across the United States, Dunn and his team identified some 80,000 species of bacteria and archaea. Most of these, he assures us, are likely to be harmless, or even beneficial. Many are found in our homes because they cover our entire bodies, patiently disposing of dead skin cells or devouring pathogens that would otherwise make us sick. But it will take decades, Dunn admits, to study even this modest sample.

It may well be worth the effort. Thermus aquaticus, for instance, turned out to be much more than a curiosity. DNA sequencing, a technology that has become indispensable to modern biology and medicine, relies on two processes. DNA's double helix structure is broken down using heat, then the separated strands are copied (to produce a sample sufficient for study) using an enzyme called polymerase. Living cells use their own polymerase to do the same job, but the stuff in most cells is destroyed by the heating process. For Thermus aquaticus, though, these conditions are positively balmy. As a result, it is at the heart of virtually every genetics test being done in the world.

Even greater discoveries may lie ahead, Dunn believes. Among the countless moulds and encrustations around our houses are dozens of species of Penicillium, the fungus that gave rise to the first antibiotics. But our antibiotics are failing, increasingly overwhelmed by resistant bacterial strains that decades of overuse have invited. Their successors may come from creatures like the thief ants found in kitchens, whose secretions can kill bacteria resembling methicillin-resistant Staphylococcus aureus, more commonly known as MRSA.

Perhaps the most persuasive argument to emerge from these essays is that such promising discoveries are not as unlikely as they appear. Organisms have wondrous abilities, Dunn points out, because they need them to survive in highly specific conditions. This means that ecologists, who puzzle out these accommodations for a living, have a unique head start in the race to find new marvels. But while we may well nod along to all this, it is surely Dunn's scientific colleagues who need to be convinced.

Still, Dunn does have some guidance to offer the rest of us, if hedged a little by scientific caution. Of the tens of thousands of species you're sharing digs with, most are likely to be harmless and many are probably doing you good. You don't need to ratchet up your household cleaning regime, and in fact, you probably shouldn't. Many cleaning products end up killing things well beyond your home, and your immune system needs to meet bacteria (especially those from outdoors) to learn what's really threatening. By explaining all this, Rob Dunn hopes to instil in us at least a tentative sense of wonder. Failing that, he suggests, we and our multitudinous lodgers should all just try to get along.

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