Some species have shared our homes for millennia (Image: Alex Wild/Visuals Unlimited/Corbis)
From cockroaches and bedbugs to fungi and microbes, your home is crawling with life. It has evolved with us for millennia, and now its fate is in our hands
THE German cockroach lives in buildings, nowhere else; outdoors it dies. Exactly when Blatella germanica threw in its lot with humans is unknown, though it is generally thought to be African in origin and may initially have inhabited caves, as many modern roach species do. Wherever it arose, it spread around the world, following our ancestors everywhere they went, being a pest, causing a nuisance. Then, in the early 1990s, it came to the attention of Jules Silverman in the city of Raleigh, North Carolina.
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An entomologist by training, Silverman’s job was to develop chemicals to kill German roaches. For a decade, the best product on the market had been roach baits – enticing sugar treats laced with pesticides. Now something had gone wrong. The baits were becoming increasingly ineffective, and Silverman needed to find out why. Discovering that the roaches in his lab still succumbed to the pesticide, he thought the simplest explanation was that their housebound counterparts had learned to avoid sugar, and were passing the lesson on from one to another. But Silverman found that newborn roaches separated from their mothers weren’t attracted to the baits either.
This left only one possibility. In 1993, Silverman published a paper concluding that the German cockroach’s aversion to glucose was the result of very rapid evolution. Pest-control companies responded by developing baits using fructose instead of glucose, and by using more pesticides with less care. The reaction of evolutionary biologists was more surprising. Silverman had anticipated a wave of research attempting to work out how the roach had evolved . Instead, nothing.
Silverman was clearly ahead of his time. The mystery of the sugar-hating cockroaches was solved just last year – more on that later – but even now, biologists rarely pay attention to the species that live in our homes. To most, the great indoors is a terra incognita, the Galapagos of daily life. Yet this is a habitat about a tenth the size of the world’s rainforest, and it is expanding rapidly. Those of us who have begun to study it are finding that its inhabitants are surprisingly diverse: your home probably contains tens of thousands of bacterial species, hundreds of fungi and dozens of arthropods and worms, among others. Some organisms have been evolving with us for millennia; some, like the German cockroach, are changing rapidly right now. You may know very little about the biodiversity within your home, but nevertheless you are responsible for its fate.
The origins of this ecosystem run far deeper than you might think. The first clear evidence for home-building is just 20,000 years old. But some of the species now under our roofs have been living with us for at least 8 million years, since our ancestors diverged from those of gorillas and chimpanzees. At that time, they would have slept in nests similar to those used by other great apes today. Many of the microbes on, in and around us originate here. So do lice, be they head, body or pubic, and probably also the mites that live in our eyelashes and the dust mites that float around us in the ether of dead skin. In my lab, we are working with primatologists to study the organisms found in chimp nests. Our aim is to understand how the many other species now found around and on us might have been with us since the very beginning.
“Some of the species now under our roofs have been living with us for at least 8 million years”
The nests of gorillas and chimps are ephemeral, typically abandoned after a single night. While they are more comfortable than sleeping on the ground or on a branch – a colleague actually tested this – the comfort is modest. Why don’t they build bird-like nests, worthy of spending weeks, months or a whole lifetime in? It is possible that chimps and gorillas choose to move on to limit the build-up of pests. Whatever their advantages, ephemeral nests were no longer an option as our ancestors increased in numbers. They were compelled to seek out new accommodation – and in doing so acquired a new throng of cohabitants.
Exactly when or how our ancestors began to spend night after night in the same sleeping places is unknown. What we do know is that as we began to settle, the number of species living with us increased. This would have happened first in caves; primates use caves occasionally, and many of the best early hominin archaeological sites are inside caves.
As our ancestors started returning to the same cave to sleep, new species evolved to take advantage. Bedbugs are close kin to bat bugs, and appear to have moved off bats and on to humans in a cave somewhere in eastern Europe or Asia, at some point in the past 100,000 years. . Their mouthparts became longer to pierce our thicker skin; they became nocturnal to avoid being detected; and their legs elongated, perhaps to make it easier to get from one sleeping spot to the next. German roaches may have moved in with us at this point, too.
The next big leap for indoor biodiversity came with farming, which began around 10,000 years ago in the Fertile Crescent east of the Mediterranean. Now domestic animals, crops, stored food, drying meat and waste would have surrounded every human action, both outside and in the growing realm of indoors. If ever nature encountered an irresistible smorgasbord, it was these early agricultural civilisations. Many species moved in with humans and, as they did, they evolved traits that made living alongside us easier.
A snapshot of these early domestic denizens comes from an . Around 1300 BC, this was home to Ranefer, who oversaw the stables of the pharaoh Akhenaten. Analysis of Ranafer’s floor revealed he shared it with an astonishing diversity of insects and other arthropods. Eva Panagiotakopulu at the University of Edinburgh, UK, and colleagues found dermestid beetles, spider beetles, houseflies, bedbugs and dozens of other species preserved in the layers of accumulated sand; some had already evolved relative to their wild ancestors. Grain beetles, for example, had lost their wings – a costly appendage, unnecessary when you can simply ride from place to place in transported grain.
Moving in
Panagiotakopulu’s list excludes vertebrates, microbes and any organism too delicate to preserve well. But they would have been there, too. , are African. At some point near the dawn of agriculture they lost their habit of migrating every spring and autumn, evolved a new beak shape and moved permanently into human settlements. They and their close relatives have spread with us ever since, evolving as we moved around the world. Similar stories emerge for house mice and brown rats, both of which have been with us for tens of thousands of years, having originally taken up with Homo erectus in India, it is thought.
By Ranefer’s time, an entire menagerie had moved into our homes. These species relied on the conditions we had created indoors for their survival, conditions we would replicate almost everywhere we went. Many followed people around the world to new continents and even to the most remote islands. Indigenous species joined in along the way, so that the sum total of organisms living indoors today is more than it has ever been. Incidentally, the movement of home-dwellers can be used to reconstruct not only their history but also ours. For example, one of the best pieces of evidence for the movement of people through the Pacific Islands is .
Just how many thousands of species live in today’s homes and backyards is anyone’s guess. But some . my colleagues and I found more than 8000 kinds of bacteria and archaea. of 50 homes, led by Michelle Trautwein at the North Carolina Museum of Natural Sciences in Raleigh, found more than 100 species of arthropod living in the average home, with 750 in the most biodiverse. Many originated as far away as Egypt and the Fertile Crescent, but others had never been seen before. And then there is the as yet unstudied diversity of fungi, protists and other groups in homes – not forgetting those species, including dozens of worms and thousands of microbes, specialised for life on our bodies. Even purified tap water wriggles with life.
We may go to great efforts to exclude these species from our homes, but they evolve to outwit us. This can happen extremely rapidly, as the story of the German cockroach makes clear. Last year, Silverman, who is now at North Carolina State University, finally resolved the mystery of how they lost their sweet tooth. With colleagues Ayako Wada-Katsumata and Coby Schal, he reported that evolution has rewired the brains of sugar-hating roaches (). Receptor neurons normally tuned to bitter tastes have become activated by glucose, so that the cockroaches perceive sugar as nasty rather than nice. Under the ferocious selection pressure exerted by pesticides, this has happened in a matter of years – perhaps as few as two.
It doesn’t end there. German cockroaches are at the base of a barely studied food web. At least two wasp species have evolved to lay their eggs inside the roach’s body and nowhere else. As their host evolved to resist roach baits, they have prospered. So too have house centipedes which prey upon roaches. Indeed there is an army of pathogens and symbionts carried into our homes by German cockroaches that will benefit from their aversion to sugar.
The story of roaches and their dependants is just one of thousands that wind through our history and into our modern homes. Although we are only just beginning to unravel them, it is fascinating to speculate about how the organisms in our homes might evolve in future. Today, indoor spaces cover about 0.5 per cent of the ice-free terrestrial surface area of Earth (see diagram). As human populations increase, so too will the area of this biome. By some measures, the extent of the indoors will double by 2100. The consequences are predictable. If there is one unassailable rule of ecology, it is that the larger the area of a biome, the more species will evolve to live in it.
Whether you intend it or not, you are almost certainly the primary agent affecting that evolution. Your cohabitants are adapted to thrive in a particular environment, with characteristic resources and climates created by you. These conditions are replicated in houses all over the developed world, so that most of us share our homes with a similar suite of species. But that is changing. These days our houses are filled with the chemicals we use to kill other species – rat poisons, roach baits, ant baits, flea powder, antimicrobials, antibiotics and, if we include our yards, herbicides. They give us a powerful way to alter our domestic environment. Depending on when, where and how often you use these chemicals, you are selectively killing those organisms least able to survive them, and favouring the surviving genes and lineages. You are, like the unwitting god of your domain, creating new forms – and they are rarely beneficial. A roach that isn’t attracted by sugar is annoying, as is a bedbug no longer susceptible to pesticides. But a malaria mosquito that fails to land on the wall where pesticide has been applied could be deadly, as could antibiotic-resistant Staphylococcus on the kitchen counter.
If we continue trying to eradicate organisms from our homes in this way, we will end up with ever more species adapted to cope with our chemical warfare. Much as we may want to, we will never create a sterile and barren environment – besides, it is an ill-conceived objective since a home devoid of non-human organisms is one in which we cannot survive. We depend on other organisms for some of our most basic needs, such as to digest the food in our guts and to defend us from pathogens. So what is the alternative? Given that we have more control over the indoor biome than any other, perhaps we can think of creative ways to exert this power.
“A home devoid of non-human organisms is one in which we cannot survive”
One option would be to create genetically engineered species to live with us. That way, we could replace harmful organisms with benign ones, or even create room-mates that would help out around the house, perhaps eating bedbugs or cleaning mould from the shower. A less radical alternative would be to cultivate conditions in our homes that favour species we like and that benefit us. Of course, we would first need to identify these organisms and find out what makes them thrive, not to mention how they interact with other organisms, both benign and malign.
It is unclear what a human-friendly indoor biome would look like, but this approach isn’t without precedent. We are already engineering our outdoor urban spaces to suit us better. In our cities, a human-friendly environment takes the form of shady trees in streets, green rooftops, peregrine falcons diving on pigeons and red-tailed hawks casting shadows on Broadway. Indoors we have barely begun to consider the possibilities. Until we do, the species that escape our chemicals will become ever more resistant to our attempts to evict them, lurking in the crevices, scurrying whenever we turn the lights on and leaving sugar pellets laced with poison uneaten.
This article appeared in print under the headline “Meet the lodgers”
