10% Human: How Your Body’s Microbes Hold the Key to Health and Happiness. Alanna Collen

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СКАЧАТЬ bacterium to live in a special cavity in its underbelly. Here, in this light organ, the bacteria, known as Aliivibrio fischeri, convert food into light, so that viewed from below, the squid glows. This obscures its silhouette against the moonlit ocean surface, camouflaging it from predators approaching from beneath. The squid owes this protection to its bacterial inhabitants, and they owe the squid for their home.

      While housing a microbial light source might seem a particularly inventive way to increase one’s life chances, squid are far from the only animal species who owe their lives to their body’s microbes. Strategies for living are many and varied, and cooperation with microbes has been a driving force of the evolutionary game since living beings with more than one cell first evolved, 1.2 billion years ago.

      The more cells an organism is made of, the more microbes can live on it. Indeed, large animals such as cattle are well known for their bacterial hospitality. Cows eat grass, yet using their own genes they can extract very little nutrition from this fibrous diet. They would need specialist proteins, called enzymes, that can break down the tough molecules making the cell walls of the grass. Evolving the genes that make these enzymes could take millennia, as it relies on random mutations in the DNA code that can only happen with each passing generation of cows.

      A quicker way to acquire the ability to get at the nutrients locked away in grass is to outsource the task to the specialists: microbes. The four chambers of the cow’s stomach house populations of plant-fibre-busting microbes numbering in their trillions, and the cud – a ball of solid plant fibre – travels back and forth between the mechanical grinding of the cow’s mouth and chemical breakdown by the enzymes produced by microbes living in the gut. Acquiring the genes to do this is quick and easy for microbes, as their generation times, and therefore opportunities for mutations and evolution, are often less than a day.

      If bobtail squid and cows can both benefit from teaming up with microbes, is it possible that we humans do as well? We may not eat grass and have a four-chambered stomach, but we do have our own specialisations. Our stomachs are small and simple, there just to mix the food up, throw in some enzymes for digestion, and add a bit of acid to kill unwelcome bugs. But travel on, through the small intestine, where food is broken down by yet more enzymes and absorbed into the blood through the carpet of finger-like projections that give it the surface area of a tennis court, and you reach a cul-de-sac, more of a tennis ball than a tennis court, that marks the beginning of the large intestine. This pouch-like patch, at the lower right corner of your torso, is called the caecum, and it is the heart of the human body’s microbial community.

      Dangling from the caecum is an organ that has a reputation for being there simply to cause pain and infection: the appendix. Its full title – the vermiform appendix – refers to its worm-like appearance, but it could equally be compared to a maggot or a snake. Appendices vary in length from a diminutive 2 cm to a distinctly stringy 25 cm, and, rarely, a person may even have two of them, or not one at all. If popular opinion is to be believed, we would be better off without one at all, since for over one hundred years they have been said to have no function whatsoever. In fact, the man who finally put the anatomy of animals into an elegant evolutionary framework is apparently responsible for this persistent myth. Charles Darwin, in The Descent of Man, a follow-up to On the Origin of Species, included the appendix in a discussion of ‘rudimentary’ organs. Having compared it with the larger appendices of many other animals, Darwin felt that the appendix was a vestige, steadily withering away as humans changed their diets.

      With little to indicate otherwise, the vestigial status of the appendix was barely questioned for the next 100 years, and the perception of its uselessness is only enhanced by its tendency to cause a nuisance. So pointless has the medical establishment assumed it to be, that by the 1950s, removing it became one of the most common surgical procedures carried out in the developed world. An appendectomy was even often tacked on as a bonus during other abdominal surgery. At one point, a man stood a one in eight chance of having his appendix removed during his lifetime, and for a woman, the odds were one in four. About 5–10 per cent of people get appendicitis at some stage in their life, usually in the decades before they have children. Untreated, nearly half of these people would die.

      This presents a conundrum. If appendicitis were a naturally occurring disease, frequently causing death at a young age, the appendix would be quickly eliminated by natural selection. Those with appendices large enough to become infected would die, most often before reproducing, and would therefore fail to pass on their appendix-forming genes. Over time, fewer and fewer people would have an appendix, and eventually it would be lost. Natural selection would have preferred those without one.

      Darwin’s assumption that it was a relic of our pasts might have carried some weight, were it not for the often fatal consequences of possessing one. There are two explanations, therefore, for the persistence of the appendix, and they are not mutually exclusive. The first is that appendicitis is a modern phenomenon, brought on by some environmental change. Thus, even a pointless organ could have persisted in the past simply by keeping out of trouble. The other is that the appendix, far from being a malign vestige of our evolutionary past, actually has health benefits that outweigh its dark side, making its presence worthwhile despite the risk of appendicitis. That is, natural selection prefers those of us who possess one. The question is, why?

      The answer lies in its contents. The appendix, which averages about 8 cm in length and a centimetre across, forms a tube, protected from the flow of mostly digested food passing its entrance. But rather than being a withered strand of flesh, it is packed full of specialised immune cells and molecules. They are not inert, but rather an integral part of the immune system, protecting, cultivating and communicating with a collective of microbes. Inside, these microbes form a ‘biofilm’ – a layer of individuals that support one another and exclude bacteria that might cause harm. The appendix, far from being functionless, appears to be a safe-house that the human body has provided for its microbial inhabitants.

      Like a nest egg stashed away for a rainy day, this microbial stockpile comes in handy at times of strife. After an episode of food poisoning or a gastrointestinal infection, the gut can be repopulated with its normal inhabitants, which have been lurking in the appendix. It might seem like an excessive bodily insurance policy, but it is only in recent decades that gut infections such as dysentery, cholera and giardiasis have been all but eliminated in the Western world. Public sanitation measures, including sewerage systems and water-treatment plants, have prevented such illnesses in developed countries, but globally, one in five of all childhood deaths are still caused by infectious diarrhoea. For those who do not succumb, possession of an appendix likely hastens their recovery. It is only in a context of relatively good health that we have come to believe that the appendix has no function. Indeed, the negative consequences of undergoing an appendectomy have been masked by the modern, sanitised lifestyle.

      As it turns out, appendicitis is a modern phenomenon. In Darwin’s day, it was extremely rare, causing very few deaths, so we can perhaps forgive him for thinking the appendix was merely one of evolution’s leftovers, neither harming nor helping us. Appendicitis became common in the late nineteenth century, with cases in one British hospital shooting up from a stable rate of three or four people per year prior to 1890, to 113 cases per year by 1918; a rise mirrored throughout the industrialised world. Diagnosis had never been a problem – the cramping pain followed by a quick autopsy if the patient didn’t make it revealed the cause of death even before appendicitis became as common as it is now.

      Many explanations were put forward to explain it, from increased meat, butter and sugar consumption, to blocked sinuses and rotting teeth. At that time, consensus opinion alighted on a reduction in fibre in our diets as the ultimate cause, but hypotheses still abound, including one that blames the rise on improved water sanitation and the hygienic conditions it brings – the very development that appeared to render the appendix almost impotent. Whatever the ultimate cause, by the Second World War our collective memory had been purged of the rise in appendicitis cases, leaving us with the impression that it is an expected, though СКАЧАТЬ