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churning vat of microbes. The rumen is a sac that branches off the rest of the deer’s gut. All food, except the mother’s milk, is sent to the rumen before it can move into the rest of the stomach, then on to the intestines. The rumen is surrounded by muscles that churn the contents. Flaps of skin inside the rumen act like baffles in a washing machine, flipping the food over as it moves.
    Most microbes in the rumen cannot live in the presence of oxygen. They are descendants of ancient creatures that evolved in a very different atmosphere. Only when photosynthesis was invented, about two and a half billion years ago, did oxygen become part of earth’s air and, because oxygen is a dangerous, reactive chemical, this poisoning of the planet wiped out many creatures and forced others into hiding. These oxygen-haters live to this day in lake bottoms, in swamps, and deep in the soil, eking out an existence in oxygen-free environments. Other creatures adapted to the new pollutant and, using an elegant sidestepping maneuver, turned the toxic oxygen to their advantage. Thus was born respiration using oxygen, an energy-liberating biochemical trickthat we have inherited. Our lives therefore depend on an ancient form of pollution.
    The evolution of animal guts gave the oxygen-hating refugees a potential new place in which to hide. Not only are guts relatively free of oxygen, they also have every microbe’s dream: a continual supply of minced food. But there was a problem. Animal stomachs are generally full of acidic digestive juices designed to tear apart living tissue. This prevented most animals from harboring plant-digesting microbes. However, the ruminants changed their stomachs, mastering the hotelier’s art, and they have been rewarded by a four-star rating of evolutionary success. The centerpiece of this hospitality is the position and friendliness of the rumen, which comes before the rest of the gut and is kept neutral, neither acid nor alkaline. Microbes thrive in this churning spa. The animal’s saliva is alkaline, so the acidic products of digestion are neutralized. Any incoming oxygen is soaked up by a small team of bacterial chambermaids.
    The rumen functions so well that scientists equipped with the most sophisticated test tubes and vats have not been able to replicate, let alone beat, the growth rate or digestive prowess of the rumen’s microbes. The rumen’s performance is due to the exquisite biological complexity that thrives in its pampered chambers. A million million individual bacteria of at least two hundred species swim through every milliliter of rumen fluid. Some of these microbes have been described; others await description or discovery. Many of the microbes are found only in rumens, presumably having diverged from their free-living ancestors during the fifty-five million years that have passed since the rumen’s origin.
    Within the rumen, the bacterial proletariat is preyed upon by a bevy of protists, all of which are single-celled but hundreds or thousands of times bigger than the bacteria. Fungi parasitize these protists, infecting then bursting the fat cells. Other fungi float free in the rumen fluid or colonize scraps of plant material. The diversity of life in the rumen makes possible the complete digestion of the plant remains. Nosingle species can fully digest a plant cell. Each species takes a small part of the overall process, chopping up its favorite molecules, harvesting the energy it needs to grow, and then sending back its wastes to the rumen fluid. These wastes become another creature’s food, building a cascading web of disassembly. Bacteria destroy most of the cellulose, aided by some fungi. Protists have a special fondness for starch grains, perhaps regarding them as potatoes to accompany their meal of bacterial sausages. Nutrients in the rumen are passed up a miniature food web, then released back into the rumen’s fluid, mimicking the nutrient cycles of larger ecosystems. The