Transcription
It doesn't just take willpower to survive; it takes guts. Life needs energy to exist, and almost all animals get their energy the same way—with a built-in power plant: a digestive system that turns food into fuel. But how does it work? Pouted animals evolved such a fantastic range of digestive systems. To understand the story of evolution, you have to go with the gut.
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This is no ordinary night out for dinner; it's a celebration of the powers of the gut. These competitive eaters may eat 20 pounds of food at a single sitting. How do they do it? With the help of a biological factory inside their bodies, the product of half a billion years of evolution. Once the food enters the mouth, the transformation begins. As food is chewed, moistened, and broken down with saliva, our tongues mold it into soft pellets that we can swallow. Food slides down the esophagus, is churned in the stomach, and mixed with powerful enzymes and acid. After four or five hours in the stomach, the food, now almost pure liquid, flows into the small intestines. Nutrients are extracted from the soup and absorbed into the bloodstream. The remainder is sent into the large intestine, where water is reabsorbed and solid waste is stored until it can be excreted.
We expect a lot from our digestive systems. Think about what it has to do: you go out, get a burger, and shove it into your face. Then you expect your body to extract the proteins, fats, and carbohydrates out of it and send those to the appropriate parts of the body. It's ridiculous what you expect of your stomach, but it's doing that for you every day. Just how complex is the human gut?
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In Luxembourg, there's a device that mimics every chemical and mechanical step required to digest a piece of food. Nearly seven tons of industrial plumbing supplies and modern electronics make up a machine called Cloaca. It is a machine that eats, that digests, and it goes through all the same processes that we have in our body. A big side of the machine shows how complex the human processes are. Each morning, a staff member feeds the machine. This particular machine can eat exactly the same that we eat, and very often it's very good things. As food falls in, a meat grinder replicates the chewing action of our teeth. Food then flows into vats of digestive juice, including hydrochloric acid—a solvent that can dissolve rock—and the same stuff we use down the line. Containers replicate the action of our intestines, the further breakdown of food, and absorption of nutrients. This part of the machine, about 26 feet long, is roughly the same length as the intestines that lie coiled inside us. After some 25 hours, digestion in Cloaca, as in our bodies, reaches the end of the line, and what comes out is really similar to what we do. And what we do is poop.
But what was the purpose of this machine? To do what our guts do. Modern technology requires a giant room full of machinery. How did our guts get to be so complex? The answer is they evolved.
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Life today is the result of billions of years of changes, mutations, and natural selection. But through all the years and all the species, one need has remained constant: to survive, every organism on Earth must have energy. To get it, animals steal it from other kinds of life—eating plants, microbes, or other animals. But once they've had their meal, they must break it down to acquire the energy and raw materials they need to grow and survive. For most animals, that breakdown happens in a gut. So, the evolution of the digestive tract is an important component of the history of life. All of these animals are able to specialize on different organisms because they have specialized digestive tracts that let them do what they need to do.
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The earliest ancestors of animals had no mouths or stomachs. They were microorganisms that simply engulfed each other for energy. Then, some 575 million years ago, life got interesting. Frozen in time on what today is the coast of Newfoundland are organisms unlike anything alive today—these were the planet's first multicellular life forms: the Ediacaran. They are so mysterious that researchers have been unsure if they were plants or animals. Some of them are tiny, but some of them range up to 2 meters in size. What we're looking at here is when life got big.
To identify these ancient organisms, paleontologists Gennaro B. and Mark Laflamme have come to the Royal Ontario Museum to study a rare trove of Ediacaran fossils. "One of our first instincts is to try and compare these to organisms that are familiar to us. Organisms that we know in terms of construction. They're very unique; they're built up of basically small little modules that have no modern counterpart, and there's absolutely nothing in modern animals that looks anything like this." Some Ediacarans were fern-like, while others were disc-shaped and lay flat on the sea floor. While their shapes have led some experts to speculate that Ediacarans were early plants, their ancient habitat tells a different story. More than half a billion years ago, the site where these Ediacarans died was a very different world. The one thing that tells us that they most definitely were not plants is the environment. Analysis of the sediments here revealed that Ediacarans were living almost one mile underwater in a place of permanent darkness.
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There wouldn't have been enough light down there to read the headline in a newspaper, let alone have active photosynthesis like a plant needs. They may have looked like plants, but their ability to thrive in a sunless world has led Narbona and Laflamme to conclude that Ediacarans were actually animals—the very first on Earth. If they were animals, how did they eat? These are the first large feeders in Earth history, and they got their food from the water. We don't see any evidence that any of them could move—no muscles, no guts, no eyes, no brains. They simply absorbed food from the water column and then transmitted it to the rest of the organism fixed to the sea floor. They could not search for food. Ediacarans competed for it by getting bigger; they were trying to maximize their surface area—how much of their body was actually in contact with water—so that they could maximize the amount of nutrients they were absorbing. In that way, for 30 million years, Ediacarans dominated the seas. But then, in a sudden evolutionary shift, life on Earth changed forever. The Ediacaran disappeared, devoured—it's thought—by new kinds of animals equipped with new kinds of organs and a whole new way of eating. For the first time in the history of life, Earth got guts.
430 million years ago, the seas were ruled by Ediacarans—simple animals that absorbed food through their skin. But meanwhile, another lineage of animals was evolving a very different feeding strategy: a gut. The first guts may have looked like the ones in jellyfish today. If you look at something like a jellyfish, it doesn't have a mouth and an anus and a tube going from one end to the other; all it has is a sack. So it can take food into the sack and then spit the food back out. That works very well for jellyfish, but life in the sea was about to become far more complex. The Ediacarans disappeared, overtaken by an entirely new array of creatures born in an evolutionary event known as the Cambrian explosion.
This chapter represents a quantum leap. This is probably the most important evolutionary event in the history of our planet. With the advent of predators, we basically start off with an arms race. As we start developing organisms that are building sophisticated teeth and means of biting and chewing and eating things, we also start developing organisms that have a means of protecting themselves against that. This eruption of forms and shapes also brings a radical new digestive design. These Cambrian creatures were quite different from those of the Ediacaran in a number of ways. One of the most important ones is mobility. Mobility allowed organisms to feed differently. Active hunting required a more efficient gut—a way to take food in one end of the body and expel waste out the other. What evolved was a tube with holes at both ends—the basic blueprint of our own gut.
But a tube alone was not enough to digest the well-armored prey of this new food chain. Predators needed extra help. That help came from something so tiny it's invisible to the naked eye. Today, a striking example of this ancient digestive secret lies a mile below the surface, deep in the Pacific Ocean. Scientists from the Monterey Bay Aquarium Research Institute, piloting an unmanned submersible, found a whale carcass. More interesting than the bones were the feathery plumes coating them. "When I first found them, I was really intrigued. This was clearly an animal that had never been seen before." These plumes were actually a bizarre new type of marine worm—an animal with no mouth and no teeth that somehow has evolved to eat whale bone.
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To make sense of this digestive oddity, specimens were sent to Greg Rouse at the Scripps Institution of Oceanography. "I spent a lot of time dissecting the bones to get the worms out and study them very carefully to try and understand their anatomy. What was really interesting was that we couldn't find any gut. There was no mouth; there was no digestive system. So we were really intrigued as to how these worms were able to get their nutrition without a digestive system." How could these worms eat solid bone? "When we started to dissect open these roots to look at them very closely, they were full of bacteria, and this gave us a vital clue." Rouse discovered that the worms themselves weren't eating the whale bone but slowly burrowing inside it. The worms were providing access to the whale bone for the bacteria. They were drilling into the bone and taking the bacteria down in their roots. Though Rouse is still investigating exactly how they do it, it's clear the bacteria are digesting the proteins and fats in the whale bone. These bacteria were working with the worms in a symbiotic relationship, breaking down the whale bone, and in turn, the worms were then eating the bacteria. So they didn't need a gut; they were actually deriving all of their nutrition through these bacteria that were devouring the whale bone for them.
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Long ago, bacteria—the very first life on Earth—evolved enzymes to break down all manner of organic material. When the first animals emerged, bacteria would find shelter in their guts. In return, bacteria broke down the food that animals could not digest themselves. So one of the intriguing questions that we have is how this cooperation could have evolved, and it appears to have occurred several times in evolutionary history. It seems to appear when there is a source of nutrition that's difficult for animals to directly exploit. Over half a billion years later, bacteria are still essential for digestion in nearly all animals. We carry some ten trillion microbes in our own guts, which help us extract nutrients from our food. Bacteria were essential for digestion, and one of the animal kingdom's most successful lineages—creatures with backbones—our own ancestors. The first fish evolved into fast-swimming predators with powerful eyes and other senses for hunting. Fish also evolved a more sophisticated gut—no longer a blind pouch or a simple tube, but the first proper stomach.
"When the stomach shows up, you get to pass food past different parts of your digestive tract in a certain order, so you could be much more efficient about breaking the food down and getting the good stuff out of it." This powerful digestive engine opened the way to bigger, more complex animals. New kinds of food created new evolutionary opportunities.
375 million years ago, one lineage of fish discovered a whole new world of opportunity on land. Some land vertebrates would evolve guts for eating leaves, while others would eat insects. They, in turn, would be eaten by other animals. Eventually, the biggest guts on dry land would evolve to fuel one of evolution's great triumphs: the dinosaurs.
Paleontologists have been digging up dinosaur bones for about 200 years. The fossils they study tell us a lot about what dinosaurs looked like and something of how they acted. But so much of their biology—what they ate, how they digested, and fueled themselves—remains a mystery. Internal organs, things like stomachs and intestines, don't typically fossilize. There are no hard parts in these organs, and so when we look for evidence of what dinosaur digestive systems were like, we have to look to other lines of evidence.
For modern paleontologists like Matt Lamanna, understanding dinosaur digestion is vital to understanding their biology. And like any successful investigation, this one relies on hard evidence. Specimens of dinosaurs like this T. rex here have been found with piles of rocks inside their rib cages. Now, why a dinosaur would have a pile of rocks inside its stomach might not be clear at first until we look at the living descendants of dinosaurs: birds.
"Well, I'm here on a beautiful chicken farm surrounded by chickens. What I'm doing here is feeding these chickens grain. Now, this grain is pretty tough stuff, and so to digest it properly, the birds need a way to break it into little bits. They can't do that with teeth; birds lost their teeth long ago in their evolutionary history. But these chickens have solved that problem a different way—by using what are called gastroliths. Gastroliths are small stones that birds intentionally swallow along with their food, lodging in the gizzard, a digestive organ. The stones act like teeth. Food enters the gizzard, which houses the gastroliths. The gizzard will contract powerfully, forcing the stones against the pieces of grain, smashing the grains into bits and enabling the birds' digestive juices to act on it more efficiently."
So what do stomach stones and chicken gizzards tell us about dinosaurs? "Because we found several groups of dinosaurs with masses of gastroliths inside their guts, I think it's safe to assume that these types of dinosaurs had a digestive system that is similar to those of birds. For instance, these dinosaurs likely had a muscular gizzard that they used to smash food into little bits." Fossil gastroliths, like hollow bones, feathers, and other traits, show that some dinosaurs had many bird-like features.
But does that mean that dinosaurs also shared birds' high-energy lifestyle—that defining characteristic known as warm-bloodedness? Paleontologists have learned a lot about dinosaurs over the last few years, but there are still a few answers that remain elusive. One of these is the question of dinosaur metabolism. In other words, were dinosaurs warm-blooded or cold-blooded? We're really not sure, even in the case of famous carnivorous dinosaurs like Tyrannosaurus rex.
To answer this fundamental question, paleontologists have been comparing dinosaurs like T. rex to their living relatives.
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On the evolutionary tree, tyrannosaurs sit between their crocodile and bird cousins. To many experts, they are less like birds and more like cold-blooded crocs and alligators—able to fast for long periods between meals and then binge voraciously, swallowing giant pieces of meat and bone, holding it all in their gut for days as enzymes break it down. But some remarkable new evidence in the T. rex's digestion is shedding light on what the king of dinosaurs was really like.
University of Colorado paleontologist Karen Chin is an expert on dinosaur digestion. She doesn't work with fossil bones; she studies something else that dinosaurs left behind. "What you see in front of you right here looks like just a bunch of large irregular rocks, but these are very special rocks because these are actually fossilized dinosaur feces. Fossil feces are actually called coprolites, and they can tell you quite a bit about ancient animals." Dinosaur coprolites are rare, but each one can reveal important clues to how dinosaur digestion actually worked. The recent discovery of two large Tyrannosaur droppings has allowed Chin to make some surprising discoveries.
To see what the tyrannosaurs were eating, Chin looks inside a piece of the mineralized dung. The specimen is packed with telling clues. "This is a piece of a bone fragment here. These white circles show where there were blood vessels going through the bone, and the pattern tells us a lot about the victim—the animal that was eaten. The evidence suggests that these are the remains of a large plant-eating Triceratops. But more important than what the Tyrannosaurus was eating is how it ate."
"The interesting thing is that they did have quite a bit of bone in them, indicating that they were not picking around avoiding tooth-to-bone contact. They were just munching anything." At first glance, the presence of bones confirmed that tyrannosaurs had cold-blooded eating habits. "Karen Chin made a remarkable discovery, and that was that the bone chunks that were in the coprolite were rounded; they were acid-etched. This is something we see in crocodilians today. So these are animals that can take bones that they ingest and actually get sustenance from them. Most animals can't do that."
But inside the second Tyrannosaur coprolite, Chin found a different clue. "These kind of soft-looking cells here are fossilized muscle tissue."
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"To me, the most interesting thing about the Tyrannosaur coprolites is the presence of the muscle tissue inside. Fossilized muscle tissue in the coprolite indicates that this passed through the gut of the Tyrannosaur relatively rapidly. In other words, they didn't hold their food for long periods of time like a crocodile can." This evidence contradicts earlier theories and suggests that T. rex may have had a fast digestive tract—more like a warm-blooded bird or mammal than a reptile. Fast digestion would also mean a fast metabolism—something that requires constant fuel. "This animal had to have been eating just a remarkable amount of flesh and bone. This is not like an alligator, an animal that can feed once every several months. This is an animal that had to feed much more frequently than that."
No doubt T. rex had croc-like ferocity, but mounting evidence suggests that it needed to eat frequently like birds and mammals. "Now, I personally favor that dinosaurs had a metabolism greater than that of living reptiles. In other words, they were at least somewhat warm-blooded."
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We may actually find that they weren't crocodile-like; they weren't mammal-like. They had their own very unique physiology, seeming to possess digestive traits of both reptiles and mammals. Tyrannosaurs had evolved the guts that helped them dominate the food chain. Yet ironically, the gut that made T. rex successful may have also led to its doom.
65 million years ago, a giant asteroid slammed into the Earth, and in the cataclysms that followed, T. rex and other big dinosaurs faced a massive food shortage. "When that asteroid hit the Earth and all of the animals that it was feeding on started disappearing, Tyrannosaurus rex was stuck. It was big; it needed meat, and there was just nothing that could do to pass on its genes." The big dinosaurs went extinct, but not all animal life would succumb to the destruction. Small mammals would survive by burrowing underground. When they began to emerge, their dinosaur predators were gone, but they weren't alone or safe. Another creature had evolved the same survival strategy and some very extreme feeding habits.
In the early days of mammals, one type of lizard evolved to take advantage of the bountiful food source. Rather than lunging after its prey, it lost its legs and slithered—cold-blooded and slow-moving. It came up with a radical new approach to feeding as well. The snake doesn't chew its food; it doesn't have the teeth or jaws for that job. Instead, snakes swallow their victims whole. How do they do it, and why have they evolved this extreme feeding strategy?
At the University of Alabama, Stephen Secor is investigating the evolution of Burmese pythons. "What's amazing about these snakes is their ability to feed upon a prey item whose diameter is several times greater than that of the diameter of the snake's head."
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"This snake can swallow whole animals because its digestive tract makes up almost ninety percent of its body. It is essentially one long gut. But once the swallowing is done, the hard work begins. How does the snake transform the rat into energy? A visit to the vet's office is one way to find out."
"Alright, let's get this shot here from the side. One of the best ways that we can quantify the breakdown in the prey item within the stomach is by simply x-raying individual snakes. Take this shot. He'll do outwardly. The snake appears calm, but inside, a powerful engine is revving."
"Oh, that's nice. That's nice. It's already looking like it's been dissolving your way. Here's the skull, and yes, you see some of that bone is missing, so the snake is already starting to break down a bit of this rat's head as well as some of its skull. Internally, what you don't see is the very dramatic increase in acid production at this point. I bet the stomach is putting a lot of hydrochloric acid as well as pepsin, and that's what's really breaking this all down. This is all literally hidden inside the snake."
"The rat will just continue to move down into the stomach, and the acid and enzymes will just constantly break it down till there's nothing left. After seven days, the rat is digested and stored as energy, which will carry the snake through a long fast. In the wild, the large snakes we call constrictors are the planet's ultimate binge eaters. These animals may spend weeks, even months, just sort of staked out in one place—perfectly still—a classic couch potato. It's been reported that some of these snakes are able to fast for years between meals. When constrictors do eat, they really know how to make up for lost time. Pythons commonly ingest prey that weighs a quarter or more of their own body weight. That's like a 200-pound man eating a 50-pound steak. But even with the immense meals these snakes are able to consume, how can they survive for so long without eating?"
Secor has discovered that snake guts come with a special feature—an on-off switch. "We see this very dramatic turning on and turning off of the gut. A number of their organs are literally shut down during these periods of fasting. Having saved precious energy in between meals, pythons can jumpstart their stomachs when opportunity walks by."
To understand how the snake actually pulls this off, Secor magnifies a section of the python's small intestine 15,000 times. The microscope reveals the snake's microvilli—the finger-like extensions that nearly all animals use to absorb nutrients. "So in mammals, including humans, the microvilli don't change with feeding and fasting; they maintain their same length, same position. But Secor has discovered that snakes have evolved microvilli unlike those in any other animals. When fasting, their microvilli are short and ordinary, but after feeding, they begin to grow and grow."
"What really was amazing was that within six hours, the microvilli had already doubled in length. Within a day after feeding, the microvilli have increased anywhere from six to seven fold in length, and that's unheard of in any other organism."
"As the microvilli grow, they increase the surface area of the small intestine exponentially, and the rat that has been turned into liquid energy by enzymes and stomach acids is thoroughly absorbed by the snake. After seven days, the process is complete, and microvilli shrink back down."
"We can do a lot of studies in the lab to figure out how things occur—how is it that the intestine turns on and turns off—but the real question is why? Why do these traits exist in a world where your prey is faster than you are? Conserving energy is vital for the constrictors. It's a very nice evolutionary mechanism to basically expend longer periods of time without eating, letting their stored energy fuel their metabolism, and it's just a good strategy for feeding, for digesting, for surviving."
Snakes aren't the only animals that have evolved extreme guts to cope with a difficult diet. 20 million years ago, in the Miocene epoch, a global cooling trend led to some dramatic environmental shifts. "There was a huge change where big forests all of a sudden turned into grasslands. Animals that were very, very good at browsing on trees had been doing great up until the Miocene. There was all of a sudden a very, very new set of rules about what was good and what was bad for the herbivores that browsed on leaves and forest plants. Their old food supply was dwindling and, in many areas, being replaced by a virtually unlimited one: grass. But eating grass posed serious problems back then. It offered little cover from predators, and even more fundamentally, grass was virtually indigestible. It's hard to eat because it's got very little nutrient in it. What nutrient it does have is bound up in undigestible starchy cellulose material."
But some mammals evolved a solution. Ruminants, including the cow, live today on a diet of grass. What was their digestive solution? You can see it inside one kind of very special cow. Just like any other cow, Rose has an amazing stomach. "How do we know? Rose offers us a window into the stomach—literally. She is a fistulated cow with a surgical opening in her side. Rose is a research animal living at Cornell University."
"You just pop the plug in there so that we can gain access to the large compartment of the stomach—the rumen. A painless process for the cow has given scientists like Debbie J. an unprecedented glimpse inside one of the most complex digestive systems on Earth. But to do its job, Rose's gut is more than intricate; it's massive. Her first and largest chamber, a giant fermentation tank called the rumen, can hold over 250 pounds of soggy grass. She also produces 55 gallons of saliva a day."
"So it's a very large compartment to hold all this material so it can digest it slowly. In an 80-hour journey, grass travels through four chambers that progressively break down cellulose, extract energy, and absorb water. In all ruminants, this digestive odyssey begins with a unique behavior known as chewing the cud. When she's done eating, she'll ruminate. She spits up her food; she spits it up in the back of her throat. She'll swallow to squeeze out any water that's in there, and then she'll chew her food."
Why this complicated eating behavior? "It may have originated as a way to avoid predators—to have a meal without becoming a meal. Since they are a prey species, if they quickly eat and then go someplace out of the open and chew in a safe place, then more of these species would survive."
But to digest grass, ruminants needed more than a big and elaborate digestive tract; they also use the Earth's original digestive aid—the tiny stowaways so many species carry inside them. "Microbes have been living inside animals ever since animals evolved, but the cow has taken it to a new level. Its gut is a giant habitat for microbes. Inside, more than 200 different kinds of microorganisms are at work, including trillions of bacteria that attack grass, turning cellulose into sugar and energy. In the rumen, large protozoa dwarf the bacteria and keep them in check by eating them, and there's even fungi that break down the fibrous forage that the bacteria can't handle."
"When you see a cow eating grass, it's pretty neat to remember that cows can't digest grass. The only reason the cow is able to do anything with that grass is because it's got a big colony of bacteria living inside its gut that will break down the grass for it. By partnering with the microbes and evolving the four-part stomach, the cows and their cousins will come to dominate the grasslands—huge thundering herds of herbivores. And that, in turn, will help another group of animals—animals that don't eat grass themselves but eat the grass eaters. New species of carnivorous mammals will evolve, some with jaws and claws, and even more fearsome predators with brains for the hunt."
What makes us uniquely human? For a long time, it seemed as if the answer would lie in the evolution of our oversized brains. But new evidence suggests that part of the answer lies elsewhere—in our guts. Harvard anthropologist Richard Wrangham has spent his career studying chimpanzees, apes, and human origins. "Before about two million years ago, our ancestors were members of a group of animals that we would think of as apes. Nowadays, if we saw them, we would not be inviting them to dinner; we would be watching them in zoos or in national parks. And they were these things: the Australopithecus. Australopithecus was short, with an ape-like brain, jaw, and teeth. And then, in this relatively short space of time, that species gave rise, among other things, to the things that became us."
These new hominids were our direct ancestors. It's believed that over hundreds of thousands of years, an evolutionary blip, the brain capacity of these new species grew to twice that of Australopithecus, while their teeth and stomach shrank. "So here we got the big problem: we got a big-toothed, small-brained ape that becomes a small-toothed, big-brained human. How did this happen?" Wrangham believes that all of these changes had the same cause: a radical change in diet.
"There's this fascinating question about what is the relationship between diet and the size of the brain. Where do you get the energy to fuel the big brain? Each organ in the body demands its share of energy. The digestive tract and brain are particularly greedy. If the species could evolve smaller guts, maybe it could evolve bigger brains."
How did our ancestors shift from big guts to little ones? Conventional wisdom says these guys learned two things: they learned to get more meat, and they learned to use tools to get at the meat. "We know they were using tools like this to cut the meat, and probably this same kind of tool could be used to pound the meat, giving you something more like steak tartare, and that would make it much easier to eat."
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"By doing work previously done by the digestive system—in effect, pre-digesting food—tools helped our ancestors survive with smaller guts. But Wrangham believes that an even more potent technology transformed digestion. It's easy to imagine that once fire was used at all by these transitional species, then they would start resting their food against the fire because they'd learn very quickly that it tasted better."
"The introduction of cooking changed everything. It allows us to get more energy out of our food with less effort. I think you can regard it as one of the greatest improvements in the quality of the diet in the history of life, and certainly among all the primates."
How could this new technology—cooking—give us bigger brains?
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To test his theory, Wrangham is collaborating with an expert in animal digestion, Stephen Secor. They've devised a simple experiment to see if there is any difference in the amount of energy it takes to digest cooked versus raw food. The guinea pigs for this experiment are two hungry Burmese pythons. Test subjects that have no problem digesting raw or cooked meat. After the snakes are fed two equal-sized pieces of steak—one raw, one cooked—they are placed in containers. As air flows in and out of the Tupperware, the amount of CO2 exhaled by the snakes will show how hard each snake is working to digest its meal.
"What we found is that compared to the raw steak meals, the cooked steak reduced the cost of digestion by about 12.5 percent. So that supports the hypothesis that a cooked meal is going to reduce the cost of digestion."
Wrangham argues that for early humans, the benefits of eating cooked meat were transformative. "We think that once you start cooking, you are able to get 10, 20, 30, 40, maybe even 50 percent more calories out of your food. Over time, evolution favored individuals with smaller guts. They're not carrying all this useless material that is diverting energy away to the gut, and it could be used for the brain. So this is the raw material of evolutionary success. This is how you make more babies; this is how you invest in your body and resist disease better; it's how you survive longer."
From the dawn of the animal kingdom—from simple pouches to tubes complete with stomachs to digestive systems that could fuel a behemoth or let a mammal live on grass—guts have been at the center of evolution. And now, a new way of digesting food has allowed a new kind of animal to walk the Earth: us.
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