It's impossible to answer a question of what humans were designed (or evolved) to do by looking at current human behavior and deducing design from that. For example, we can ask if humans are designed to travel into space and see that humans do travel into space, and conclude that humans are designed to travel into space. That conclusion ignores the fact that it takes decades of engineering and life support systems to allow humans to travel into space. It is not a natural activity at all, but one that is enabled by technology.

An omnivore is an animal that eats food of both animal and plant origin. Most commercially available cat food contains plants, and many cats thrive on such food. Cats can also be observed eating grass. But we would never classify a cat as an omnivore based on those observations. Similarly, we would never declare that cats are designed to eat plants.

Dr. Milton Mills argues that humans are designed to be herbivores. The following is a synopsis of his reasoning. His full presentation can be seen here.

All animals must procure their food in an energy-efficient manner. Survival in nature is fundamentally a function of energy in versus energy out. If animals expend more energy procuring their food than they can extract from the food they obtain, they will starve to death and eventually cease to exist as a species.

The theories that somehow humans survived by hunting before modern technology don’t add up. It takes so much energy for humans to find and take down animals, we would always be at an energy deficit, and we wouldn’t survive.

Humans, like other herbivores, are optimized for foraging.

Animals that are designed to eat plants have very different issues to confront than animals who are designed to eat other animals. Plants are anchored in the ground; they don’t kick you; they don’t spit at you; they don’t try to stab you; so they’re pretty easy to eat when you find them. The problem is, plants tend to be spread out over very vast distances.

So animals that are designed to eat plants are designed to forage. That is, they are designed to cover large areas at very low energy cost.

Humans, unlike carnivores and omnivores, are not optimized for predation.

Animals don’t want to be eaten. They will run away, and if you catch them, they will kick you, they will try to stab you with their horns, and they will do everything they can to get away from you.

So animals that are designed to eat other animals are designed to run very fast for short periods of time, tackle other animals, take them down, and kill them without being injured themselves.

Humans and other herbivores, unlike carnivores, are attracted by beautiful food.

Carnivores do not go out and look for the biggest, strongest, most robust antelope on the savanna, because that antelope is likely to outrun them. If they catch up to it, it is likely to injure them so that they won’t be able to hunt anymore. So instead, they look for weak, diseased, or defective animals because they are easier to catch.

Unlike humans and other herbivores, all true carnivores seek “ugly” food. That is, they desire food that requires a minimum expenditure of energy to obtain. This means that they do not waste time and energy pursuing the strongest and healthiest members of prey species, because they are more likely to be outdistanced by those animals, and, if they catch up to them, be injured by those animals. Instead, they pursue the sick, old, lame, mentally challenged, or very young, and they look for food that is already dead (carrion), because they have the physiology that allows them to eat rotting tissue without any illness. While humans may find it disgusting to think of eating a rotting carcass, the energy content of a rotting carcass is exactly the same as that of a fresh one. The energy content of a massive tumor is exactly the same as healthy tissue. So carnivores don’t mind eating an animal with cancer, because it’s going to be easier for them to catch, and it provides the same amount of energy as an animal without a disease. Behaving in this manner ensures that they will ultimately strengthen the gene pool of their prey species by weeding out the “less fit.”

Humans, like other herbivores, seek the most beautiful plants because they are the most nutritious. They don’t want the defective, dried out, broken-down plants because those plants are not as nutritious. Eating the healthiest plants is actually a good thing, because every time an herbivore defecates, it spreads the seeds of plants and helps them to multiply and spread.

When we humans, using our herbivore mindset (that is, looking for the biggest, the best, and the healthiest) decide to hunt animals, we kill the most beautiful and robust members of a species, which weakens the prey species’ gene pool and drives them toward extinction.

Unlike humans and other herbivores, carnivores have a streamlined, torpedo shape so that they can run very fast. They have an armored front, meaning that, when you look at their chest and shoulders, they’re heavily padded, so that if an animal tries to kick them, the kick will likely land on their shoulders. The most vulnerable parts—the unprotected abdomen and genitalia—are way at the back, where they’re the most protected from injury. They have a very thick, muscular, sturdy neck, and forward deployed weapons to facilitate attack.

Unlike humans, carnivores’ top speed (excluding the cheetah, which is an extreme example) is roughly 35 miles per hour, which enables them to ambush and capture sick, injured, and unwary prey.

Unlike humans, carnivores have a digitigrade stance, which means that they are permanently on their toes. Their heels are off the ground, and about 1/3 of the way up the leg. Their stance lengthens their legs, which helps them run faster.

Unlike humans, carnivores’ nails are sharp claws, which not only helps them to grapple with and wound prey, but also act like sprinters’ spikes and allows them to grab the ground as they run. Their joints are permanently flexed, which facilitates rapid acceleration for pursuit, but it also means that they must use muscle energy to resist gravity. Therefore, when carnivores are not actively chasing prey, they lie down, because it takes too much energy for them to stand. They have lightened limbs and small feet, which reduces the energy cost of running.

Unlike humans, carnivores have super-acute hearing. Their ears swivel like mobile radar to help localize their prey.

Unlike humans, carnivores’ eyes are optimized for night vision and for following movement. Their night vision is more than six times better than humans, but it has very low resolution. They have a reflective layer at the back of their eyes called the tapetum that helps to amplify low light levels. That is why they hunt primarily at night, when prey animals are asleep. If you can sneak up on a sleeping animal, it requires less energy to obtain that animal.

Unlike humans, carnivores have a linear streak along the backs of their retinas, where the highest concentration of photoreceptors are located, which allows them to detect and follow movement very easily. That is why, if you drag something in front of a carnivore (like a string in front of a cat) they have an instinct to chase it, because it’s moving across that linear streak, and their brain tells them to chase it.

Carnivores’ sense of smell can be 100,000 times more powerful than that of humans. It is sensitive enough to detect and track prey at great distances. It can also identify prey with cancers, infections, and metabolic disorders. Those are the ones they chase, because the sick ones are easier to catch. Humans are now using this ability by training dogs to detect humans with melanoma recurrences and with colon cancer recurrences, because their noses are sensitive enough that they can detect tumors well before they would show up on a cat scan.

Humans, like other terrestrial herbivorous mammals, tend to live much longer than carnivores.

This may be because herbivores’ diets are much higher in antioxidants and other things that help offset the ravages of aging. These animals are a living testament to the adequacy of plant protein. All protein is initially made by plants. Any protein found in animal tissues is actually second-hand plant protein. The largest, strongest animals on this planet are all herbivores.

Humans, like other herbivores, are designed for walking while foraging.

Humans are clearly not designed to chase other animals down and kill them. We have the widest part of our bodies presented to the wind, so they’re non-aerodynamic. We have the most vulnerable parts of our anatomy right up front to get injured. We have lithe, flexible necks. We have straight, heavy, pillar-like legs that allow us to stand with minimal muscular support. We have large, heavy, flexible feet, which helps us stand up, but when we try to run, we’ve got to lift our large, heavy feet and that requires a lot of energy, and that’s why we’re not good runners. Our nails are flat and blunt. Our lifestyle factors make humans relatively invulnerable to predation: we live in social groups; we’re active during the daytime; and we sleep at night when predators are active. In the back of human retinas, there is a pit called the fovea, which is lined with cones. Cones are the cells that give you very precise, discrete vision and color vision. Carnivores don’t have a fovea.

Humans' slow speed and poor endurance are most useful for escaping stinging insects. The fastest humans run somewhere between 19-22 miles per hour, and they can only keep that speed up for about 150 meters. That is clearly not fast enough to allow us to chase down prey, but what it does allow us to do is escape bees and wasps. Most bees and wasps will only chase an intruder for about a 200-meter radius around their hive, and their top speed is about 19-20 miles per hour. So basically, our top speed allows us to escape bees when we try to steal their honey.

Pregnancy and upper body strength make the concept of “stone age” hunting impractical, if not impossible. The female of every species is always able to procure the diet she needs to live, carry a pregnancy, and raise young.

Like other large herbivores, humans have a long gestation period, and deliver large, typically single, open-eyed infants.

The long gestation period of humans is typical of large herbivores. Human fetal development suggests that being an herbivore was a necessary prerequisite for human brain development. As with all other large herbivores, single births are the rule, with the baby’s birth weight being approximately 6-8% of the mother’s ideal weight. Human babies are born with their eyes open, which is a gauge of brain development.

The gestation of humans and other large herbivores is typically 2.5-3 times that of carnivores. All are greater than or equal to 34 weeks. All large apes have a gestation of 34-40 weeks. All tend to have single, large babies that are born with their eyes open and at an advanced stage of development.

Unlike humans and other large herbivores, carnivores have extremely short gestation periods. Multiple births (litters) are the rule. They all have their babies at a very early stage, because a very pregnant female cannot hunt effectively. She would risk injuring herself and her developing fetuses.

Unlike humans and other large herbivores, carnivores have tiny babies. They are born at an extremely early stage of development. They cannot regulate their body temperatures. Their eyes are closed. In effect, carnivore newborns complete their “fetal” maturation outside the womb.

Because they are born at a very early stage of development, carnivore infants require milk that is much richer than the milk of humans or other herbivores. Carnivores’ milk contains 2-10 times more fat, 2-4 times more protein, and 1.5-2 times more solids than the milk of humans or other herbivores.

Unlike humans and other herbivores, carnivores have reduced facial muscles, which give them a very wide mouth gape so that they can run up to another animal, grab it, and start to rip it apart. Their temporalis muscles on the sides of their heads are massive and are the main jaw muscle. Their teeth are designed for ripping, tearing, and cutting. They do not chew their food. Their jaws have minimal side-to-side motion and cannot move forward. Their jaw joint is on the same plane as their cheek teeth, so that their jaws function like a pair of shears to slice flesh off bone and cut through hides. The molars are blade-shaped, and the upper molars slide completely past the molars in the lower jaw in a slicing motion. The angle of the mandible is very reduced in carnivores, because the muscles that attach there (the masseter and pterygoids) play a very minimal part in the operation of carnivore jaws. Carnivores have extremely powerful jaws capable of wrestling half-ton animals to the ground, dismembering them, and crushing bones without being dislocated. The bite force of carnivores is 500-1,000 pounds per square inch, while that of humans is only 135-150 psi. Carnivores’ saliva has no enzymes, so they have no need to chew. They rip off a chunk of an animal and swallow it whole. If the animal is small enough, they will simply swallow the entire animal.

Unlike humans and other herbivores, the esophagus in carnivores is wide and distensible, which allows them to swallow large chunks and bones without choking or lacerations. They have a voluntary muscle along the entire length of their esophagus.

Unlike humans and other herbivores, carnivores’ stomachs are designed for intermittent feeding. For this reason, they are huge. Although carnivores try to hunt every day, they typically make a kill once every 7-10 days. That means that when they make that kill, they need to be able to absorb enough calories from that kill to make up for all the energy wasted over 7-10 days, and to last them until they can make another kill. Their stomachs alone hold 60-70% of their total gut capacity, and that allows them to consume 20-30% of their body weight at one meal. A 50-kilogram (110-pound) wolf can consume up to 15 kilograms (34 pounds!) of animal flesh and bones, which at 1.41 calories per gram, equals 21,500 calories at a single meal! This huge capacity is necessary because hunting is inherently inefficient.

Unlike humans and other herbivores, carnivores can eat carrion (putrefying, bacteria- and toxin-laden flesh), so they can recover additional energy from a single successful hunt over the course of several days if they can fend other predators off their cached prey. That’s because, when food is present, carnivores’ stomachs are extremely acidic, with a pH less than or equal to 1. This powerful acid dissolves bones, hooves, and connective tissue. It is also strong enough to dissolve a penny!

Animal tissues contain no fiber: there are only the cell membranes (not cell walls), which are made up mostly of fatty acids. They are mostly protein, water, and fat on the inside. Animal tissues are very quickly and easily broken down, so, unlike humans and other herbivores, carnivores do not need long, complicated digestive tracts. Their small intestines are very short, only 3-4 times the body length. They are mainly lined with protein and fat-digesting enzymes. Unlike humans and other herbivores, carnivores can go for long periods without eating without their mucosa involuting and becoming non-absorptive. They have a poor capacity to digest and absorb a moderate-to-large carbohydrate load per meal.

Unlike humans and other herbivores, carnivores’ colons are very short, straight, and non-pouched. Their only function is elimination. The lack of fiber in the digestive contents means that bacteria in the colon can only use protein as an energy substrate. Therefore, meat residues will putrefy, releasing toxic metabolites if not rapidly eliminated.

Unlike humans and other herbivores, carnivores do not develop heart disease, no matter how much fat and cholesterol they are fed. They don’t develop gallstones because of the special emulsifying properties of their bile. Carnivores can detoxify preformed vitamin A and manufacture vitamin C. Their urine is up to 2.5 times more concentrated than that of humans and other herbivores, which means that they don’t get dehydrated when they eat high-protein diets. The can also metabolize excess animal protein without damaging their bones.

The facial muscles, jaw, mouth, and teeth of humans, like other herbivores, are optimized for batch feeding on plants.

Humans, like other herbivores, are batch feeders with well-developed facial muscles and a small opening to the oral cavity. The angle of the mandible is greatly expanded, because the muscles that attach there, the masseter (on the outside) and pterygoids (on the inside), are the main jaw muscles. The area at the top, where the temporalis attaches, is very small, because it does very little in herbivores. Because humans and other herbivores have well developed facial muscles and a small oral cavity, they can do something that carnivores can’t do. They can create a vacuum to suck liquid. We have fleshy, muscular lips to help move food into our mouths. The tongues of humans and other herbivores are thick and muscular to aid in chewing, along with our cheek muscles.

Humans have spade-like incisors, designed for cropping and peeling fruit, but useless for trying to bite into an animal carcass. The cuspids (“canine teeth”) are reduced in size and have a flattened form. They function like accessory incisors, and are useless for killing an animal. Our molars are flattened to provide a grinding surface to shred fibrous plant foods, and they slide across each other horizontally. The jaw joint is above the plane of the cheek teeth. The angle of the mandible is expanded to the typical “L” shaped jaw of herbivores. The masseter (on the outside) and pterygoids (on the inside), are the main jaw muscles. They hold the lower jaw in a sling-like arrangement, and move it forward and side-to-side in a rotary fashion to help you grind your food. This mobile type of jaw structure is only found in animals that are committed herbivores because it can easily be dislocated.

The digestive tract of humans, like that of other herbivores, is optimized for digesting starches and fiber, as well as protein and fat.

Humans, like many herbivores, are hind-gut fermenters who have the enzyme salivary amylase present in their saliva, which is produced by the parotid salivary gland, and which begins the digestion of starches. When we chew our food thoroughly, most of the carbohydrate digestion occurs because of this enzyme in our saliva, not the enzymes from the pancreas. Humans and other herbivores have a narrow and muscular esophagus, so they can swallow only a small bolus of soft food. This is why 90% of people who choke to death every year do so by trying to eat meat, and the number-one culprit is hot dogs. Humans who eat a lot of meat also tend to get gastro-esophageal reflux disease (GERD), which if untreated can lead to esophageal cancer.

Plant tissues are composed of protein, fat, and carbohydrates. Because plants don’t have bone, they rely on fiber to stiffen and protect their tissues and resist the force of gravity. This means that all plant cells, in addition to having a cell membrane, have a cell wall that is made up of cellulose, which requires prolonged digestion. Cellulose means that herbivores need much longer, more elaborate digestive tracts.

Like the stomachs of other large herbivores, human stomachs hold about 25% of the total gut capacity. Based on the energy content of wild foods, humans can only consume 800-1270 calories per meal, which is less than our daily caloric needs. Therefore, we must eat multiple meals each day. When food is present, the pH of our stomachs is about 4-5, which is only moderately acidic.

Because of the small capacity of our stomachs and our inability to eat carrion, humans cannot recover large amounts of energy from a single carcass the way carnivores can, so hunting is much less efficient for us. Because if you go around chasing an animal, trying to kill it, and you finally succeed after 10 days in killing something, you can’t even consume enough calories at that one meal to replace what you just expended trying to hunt. Furthermore, once that carcass starts to rot, you’re done. So for humans, the metrics of hunting don’t add up.

Thus, in the absence of reliable, large-scale preservation methods, hunting was not an efficient way of obtaining energy for stone age humans. This is why, throughout history, crop failures have resulted in famine, starvation, and death for human populations, and why plant-eating locusts are considered a plague and not a food bonanza. It’s different in cold areas, where meat can be preserved, but in equatorial regions where humans are believed to have developed, hunting is just not a good way of trying to obtain food.

Like other herbivores, human small intestines are very long. Human small intestines are typically 25-35 feet long. Body length in animals is measured from the top of the head to the base of the tail. The human body (measured head to tail bone) averages 2.5-3 feet. The human small intestine therefore is, like that of other herbivores, 10-11 times the body length. The mucosa of the intestines are further lengthened by being compressed like an accordion, and its surface area is further increased by villi and microvilli. The total surface area is equal in size to a singles’ tennis court. The enzymes are a mixture of protein, carbohydrate, and fat-digesting enzymes. Because fiber slows down digestion, the extended length allows sufficient time and surface area to extract absorbable nutrients from fibrous plant tissues. Like other herbivores, humans have an adjustable mix of carbohydrate, fat, and protein-digesting enzymes, with an unlimited capacity for carbohydrate digestion and absorption. We have a very long colon, which, as in many other herbivores, has a pouched structure with an annular configuration.

Like other herbivores, and unlike carnivores, humans also have an appendix, which is part of the gastrointestinal tract immune system. Our colons perform functions including water absorption, and fiber fermentation (hind-gut fermentation). They produce short-chain fatty acids that give us additional energy. They also produce vitamins and enhance immune function. Like other hind-gut fermenters, we eliminate semi-solid, fibrous stools.

In humans, food spends more time in the colon than in the stomach and small intestine combined. Food stays in your stomach a maximum of three hours. It then passes through the entire length of the small intestine in two hours or less. It then spends up to 13 hours in your colon, where a bacterial population breaks it down into short-chain fatty acids (STFAs) that are vitally important to human physiology. One of the short-term fatty acids, butyrate, is four carbons long, and is the preferred fuel for colonic cells. They don’t like to get their energy from your bloodstream; they prefer to get it from the intestinal lumen. There is also a three-carbon STFA called proprionase that is absorbed, goes to the liver, and inhibits an enzyme called HMA co-A reductase, which is the same enzyme that statin drugs inhibit. So healthy intestinal bacteria can do what medications do without the side-effects. Proprionase also helps reduce your liver’s production of glucose, like another drug, Glucophage. A two-carbon STFA called acevate is used by your body cells in lieu of glucose, which means that your glucose stores last longer, your brain is happy, you don’t get hungry as often.

Your colon also activates phytoestrogens and lignans that makes them much more bioavailable and bioactive. Breast cancer risk is decreased by 22% in women with the highest lignan intake. Whole grain rye decreases PSA levels by 14% in men with prostate cancer. Consumption of soy phytoestrogens reduces breast cancer and colon cancer risk in a dose-dependent fashion, because it increases sex-hormone-binding globulin, which goes into your bloodstream and latches on to estrogen in women and testosterone in men. The lower your levels of unbound estrogen and testosterone, the lower your risk of breast cancer and prostate cancer, respectively. The metabolism of phytoestrogens by specific bacterial strains in the colon can make phytoestrogens more bioavailable. Bacterial strains that are most effective at activating phytoestrogens are those associated with plant-based diets.

Diets high in fiber stimulate beneficial Clostridial species that give you a much thicker layer of mucus, which prevents pathogenic bacteria from getting into your bloodstream. Cruciferous vegetables stimulate immune cells to be much more active. SCFAs and other fermentation products induce T-reg cells and help calibrate immune function to dampen inflammation. Fiber-poor diets produce a state of simmering hyperactivity that promotes higher levels of inflammation. Changes in bacterial populations brought about by a diet too poor in fiber and too rich in meat and fat may select for species that are less efficient at fermenting fiber and tend towards putrefecative activity. These aberrant strains may also be less adept at producing neurotransmitters like GABA, serotonin, and others. The thinner mucus level also leads to “leaky gut,” whereby bacterial endotoxins that increase total body inflammation seep out of the colon and into the bloodstream. In addition, these heat-stable bacteria remnants are also found in and absorbed from raw and cooked meat. Leaky gut can also increase lipo-polysaccharide antibodies, a bacterial antigen, which can cause depression. Inflammatory markers correspond with major depression. Lipo-polysaccharide (LPS) increases these inflammatory chemicals. Studies show that major depression is accompanied by an activation of the IRS and elevated levels of pro-inflammatory cytokines, and LPS may induce depressive symptoms. LPS and also bacterial lipoprotein (BLP) were found to be highly resistant to cooking, low pH, and protease treatment. Studies have also shown that a single meal high in animal fat can induce endotoxemia.

Countries with the highest level of meat consumption have the highest level of Alzheimer’s. Worldwide, the prevalence of Alzheimer’s correlates with the per capita consumption of meat and animal foods. Research suggests that risk for Alzheimer’s disease can be decreased by reducing intake of saturated and trans fats, animal protein, alcohol, maintaining a healthy weight and stopping smoking. Fruits, vegetables, fruit and vegetable juices, antioxidants, vitamins C and E, exercise, and omega-3 fatty acids have all been shown to protect against this disease. As with other chronic diseases common in western countries, inflammation is associated with increased risk for dementia. Markers of inflammation, such as TNF-a, C-reactive protein, and IL-6 have all been shown to be elevated in individuals with dementia. In fact, some scientists believe that levels of inflammatory markers can actually be used to predict cognitive decline and the development of dementia.

Thus, our digestive system is designed to quickly and efficiently extract most or all of the readily absorbable nutrients from ingested food through absorption in the small intestine and then allow an extended period of time for fermentation of the remaining fiber in the colon. Fiber fermentation is essential for normal physiology, optimum health, and brain function and protection.

Only herbivores have carbohydrate-digesting enzymes in their saliva. Only herbivores have an appendix, which is part of their gastrointestinal immune system. Herbivores cannot detoxify pre-formed vitamin A, but they can take beta carotene and convert it into vitamin A. The excess is stored in the skin as a natural sunscreen. Herbivores can detoxify a wide range of plant alkaloids. Most herbivores require a dietary source of vitamin C. Herbivores cannot eat rotting flesh. They easily develop heart disease when fed diets high in saturated fat and cholesterol.

A variety of plant foods contain abundant calcium. Cows get calcium from green plants, and moose (and other deer) grow antlers of solid bone weighing 80 pounds, (a human skeleton weighs only 35 pounds by comparison) in only 3 months from a diet composed entirely of plants.

Eating plant foods actually modifies the proteins that surround our DNA in ways that help regulate the function of our genes. It turns off cancer-causing genes, and turns on protective genes. In other words, your diet helps control your DNA. All DNA is coated and protected by histone proteins that can be modified via methylation and acetylation through plant foods consumed in the diet. These changes affect gene transportation rates for many genes, including oncogenes and tumor suppressor genes.

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