Is Meat Unhealthy? Part II

Over time, animals adapt to the foods they regularly consume.  This is how archaeologists can, for example, determine that Triceratops was an herbivore and Tyrannosaurus was a carnivore just by looking at the structure of the skeleton.  Adaptations to diet extend beyond skeletal structure, into digestion, metabolism, the brain, musculature, and other aspects of physical function.  What is our evolutionary history with meat?

Human Evolutionary History with Meat: 200 to 2.6 Million Years Ago

Mammals evolved from ancestral "mammal-like reptiles" (therapsids, then cynodonts) approximately 220 million years ago (Richard Klein. The Human Career. 2009).  Roughly 100 million years ago, placental mammals emerged.  The earliest placental mammals are thought to have been nocturnal shrew-like beasts that subsisted primarily on insects, similar to modern shrews and moles.  Mammalian teeth continued to show features specialized for insect consumption until the rise of the primates.

65 million years ago, coinciding with the evolution of the first fruiting plants, our ancestors took to the trees and became primates.  For most of the time between then and now, our ancestors likely ate the prototypical primate diet of fruit, seeds, leaves/stems, and insects (1).  Some primates also hunt smaller animals and thus eat the flesh of mammals, birds, reptiles, amphibians, and fish in addition to insects.  However, the contribution of non-insect meat to the diet is usually small.

Our ancestors diverged from those of chimpanzees, our closest living relatives, approximately 5-7 million years ago.  This suggests that our ancestors 5-7 million years ago were likely still eating a prototypical primate diet similar to modern chimpanzees: fruit, seeds, leaves/stems, insects, and non-insect meat, in descending order of importance.

Modern chimpanzees hunt and eat other animals regularly, although these foods only provide a small percentage of total calories.  In certain areas and seasons adult chimpanzees can average as much as 65 grams of meat per day (2).  This is 2.2 oz of meat per day, or, if we assume a chimp weighs 100 lbs and a human weighs 150 lbs, the equivalent of a human eating 3.3 oz of meat per day (1/5 lb). This is in addition to their consumption of insects and eggs.  However, these figures should be viewed as a maximum rather than a representative value.

The regular consumption of animal foods by chimpanzees is confirmed by stable isotope analysis, a technique that tells us what type of food "building blocks" the animals used to construct their tissues.  This technique is based upon the principle that you are, quite literally, what you eat.  Fahy and colleagues found a stable isotope signature consistent with regular animal food consumption in chimps in a Côte d'Ivoire park, although as expected plant foods were by far the main calorie source (3).  Males ate more meat than females, and successful hunters ate more meat than unsuccessful hunters.  A separate stable isotope study suggested that one population of chimpanzees ate a significant quantity of meat, while a second population did not (4).  The picture that emerges is one of sporadic, modest meat intake, with substantial population and individual variability.  It seems likely that the diet of our ancestors shortly before they evolved bipedalism was similar to this.

Human Evolutionary History with Meat: 2.6 Million Years Ago to the Historical Period

Our genus Homo emerged 2.6 million years ago in East Africa (Richard Klein. The Human Career. 2009).  This transition is marked by the appearance of stone tools in the archaeological record.  Our ancestors didn't waste any time using these tools to eat other animals, as demonstrated by the piles of bones with tool cut marks they left behind.

We really have no idea what proportion of the early Homo diet came from meat.  It's tempting to look at piles of bones and imagine a meat-heavy diet, but since plant foods don't leave many traces, there's no way to determine from a two-million-year-old archaeological site how important meat was in the diet.  All we know is that they ate some meat.  Although humans eventually became top-level predators, we also don't know whether these early humans were actively hunting, or simply scavenging what other predators left behind-- perhaps using their tools to access gristle, brain, and marrow inaccessible to other animals.

At the same time as tool-marked bones appear in the archaeological record, early humans began undergoing a remarkable physical transformation, which represented (in large part) a progressive genetic adaptation to a new subsistence strategy.  Our brain doubled in volume, our gut became smaller, and the proportion of small intestine to large intestine increased.  Our teeth and jaws became smaller and less robust (Daniel Lieberman. The Story of the Human Body. 2013).

What does this signify?  The consensus is that these changes occurred in response to a shift toward a so-called "high-quality" diet.  This means a diet that has a higher calorie density and contains less fiber, relative to the typical primate diet of leaves and low-calorie fruit (the latter is not at all suitable for a modern human).  The small intestine is what breaks down and absorbs protein, carbohydrate, and fat, while the large intestine ferments fiber to extract calories from it.  The shift from a large-intestine-dominant gut to a small-intestine-dominant gut signifies a shift from getting most calories from intestinal fiber fermentation, to getting most calories from direct absorption of protein, carbohydrate, and fat.

What constituted this "high-quality" diet?  No one knows for sure, but it's thought to have been some combination of meat and starchy foods such as tubers, gradually displacing leaves and low-calorie fruit.  If starchy tubers were on the menu, that implies that we may have been cooking our food much longer ago than previously thought, which is possible but highly speculative.  The first clear example of fire use by humans is dated to 780,000 years ago-- long after we had begun adapting to a high-quality diet (Richard Klein. The Human Career. 2009).  That does not rule out earlier use of fire.  It's unclear to what degree our Homo ancestors relied on starchy tubers before the widespread use of fire for cooking food (tubers have a much lower food value when eaten raw).  We know for certain, however, that meat was on the menu.

A number of studies have been conducted on the diet of ancient humans and related species, using stable isotopes and other methods.  Due to the complexity of interpreting these data when little is known about the diet or ecological context, the best we can say is that our ancient ancestors ate a diverse diet (5, Richard Klein. The Human Career. 2009).  Data from more recent Paleolithic human hunter-gatherers in Europe (both Homo sapiens and Homo neanderthalensis) suggest that they probably consumed a substantial amount of meat, as part of a diverse omnivorous diet.

Human Evolutionary History with Meat: Historical Period to Present

Throughout all of recorded history including today, virtually all cultures have eaten meat.  The only exceptions I'm aware of are cultures whose religion forbids meat consumption.  Craving and seeking meat is a trait that nearly all humans share, and many go to great lengths to obtain it.  It seems we have a natural affinity for meat.  However, meat consumption varies dramatically between populations, mostly as a result of restricted availability in some parts of the world.

Current and historical hunter-gatherers offer us a window into the possible diets of our ancestors, allowing us to flesh out the rough outline that archaeology provides us.  The most comprehensive analysis of animal food consumption by hunter-gatherer cultures was published by Loren Cordain and colleagues in 2000, with University of Michigan anthropologist John Speth as senior author (6).  They sorted through data on 229 hunter-gather cultures and arrived at the following conclusions:

1. None were vegetarian or vegan.
2. Animal food consumption varied widely between cultures.
3. On average, animal foods supplied more than half of all calories.

Some have criticized the conclusions of this paper, arguing that the underlying data are poor quality because they often rely on anecdotal and non-systematic accounts of diet, and sometimes include cultures that had access to guns.  This is true-- much of the underlying data are poor quality-- but this paper is the best estimate we currently have.  It is also roughly consistent with the few available detailed analyses of hunter-gatherer diets (e.g., Richard Lee's work with the !Kung).

I think we can tentatively state, based on historical data, that our more recent hunter-gatherer ancestors probably ate a substantial quantity of animal foods.  However, it's hard to say exactly how much.  As we see among contemporary hunter-gatherer societies, meat intake probably varied widely based on local availability.

Beginning about 12,000 years ago, humans around the globe independently developed agriculture.  In most locations, this eventually resulted in an extreme dietary shift away from animal foods and toward starchy grains, tubers, and legumes.  This dietary shift, when added to the infectious disease burden resulting from higher population densities and sedentism, was not good for our ancestors' health.  Early agriculturalists were typically smaller, sicker, and shorter-lived than the hunter-gatherers that preceded them (Cohen and Armelagos. Paleopathology at the Origins of Agriculture. 1984; Cohen. Health and the Rise of Civilization. 1991).  However, over time agricultural populations bounced back, presumably as their cultures and genomes adapted to the agricultural way of life.

Today, non-industrial agricultural populations tend to be of similar height and health status as hunter-gatherers, as long as they have access to a diverse diet providing sufficient calories and protein.  All non-industrial agricultural populations I'm aware of eat meat, although typically in modest quantity.

Conclusions

It's likely that our ancestors have been eating animal foods continuously for at least 100 million years,  and probably longer than that.  Due to this extremely deep evolutionary history with meat, we almost certainly bear genetic adaptations to animal food consumption.

Our strong affinity for meat drives humans around the world to accept substantial risk and expense to obtain it.  This suggests that meat may play an important role in our reproductive success.  However, exactly what role it plays is controversial.  It's easy to make a case that meat's value lies in its nutritional qualities, due to its high density of calories, protein, and micronutrients.  However, others have argued that hunting and meat consumption play a social role-- as a way for males to demonstrate their cleverness, bravery, and physical prowess to others.

Both explanations make sense to me, but it's clear that the nutritional explanation is part of the picture.  The reason is that humans are obligate omnivores: we have an absolute dietary requirement for vitamin B12.  Besides modern nutritional supplements, animal foods are the only food category that has been convincingly demonstrated to supply this nutrient in sufficient quantity.  This strengthens the argument that 1) our ancestors have been eating animal foods continuously for a very long time, and 2) we are genetically adapted to animal food consumption.

That said, the evidence also suggests that we're probably adapted to diets of widely varying meat content.  We aren't carnivores, and our ancestors have often gotten by on omnivorous diets containing only small amounts of meat.

Take-away points:

• Our ancestors have been eating meat for at least 100 million years.
• The prototypical primate diet is low in meat.
• Humans are obligate omnivores.
• Human cultures vary widely in meat intake, but they nearly all eat some meat.