Meat, Plants, and Humans..

This week on ASU campus I managed to attend a fascinating talk: Reconsidering the Role of Plant Foods in Hominin Diets by Dr. Chelsea Leonard. Screen Shot 2016-02-17 at 11.41.44 AM

It was a job talk for the Evolutionary Anthropology department here at ASU and Dr. Leonard is an evolutionary ecologist interested in “human foraging decisions & diet reconstruction”(so- her work would help to clarify what humans ate in the past!) working with Twe populations in Namibia (southwest Africa).

Why does Dr. Leonard study the role of plants? Since shifting towards more meat in diets of early humans has been suggested to be crucial for the unique adaptations in our genus (e.g. large brains), animal foods appear to be very Screen Shot 2016-02-17 at 12.00.04 PMimportant. There is indeed a strong case for meat in a human diet- in comparison to chimpanzees who are mostly herbivorous (eat plants), the human gut has opposite proportions- our small intestine is much longer, while the colon is a lot shorter. The colon is where fiber fermentation occurs- something crucial if you are eating lots of plant foods (and wild plant foods are very high fiber!). What Dr. Leonard suggests, though, is that meat’s importance in human diets may be quite overstated (especially in meat-heavy “paleo” diets popular now).

The people she studies- Twe- are “forager-horticulturalists”; while the Namibian government has been providing maize for them (this started very recently, in the last 7 yrs or so), they mostly forage for wild foods and have very low intake of animal products. Apparently, historically this population hunted large game and had a higher meat intake.. but the area is very poor in large animals now (and has been this way for ~200 yrs).

While I wont’ be able to describe everything Dr. Leonard discussed, I found the following fascinating.. Based on her observations and interviews with the Twe, she constructed and analyzed a hypothetical (yet realistic) diet for this region. Since Twe seem to be doing just fine health-wise with an extremely low animal food intake (there might be some birds, insects, rodents eaten from time to time), she wanted to test if their meatless diet truly meet basic nutritional requirements. FullSizeRender 9

Based on the plants the Twe regularly eat, her analysis showed that such meatless diet can realistically provide enough protein (it can reach minimum levels of essential amino acids our body can not produce without foods that contain them), it can also provide enough fat (while most plant sources were extremely low in fat, the grass seeds often eaten are rather high in it). The main issue with this meatless diet was calories. Getting enough calories to survive would be improbable : while the hypothetical food intake reaches 1774 calories a day.. only 772 of them are metabolized. What this means is that a lot of these calories are not available to the human body- since humans can not ferment fibers very efficiently, a lot of this rough wild plant fiber is indigestible and does not provide our body with energy.

The main issue with this meatless diet was calories.

Since foraging for wild plants  is very labor intensive (and this does not really mean standing around picking berries, but e.g. digging up roots that are about 1 meter (~40 inches) into the ground, or grinding grass seeds and cooking them into porridge), there isn’t enough time in a day to get enough digestible calories from foraging. So animal products are more efficient and provide a concentrated mix of not only essential nutrients, but fat, protein, and calories. While the speaker couldn’t quite estimate the % of calories coming from small game (the birds, insects, etc.), it was very small but still was a part of this population’s diet [note: any time honey was available, it was eaten in large amounts and rather adored, apparently!]. Thus, while a  vegetarian diet can be maintained in our modern world with plentiful food supply (and supplementation), it was not possible for non-industrialized populations.

humans are highly adaptable as we span huge geographical areas, and thus no single “diet” “made us human”

We know humans are highly adaptable as we span huge geographical areas, and thus no single “diet” “made us human” (thus, there is no one Paleo Diet). Yet plants are extremely important in our history- we see that they can sustain populations in good health to a very large degree. One issue with studying the role of plants in human diets is that they do not last well archeologically (e.g. it’s much easier to find evidence of large game being consumed, because their remains last well).

while a  vegetarian diet can be maintained in our modern world with plentiful food supply (and supplementation), it was not possible for non-industrialized populations.

Overall, this was a really great talk! It also reminded me of a paper I read on the significance of plant foods in human evolution, which I talked about HERE.

[note: if you are an evolutionary anthropologist sand have any edits/clarifications to my post, please comment! I am not an evolutionary anthropologist :)]

The Sci Files #1: Importance of Carbs in Human Evolution

Note: This Fall I decided to attempt even more science communication! The Sci Files (imagine the x files theme playing) will be a collection of health & food-related research articles that I summarize in plain(er) language. I became quite passionate about breaking down hard-to-understand research for the public audience and I’ll try to do my best, considering I’m no expert! Yet 5 years of graduate courses- statistics, research methods, nutrition psychology, evolution & medicine- at least give me skills to understand a lot of the material that might be overwhelming to a lay reader. I will try to keep the summary to one page (~500 words), possibly followed by extra material that could be interesting 😉

For the first Sci File, i’m looking at a paper discussed yesterday during a lecture on the paleolithic diet. It’s published in 2015 in The Quarterly Review of Biology and the title intrigued me “The Importance of Dietary Carbohydrate in Human Evolution”. I’ve heard multiple talks on how the various “paleolithic” diets could have included starchy foods, but I didn’t think they were substantial parts of such diets.
Original paper: Hardy, K., Brand-Miller, J., Brown, K. D., Thomas, M. G., & Copeland, L. (2015). The importance of dietary carbohydrate in human evolution. The Quarterly Review of Biology, 90(3), 251-268.

Short summary:  

Apparently, you can delete the “NO” and still keep calm 😉

The authors propose that carbohydrates- particularly cooked high starch plant foods like tubers & roots- were essential in the evolution of our species- especially for the quick expansion of the human brain. They support this by showing that (1) critical development of this large glucozse-hungry organ required digestible carbohydrates, and eating cooked starch would really increase this energy availability to the brain (+ other glucose-hungry tissues such as red blood cells and the developing fetus).

They also show that the mutation in the enzyme for digesting carbs (salivary emylase, AMY1) co-evolved with both cooking and eating starchy carbs, giving an advantage to early humans. To put it in simpler terms: carbs were quite important, as shown in our increased ability to digest cooked starch (otherwise, why retain this mutation if we did not rely on cooked starches for a substantial amount of time?). A meat-heavy diet wouldn’t have provided sufficient glucose or energy to the growing brain + 1) large amounts of protein are in fact toxic and 2) providing sufficient amount of animal-based food would require too much effort:

“the energy expenditure required to obtain it may have been far greater than that used for collecting tubers from a reliable source”

Some Context: 

There is no clear agreement on what constituted a “Paleolithic diet”, but it makes sense to assume that our current physiology should be optimized to the kind of diet we had during our evolutionary past. Some important features in our evolution are considered linked particularly to key changes in diet: smaller teeth, smaller digestive tract (1.8 mln years ago), larger brain size (began ~2 mln yrs ago; accelerated around 800,000 yrs ago), and better aerobic capacity (ability of the heart and lungs to get oxygen to the muscles) about 2 mln years ago.

Early hominins include modern humans, extinct human species, and all our immediate ancestors

Some have argued that these changes happened because  humans transitioned from a diet based on fibrous plants to mostly meat-based diets.. But this paper offers evidence that both plant carbohydrates (carbs) and meat were crucial in human evolution. In their words:

We contend that in terms of energy supplied to an increasing large brain, as well as to other glucose-dependent tissues, consumption of increased amounts of starch may have provided a substantial evolutionary advantage to Mid-to-Late Pleistocene omnivorous hominins“.

This photo is missing some starches!

Actual physical remains of early hominins are quite rare, so there is a lot of uncertainty about their lives. As already mentioned, there were several important changes in hominin morphology (size, shape, and structure of an organism) related to the appearance of Homo erectus (teeth, digestive length, brain). Anthropologists propose that they occurred with a change from a “high-volume, low-energy diet” (lots of fibrous plant material that’s not very calorie rich), to a low-volume, high-energy diet (so foods that are more packed with energy like meats and starchy roots & tubers). 

It looks like climate fluctuated between moist and dry periods, which required flexibility in diet (omnivory).. Increased meat consumption has been suggested as an important buffer against such environmental change (and helped expend into new unfamiliar environments), but high starch plant foods might have also been a very common and important part of the diet- especially when cooked. The timing of widespread cooking is not known, but it is argued that it was long enough ago to allow for biological adaptations to take place.

Note: Secure evidence of the use of fire to cook dates to about 400,000 years ago, though some suggestive evidence for a relationship between humans and fire dates to at least 1.6 mln years ago.

The fact that early hominins ate starchy foods is supported by various evidence (the paper goes through rather wordy technical anthropological examples that I fail to summarize in a simpler way). But while meat-eating evidence usually survives (e.g. animal remains with cut marks suggesting being butchered), evidence for plant foods doesn’t, which makes it hard to reconstruct ancestral diets based on physical remains alone (and biases them towards exaggerating meat eating).

Co-evolution of cooking & carb-digesting genes

Humans have the ability to digest starches with the help of enzymes in saliva- salivary amylase! AND humans are quite unusual as we have high levels of these enzymes, suggesting an adaptation to diets rich in cooked carbohydrates. Also, people from populations with high-starch diets have generally more AMY1 copies than those that have traditionally low-starch diets (hey! adaptation!).

Amylase (salivary amylase or AMY1)- enzyme that begins digesting starches in the mouth as it’s present in the saliva. Authors hypothesize that cooking and variation in the salivary amylase gene copy number are correlated.

The variation in copy numbers of salivary amylase genes is an important point of the paper – these enzymes are pretty much ineffective on raw starch, but cooking substantially increases their potential to provide energy/calories. So multiplication of the salivary amylase (AMY1) would become selectively advantageous only when cooking became widespread. (It’s been estimated that the three human AMY1 genes have been evolving separately for less than 1 million years). The authors theorize a gene-culture co-adaptation scenario here: cooking starch-rich plant foods (cultural evolution) coevolved with increased salivary amylase activity in the human lineage (gene evolution). Without cooking, eating starch-rich plant foods probably couldn’t meet the high demands for preformed glucose noted in modern humans.

Note: A mutation that is selectively advantageous means a change in DNA that gives a survival advantage to a particular genotype under certain environmental conditions. SO in an environment where starches are available (e.g. you can find a lot of roots and  tubers) and humans have learned to cook, having more copies of the AMY1 gene that aids in digesting cooked starch would allow those folks to survive more (e.g. in times of food crisis when they can’t hunt or gather other sources of food, etc.) vs. folks who don’t have that mutation.

To further test the paper’s hypothesis, we need “a convergence of information from archeology, genetics, and human physiology”. So let’s stay tuned 🙂


Well, i’m at around 900 words, which is more than the summaries i hope to do in the future! In my defense, this paper was FULL of fantastic information, often rather technical and challenging to explain in less words. I do have some extra content below i found fascinating if you found this summary interesting!

Continue reading

No, we are not carnivores. But we’re no apes either.

I often read on vegan health groups that humans are not meant to eat meat because our guts are long like those of the apes (who are predominantly plant-eating) vs. the short guts of carnivores (meat-eating animals). In fact, I used to argue this way myself in my vegan days. 🙂

However, once you understand a topic a bit better, the simplified and incorrect statements about it simply irritate you.


Human guts, when compared to those of existing apes, have similarities AND differences. Humans and apes show the same gut anatomy- simple acid stomach, a small intestine, etc.. However, humans stand apart from all apes: more than half of human gut volume is found in the small intestine while all apes have by far the greatest total gut volume in the colon; also the overall size of the human gut in relation to body size is small in comparison to that of apes.


What this means is that humans have adapted to a “high quality” diet. Here high quality means a calorie and nutritionally DENSE diet, which includes animal foods and tubers. A high quality diet for humans means that we need to eat a smaller volume of food to obtain the nutrients and energy we need.

The way our gut differs from apes says a lot. Most large primates have expanded colons, which is an adaptation to fibrous low-quality diets [“low-quality” here = highly fibrous foods such as leaves and bark]. The large colon allows fermentation of low-quality plant fibers (which allows extraction of additional energy in the form of volatile fatty acids). Our relatively enlarged small intestine (the principal site of nutrient digestion & absorption) and smaller colon reflects an adaptation to an easily digested diet that is nutrient-rich.

There is a general consensus that current hominoids (apes and humans) come from a strongly plant-eating ancestry. Apes, however, evolved into larger bodies that allowed to sustain themselves on lower quality diets. By eating both animal matter [to satisfy requirements for essential nutrients] and plant sources [primarily for energy] humans were able to avoid the constraints imposed by body size increases in apes (such as lower mobility and sociality in apes). Simply speaking: the bulkier they get and the more time they spend eating, the less they move around and the less social they are, which is a disadvantage compared to humans.uuuu2


This dietary change in humans (adding animal and other dense foods), which departed from known plant-dominated diets of the apes, was eventually reflected in our brain size (much larger), overall form of our guts (shift in gut proportions, overall gut size) as well as dentition (smaller teeth, jaws). Concerning our particularly large brains: our brains are particularly energetically “expensive” as we expend a larger proportion of our daily energy on brain metabolism than other primates (in comparison to other primates, our brains are 3 times the size). Paleontologists believe that fast brain evolution happened about 1.8 mln. years ago and was associated with important changes in diet and foraging behavior (some argue it is specifically the addition of meat that allowed for such large brains to evolve). Apart from switching to high quality nutrition, humans show other adaptations to having a larger brain- compared to other primates we are “undermuscled” (less skeletal muscle) and fattier. Greater level of body fatness in human infants in fact helps grow a large brain by having stored energy and reducing energy requirements of the rest of the body (that has less muscle mass).


The point is: no, we’re definitely not “meant” to be vegetarian. Also, the point is not to say that vegetarianism doesn’t make sense for many of us. There are plenty of great reasons to avoid animal foods (ethics, environment, etc.), plus it’s easy to have an adequate veg. diet for adults with availability of supplements (vegan diets are not recommended for small children, though, considering brain development; the several vegan PhDs I know did not raise their children vegan specifically because of this) … but stating that we are not meant to thrive on both animal and plant sources is incorrect. Contrasting us with true carnivores [like cats] to show how very different we are (e.g. hey we don’t have claws and sharp teeth… um.. we however do have large brains to allow for sophisticated tool creation that replaces those) is also a terrible idea- we are not true carnivores either and have a dual dietary strategy [plants + animal sources].

Lastly, all this material should not support the notion that we ought to eat bacon 10 times a day.. meat clearly has a place in our diet, but this shouldn’t be used to justify a purposeful meat overload (I’m not sure what the benefit is for advocating heavily animal-based diets, considering modern animals are fattier and less packed with phytochemicals than the wild ones + there’s the whole issue of antibiotic and hormone use at the minimum. Unless, of course, you go on the “carbs are evil” side, but I am not in favor of that view… or you imagine our ancestors ate predominantly meat, which does not look to be the case since the diets varied dramatically depending on environmental circumstances). 

(Note: I’m not pretending to actually be an expert on any of these topics; I simply read peer-reviewed articles and hang out with evolutionary anthropologists 🙂 ).


Milton, K. (2003). The critical role played by animal source foods in human (Homo) evolution. The Journal of nutrition, 133(11), 3886S-3892S.

Leonard, W. R., Snodgrass, J. J., & Robertson, M. L. (2007). Effects of brain evolution on human nutrition and metabolism. Annu. Rev. Nutr., 27, 311-327.

P.S. The bottom line is: these t-shirt designs are both incredibly dumb (disclaimer- i laughed a bit..but immediately felt guilty :D)

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