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.
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”
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“.
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!
- Modern humans need a reliable source of glycemic carbohydrates (glycemic carb is really digestible carbs that actually contributes to the rise in blood glucose, not the resistant starch that passes through our system undigested) to support normal functioning for the brain, kidney medulla, red blood cells, and reproductive tissues. In addition to the demands of the brain, red blood cells alone require about 20 grams of glucose per day directly from the bloodstream.
- When dietary carbs are absent from the diet (or during starvation) the brain begins to use ketones byproducts of high levels of fat oxidation. In this situation, ~80% of brain’s energy needs can be met from ketones, but even for those adapted to eating very low-carb, there still remains an absolute requirement for 30-50 grams of dietary glycemic carbs/day.
- While some glucose from eating is immediately used for energy, the rest is stored in muscle and liver tissues. Body reserves of glycogen are limited, though, and provide glycemic carbs (for brain, red blood cells, other tissues that can’t use fatty acids as their energy source) for only 18-24 hours of fasting (aka Not eating). With longer fasting or under starvation, there’s considerable loss of tissue protein (death due to starvation is not just due to reduction in body’s fat stores, but also critical loss of this protein to supply energy to the brain). So you’d have to generate large stores of glycogen during periods of sustained fasting/hardship. The diet for building such reserves must consistently provide caloric surplus.
Why are carbs important here? There is a limit (35-40%) to the amount of calories we can get from proteins. Above this ceiling, protein toxicity can occur and can cause death quite quickly (hunter-gatherer dietary practices suggest humans indeed avoided diets that exceeded this protein limit!).
- When eating animals- carbs can actually be obtained from the stomach contents of prey animals (makes sense!) and when quickly frozen meat after slaughter, you also retain a lot of its muscle glycogen (so another source of carbs). Another note- while a very high fat intake will prevent protein toxicity (just be reducing the % of protein from your overall diet), it also has a serious disadvantage (especially in the context of evolution!)- compromising reproductive function due to high levels of ketones in blood (glucose is the main energy source for fetal growth) [a link has been made between maternal gestational ketonemia & reduced offspring IQ].
- I mentioned increased aerobic capacity already.. so by 2 mln years ago, the early Homo became capable for endurance running- something considered important to exhaust prey, for example… glucose is the only energy source sustaining running speeds above 70% of maximum oxygen consumption.