Did the Umami Taste Bud Evolve?

“Umami Taste Receptors Evolved with Primates’ Diets”:
A Claim Promising Much, but Delivering Little

 

by Jerry Bergman, PhD

Did our umami taste receptors evolve from ape ancestors? [1] That’s a recent evolutionary claim deserving examination.

The evolution of the ability to taste different foods has always perplexed evolutionists for two reasons.[2] First, “taste is amazing. The human sensory systems allow us to distinguish about 100,000 different flavors. Flavors emanate from our bodies’ ability to discern one taste from another.”[3] Second, is the fact that the taste cell system is enormously complex and will not function until all of the parts are designed, produced, and properly assembled.

Missing a single part of the and taste cell system and the person will not be able to taste foods, although certain stimuli of food may be sensed, including temperature, texture, and liquid traits. In addition to the standard sweet, sour, salty, and bitter flavor sensations, a new taste has been added to the set called umami. Although discovered in 1907 by a Japanese scientist. only recently has research been done on its importance. The term ‘umami’ means “essence of deliciousness” or “pleasant savory taste” in Japanese.

Scientists now consider umami to be a distinct taste. Umami is sensed by dedicated receptors and not by a combination of the traditionally recognized taste receptors. It is a “meaty” taste sensed through taste receptors that typically respond to the amino acid L-glutamate and 5′-ribonucleotides, such as in glutamates and nucleotides which are widely present in meat broths and some fermented products. To accent the umami taste glutamates are commonly added to foods in the form of monosodium glutamate (MSG), and nucleotides in the form of inosine monophosphate (IMP) or guanosine monophosphate. Foods that have a strong umami flavor include red meats, fish, and even shellfish, but some non-meats have the taste including tomatoes, mushrooms, cheeses, and soy sauce.

The evolution of  umami taste

The Toda, et al., study considered in this review assumed the existence of the entire complex taste system. They only attempted to explain the gene that encodes the taste receptor that allowed primates in the past that relied on insects for protein to transition to eating leaves and fruit.[4] In short, “Taste perception plays an essential role in food selection. Umami (savory) tastes are sensed by a taste receptor complex, T1R1/T1R3, that detects proteinogenic amino acids.”[5] Many primates including humans have taste receptors, but some animals, like mice, lack them.

The study reviewed here found the molecular basis for this difference in umami tasting. After that, they went off into a narrative about evolution: “the receptor evolved in humans and some other primates away from mostly binding free nucleotides, which are common in insects, to preferentially binding glutamate, which is abundant in leaves. The authors argue that the change facilitated a major evolutionary shift in these primates toward a plant-heavy diet.”[6] The question at hand is whether the umami taste evolved.  The human receptor is narrowly tuned to respond to glutamate, and scientists did not have a good answer for why this was true.

The T1R1/T1R3 receptor is responsible for sweet taste in a variety of creatures, such as hummingbirds. Furthermore, baboons and macaques have very high sensitivity to glutamate but squirrel monkeys had a very low sensitivity to glutamate, just like mice do. Thus, the researchers “hypothesized that the glutamate taste perception was acquired during primate evolution.”  The only explanation for its existence in seemingly random types of life-forms is “that this narrow tuning has evolved convergently multiple times.”[7]

The Convergence rescue device

When differences exist, evolution is assumed, and if no pattern of the receptor exists as they progress up the evolutionary tree, convergent evolution is assumed. Convergent evolution is the idea that some structure or organ evolved from scratch numerous times in history. The problem is, if evolution is improbable for one occurrence, then evolution numerous times to reach the same outcome is far more improbable. A search for theories of how the taste system itself evolved turned up nothing. The research study reviewed here determined that the

receptors from all but four nonhuman primates—marmosets (Callithrix jacchus), tarsiers (Carlito syrichta), squirrel monkeys, and greater galagos (Otolemur crassicaudatus)—were sensitive to glutamate. These four species primarily rely on insects for protein. In contrast, the receptors from humans and gorillas were much less responsive to the free nucleotides inosine monophosphate and guanosine 5’-monophosphate than were the receptors from the rest of the primates.[8]

Thus humans in this test were closer to gorillas than the other four primates tested. The authors concluded that the

four nonhuman primates with a taste receptor specialized to detect free nucleotides primarily rely on insects as a protein source, while most of the other primates the researchers studied include leaves and fruit as a large proportion of their diets. In a final test of one species’ preferences, the researchers showed that squirrel monkeys prefer to drink water that has added free nucleotides, but show no preference for water with monosodium glutamate added.[9]

Analysis of the Toda et al. research

All that the researchers found is the obvious fact that taste-receptor design greatly affects food preference. The study did nothing to document the evolution of taste receptors in spite of the article’s title, “Umami Taste Receptor Evolved with Primates’ Diets.” The correct title should have been “Umami Taste Receptor Affected Primate Dietary Preference.” Nor did they find any evidence to support their sweeping conclusion that  “sensory systems … can evolve new adaptive sensory capabilities. …  To use leaves as a new protein source, the ancestors of large primates (including humans) evolved their umami taste receptor as a sensor for glutamate.”

The specialization for glutamate may have helped some primates to overcome the bitter and aversive tastes also present in leaves. Another evolutionary possibility is dietary preference (or constraints due to food limits in an animal’s environment) influenced the evolution of the umami taste receptor. Yet another possibility is animals with high concentrations of the umami taste receptors moved into environments which provided sufficient levels of food that their genetic taste preference that was favored. In the case of the umami taste receptor, environments with a large insect population might be an example.

Summary

The researchers claimed that they tested the “evolutionary plasticity of T1R1/T1R3 as a sensor for detecting proteinogenic amino acids.” In fact, all they did was to find that the “Umami Taste Receptor Affected Primate Dietary Preference” and nothing more. The study was well done and carefully executed, but it did not provide evidence for the evolution of the genes involved in umami tasting. They found differences in animals, but nothing about how these differences evolved. They only claimed that they did.

This should not surprise readers. Theories of the evolution of tastes are based largely on speculation about how they may have evolved. Here is an example of speculation that personifies evolution: “rather than evolving a specific dietary preference for fried fish or chocolate, maybe evolution has furnished us with a tendency to eat whatever happens to taste good, as our taste buds have evolved to detect foods with the energy and nutrients to promote health and wellbeing.” [10]

Similar statements are common in the literature on the evolution of the taste buds themselves.

References

[1] Toda, Yasuka, et al., 2021, Evolution of the primate glutamate taste sensor from a nucleotide sensor, Current Biology, August 26, https://doi.org/10.1016/j.cub.2021.08.002.

[2] Breslin, Paul, 2013, An evolutionary perspective on food and human taste, Current Biology 23(9):R409-R418, May 6.

[3] Food Insight, 2018, The Science of Taste. How did our sense of taste evolve?, March 13.

[4] Olena, Abby, 2021, Umami taste receptor evolved with primates’ diets. The Scientist September 6 .

[5] Toda, et al., 2021.

[6] Olena, 2021.

[7] Olena, 2021.

[8] Olena, 2021.

[9] Olena, 2021.

[10]  Laland, Kevin N., and Gillian R. Brown, 2002, Sense and Nonsense: Evolutionary Perspectives on Human Behaviour, Oxford University Press, New York, NY, pp. 16-17. Emphasis added.

Diagram of a cell cluster containing the taste bud receptor cells. Note the taste hairs on top of the cells. From wikicommons

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Dr. Jerry Bergman has taught biology, genetics, chemistry, biochemistry, anthropology, geology, and microbiology for over 40 years at several colleges and universities including Bowling Green State University, Medical College of Ohio where he was a research associate in experimental pathology, and The University of Toledo. He is a graduate of the Medical College of Ohio, Wayne State University in Detroit, the University of Toledo, and Bowling Green State University. He has over 1,300 publications in 12 languages and 40 books and monographs. His books and textbooks that include chapters that he authored are in over 1,500 college libraries in 27 countries. So far over 80,000 copies of the 40 books and monographs that he has authored or co-authored are in print. For more articles by Dr Bergman, see his Author Profile.