Scientists have learned that humans spend much less time chewing food than did the hominins—a find that carries implications for design.
Animals began chewing their food about 260 million years ago. Chewing has been observed among many vertebrates, but it’s especially evident in mammals. Mammals possess the bone and muscle structure for the precise occlusion and vertical and horizontal jaw movements required for efficient mastication of food. These unique jaw, mouth, and teeth designs contributed to mammals’ global radiation (spread), both on the continents and in the oceans.1
Mastication (the chewing of food) makes swallowing and digestion easier and more energy efficient. For efficient mastication, animals need to reduce food into small particles and then mix and lubricate these particles with saliva. The energy needed to reduce food from its ingested size down to what’s needed to be swallowed and digested defines the efficiency of the process.
Mastication involves teeth, jaws, mouth, tongue, pharynx, and masticatory muscles in a highly organized and complex fashion.2 Teeth in the mouth mechanically break down ingested food. Several processes combine to force food onto the working surfaces of the teeth: (1) cyclical vertical and lateral movements of the mandible with repetitive opening and closure of the jaws; (2) the tongue continually pushes food onto the incisors and molars; and (3) masticatory muscles cause the cheeks to tense, thereby also pushing food onto the teeth. Multiple pairs of salivary glands and their ducts ensure that just-right kinds and amounts of saliva are mixed into the food being chewed at just-right times.
Metabolic Cost of Chewing
A multidisciplinary research team led by biologist-anthropologist Adam van Casteren endeavored to measure, for the first time, the true energetic cost of chewing for modern-day humans compared to present-day great apes and the hominin species that predated humans.3 The team had to eliminate confounding metabolic costs (for example, costs associated with digestion, taste, smell, and sight). They overcame these confounding factors by having human subjects chew odorless, tasteless gums of varying stiffness and viscosity. The team determined the metabolic costs by having their subjects chew gum in a ventilated hood system where they measured oxygen consumption and carbon dioxide production.
For the human subjects, their average basal metabolic rate of oxygen consumption was 195 milliliters/minute. When they chewed soft gum, it rose to 214 milliliters/minute; when they chewed stiff gum, it rose to 236 millimeters/minute. Thus, chewing a tasteless, odorless gum elevated their metabolic rates by 10 to 20%. Of the human subject variables (weight, height, age, sex, time of day, gum order), only sex had any influence on the results, with women showing a slightly greater metabolic rate increase than men.
Humans’ Chewing Advantage
Van Casteren and his colleagues were the first scientists to demonstrate that energy expended in human chewing is substantial and that the stiffer the substance being chewed the greater the energy expenditure. However, for mammals, the time spent chewing per day is a more important factor than the energy expended per minute of chewing. Previous researchers have established that the daily chew times for present-day humans are low.4 The lowest observed daily chew time is 7.2 minutes, the maximum is 75.7 minutes, and the mean is 35.3 minutes.
The great apes spend much more time chewing their food than present-day humans. Chimpanzees and bonobos have an observed daily chew time of 4.5 hours.5 Orangutans’ daily chew time is 6.6 hours.6 Furthermore, the great apes expend much more energy per minute chewing than present-day humans. As van Casteren’s team noted, the energy that extinct hominin species spent chewing food must have been similar to that spent by the great apes.7
The researchers provided a comprehensive explanation for modern humanity’s enormous chewing advantage. First, they point out that plants typically protect their valuable nutrients with lignified (woody) structures that require great masticatory effort to break.8 Such plant tissue is what the great apes are observed to consume and is most likely the plant tissue that hominins consumed. Early modern humans, on the other hand, engaged in the cultivation of easy-to-chew plant species rich in nutrients and energy. Second, van Casteren’s team showed that early modern humans’ ability to grind, roast, process, and cook their food liberated them from energy-intensive chewing and lengthy daily chew times. Third, humans possess a unique anatomy designed to take advantage of easier-to-chew food sources.
Modern humans’ large brains and the long development times of human offspring demand a lot of energy to support them. This extra energy demand, though, is offset by how little energy humans need to chew food. The daily cost of chewing for present-day humans is only about 0.1% of their total energy expenditure. Modern humans’ large complex brains, bipedal capability, and manual dexterity also give them a huge advantage in their ability to acquire calories and nutrients.
Such efficient procuring, processing, and chewing of their food means that, unlike other animals, humans can devote most of their energy and time to pursuits that have nothing to do with keeping their bodies alive, healthy, and functioning. This efficiency sets them free to engage in science, engineering, mathematics, art, music, literature, recreation, social discourse, philosophy, and theology. Stated another way, it sets them free to express the image of God. Thanks to the research by van Casteren and his colleagues, we have yet another significant piece of evidence for the biblical doctrine of human exceptionalism.
A. G. S. Lumsden and J. W. Osborn, “The Evolution of Chewing: A Dentist’s View of Paleontology,” Journal of Dentistry 5, no. 4 (December 1977): 269–287, doi:10.1016/0300-5712(77)90117-8; Callum F. Ross et al., “Modulation of Intra-Oral Processing in Mammals and Lepidosaurs,” Integrative & Comparative Biology 47, no. 1 (July 2007): 118–136, doi:10.1093/icb/icm044.T. M. van Eijden, “Biomechanics of the Mandible,” Critical Reviews in Oral Biology & Medicine 11, no. 1 (January 2000): 123–136, doi:10.1177/10454411000110010101; Lumsden and Osborn, “Evolution of Chewing,” 269–287; Ross et al., “Modulation of Intra-Oral Processing,” 118–136.Adam van Casteren et al., “The Cost of Chewing: The Energetics and Evolutionary Significance of Mastication in Humans,” Science Advances 8, no. 33 (August 17, 2022): id. eabn8351, doi:10.1126/sciadv.abn8351.Chris Organ et al., “Phylogenetic Rate Shifts in Feeding Time during the Evolution of Homo,” Proceedings of the National Academy of Sciences USA 108, no. 35 (August 22, 2011): 14555–14559, doi:10.1073/pnas.1107806108.Callum F. Ross et al., “Ecological Consequences of Scaling of Chew Cycle Duration and Daily Feeding Time in Primates,” Journal of Human Evolution 56, no. 6 (June 2009): 570–585, doi:10.1016/j.jhevol.2009.02.007.Ross et al., “Ecological Consequences of Scaling,” 570–585.Van Casteren et al., “Cost of Chewing,” p. 3.Peter W. Lucas et al., “Mechanical Defenses to Herbivory,” Annals of Botany 86, no. 5 (November 2000): 913–920, doi:10.1006/anbo.2000.1261; Adam van Casteren et al., “Unexpected Hard-Object Feeding in Western Lowland Gorillas,” American Journal of Physical Anthropology 170, no. 3 (November 2019): 433–438, doi:10.1002/ajpa.23911.
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