Hard chews: why mastication played crucial role in evolution
A preeminent study has, for the first time, directly measured the energetic cost of chewing in humans and has suggested it may have played a crucial role in the evolution of human jaws, chewing muscles and teeth.
The University of Manchester led study - published in the journal Science Advances - indicates that you exert more energy when chewing hard to process foods like nuts and raw fruit.
However, the speed at which we chew, they find, has much less of an effect on the energy exerted than the hardness of the material we chew.
The scientists recruited 15 women and 6 men aged between 18 and 45 and asked them to chew two different types of odourless and tasteless gum: soft and stiff.
They then measured the volunteers’ energy expenditure when chewing by making precise measurements of oxygen consumed, carbon dioxide production and linking this to chewing muscle activity.
When chewing the softer gum, the subjects increased their energetic expenditure by an average of 10.2% relative to their basal metabolic rate, rising to 15.1% above for the stiffer gum, respectively when compared to their basal metabolic rate.
Scientists believe the ability to chew evolved in some vertebrates around 260 million years ago.
The evolution of complex chewing and teeth was a key evolution step that allowed mammals to eat a variety of different foods, however, humans have developed ways to process food, so it is easier to consume.
Many modern foods are the end products of thousands of years of artificial selection, increasing the ease at which they are consumed. However, foods consumed by foraging peoples and early humans would have almost certainly required a greater masticatory effort to orally process food.
The results suggest that because early humans probably ate harder to process foods, the evolutionary response of natural selection would be to produced jaws, muscles, and teeth which, to some extent, optimise the energy used to chew.
Adam van Casteren, an evolutionary biomechanist from The University of Manchester was the lead author on the study collaborating with scientists from the Max Planck Institute for Evolutionary Anthropology (Germany), The University of Chile (Chile) as well as Maastricht University Medical Centre and Leiden University (the Netherlands).
He said: “We are the first to demonstrate that the energy expended in human chewing is substantial and that the harder the material which is chewed, the higher the impact on the metabolic costs.
“We believe the need to extract maximal energy from food sources without wasting it on processing costs, could be one of the driving forces behind the evolution of mammalian mastication.”
“That is because in some foods, nutrients are not readily available and the ability to chew effectively is probably vital for any animals’ survival.”
He added: “For modern humans, it is likely that mastication represents a small part of the daily energy budget.
“But before the onset of cooking and sophisticated food processing methods, when food needed to be chewed for much longer, the costs to our ancestors have likely been relatively high.”
“This research brings a novel energetic dimension to the interpretation of early human dentofacial fossils.”
The study was funded by the Max Planck Society and the Biotechnology and Biosciences Research Council and the European Research Council.
Case study: the energy cost of chewing to great apes
Chimpanzees consume raw meat at an estimated 400 kcal/hour and orangutans at a substantially slower rate of 185 kcal/hour (44, 45).
If we assume that chewing meat exerts a similar energic cost to the stiffer gum, then chewing meat would cost 9.2 kcal/hour.
This would result in a 2.3% reduction in energy gained from meat per hour for chimpanzees and a 5.0% decrease in the case of the orangutan.
These numbers are speculative, but given the high mechanical challenge provided by the connecting tissues of actual meat, they are likely to be conservative.
Image 1: The ventilated hood system at Maastricht University used to measure oxygen consumed and carbon dioxide produced during activities such as chewing.
Image 2: Measuring the chewing muscles with an ultrasound wand.