In a review published in the journal Communication Biologyresearchers in the United States of America described the potential role and benefits of externally fermented foods in inducing human brain expansion. They further discussed the explanatory power of their ‘external fermentation hypothesis’ and examined relevant food practices in human cultures.
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Background
The volume of the human brain has tripled over the course of evolution. Although brain expansion has been studied with regard to its magnitude and timelines, the mechanisms underlying this modification are poorly understood. Several hypotheses have been proposed in this regard. The ‘expensive tissue hypothesis’ suggests that the enlargement of the human brain required a reallocation of resources from the digestive system. This is evident from the fact that the human gastrointestinal tract is 60% smaller compared to that of primates. However, since the gut itself is responsible for nutrient absorption, other studies suggested that increased brain size could only support net fitness if energy costs were adjusted through dietary changes, such as increasing consumption of meat or tubers . Another diet modification could have been cooking, which provided the right calories and nutrition to support a larger brain size and smaller gut size. However, the controlled use of fire would require a reasonable level of cognitive skills, which may not be present in hominids with a lower brain-to-body ratio.
Given the limitations of the above hypotheses, a gap remains in our understanding of the causes of initial encephalization. To address this need, researchers in the current review proposed the “external fermentation hypothesis” and discussed the evidence supporting it.
Internal fermentation
In the human gastrointestinal tract, especially the large intestine, symbiotic bacteria break down organic food material into nutrients such as short-chain fatty acids, a process called internal fermentation. It provides additional energy from undigested fiber, improves the absorption of vitamins and minerals and also enables the breakdown of anti-nutritional factors (ANFs) in food.
External fermentation and its role in brain expansion
On the other hand, external fermentation occurs when the breakdown of food is caused by bacteria in the environment or on the food surface. External fermentation offers benefits similar to internal fermentation. It improves the host’s intestinal health by contributing to microflora, improves nutrient absorption, improves nutrient bioavailability by breaking down ANFs and helps convert toxins into edible material. Furthermore, external fermentation improves host immunity as ingested probiotic bacteria colonize the intestines and prevent the colonization of pathogens in the area.
A schematic representation of the external fermentation hypothesis.
According to the researchers, the large intestine experienced a significant 74% reduction over evolution, indicating a reduced need to break down plant foods. The external fermentation hypothesis states that this change could possibly be an adaptation following the ingestion of externally fermented foods.
The feasibility of external fermentation was further discussed, with the hypothesis that early hominins may have transported and stored food, inadvertently initiating external fermentation. Over time, this practice may have developed into a culturally reinforced phenomenon, contributing to brain expansion and cognitive development in hominids.
Explanatory advantages of the current hypothesis over others
Several explanatory advantages of the external fermentation hypothesis have been identified over previous hypotheses. Compared to dietary changes such as harvesting tubers, eating meat and cooking, consuming externally fermented foods requires much lower cognitive skills. Fermented foods provide all the benefits of cooked foods while requiring no special planning, social coordination, or attention. Fermentation seems to be discovered more often than, for example, the fire needed for cooking. Furthermore, fires must be actively maintained, while fermentation is a passive process. Fermentation is also an easier alternative to other intensive food preservation techniques. The researchers suggest that hominids with lower cognitive skills and smaller brains may have accomplished the fermentation more easily than other methods.
Current fermentation practices
Today’s fermentation technology is highly developed and widespread. People all over the world manage to ferment all kinds of foods from different sources under different climatic conditions and time scales. The researchers collected a list of such examples and used them as evidence to support the plausibility, cultural acceptability and universality of fermentation.
Testing the hypothesis
To test the external fermentation hypothesis, the researchers suggest several possibilities, including examining genetic shifts related to metabolic, digestive and immune processes affected by external fermentation, analyzing olfactory receptor genes for possible positive selection related to the detection of fermented foods, and examining shifts in the human microbiome compared to simian relatives. They emphasize the need for empirical research, including microbiological research, comparative analyses, and genetic and genomic research, to support or refute their hypothesis.
Conclusion
The researchers in the current review propose the ‘external fermentation hypothesis’, which suggests that the adoption of fermentation technology by early hominids was a key mechanism for the expansion of the human brain and the reduction of the gut. They suggest that the conversion of enteric fermentation into an external practice may have been an important innovation, creating the metabolic conditions for the selection of brain expansion. The current review provides new insights into the evolution of the human diet and the anatomy of the gut and brain, and invites commentary and experimental testing to further validate the hypothesis.