In a review published in the journal Communication BiologyResearchers in the United States have described the potential role and benefits of externally fermented foods in stimulating the expansion of the hominid brain. They also discussed the explanatory power of “external fermentation hypotheses” and explored relevant food practices in human cultures.
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Background
Human brain volume has increased threefold over the course of evolution. Although brain expansion has been studied in terms of extent and timeline, the mechanisms underlying this change are not fully understood. Various hypotheses have been put forward on this subject. The “expensive tissue hypothesis” proposes that human brain expansion requires a reallocation of resources in the digestive system. This is evident in the fact that the human gastrointestinal tract is 60% smaller compared to primates. However, given that the gut itself is responsible for absorbing nutrients, other studies have suggested that a larger brain size may support net fitness only if energy costs are adjusted by dietary changes such as increasing consumption of meat or tubers. Another dietary change may be cooking that provides appropriate calories and nutrition to support increased brain size and decreased gut size. However, the controlled use of fire would require modest cognitive abilities, which may not be available in hominins with lower brain-to-body ratio.
Given the limitations of the above hypotheses, there remains a gap in our understanding of the triggers of initial encephalization. Addressing this need, researchers have proposed the “external fermentation hypothesis” and discussed the evidence supporting it in this review.
internal fermentation
In the human gastrointestinal tract, particularly the colon, symbiotic bacteria break down organic foodstuffs into nutrients such as short-chain fatty acids through a process called internal fermentation. It provides additional energy from undigested fibre, increases the absorption of vitamins and minerals, and also enables the breakdown of anti-nutritional factors (ANFs) found in food.
Role in external fermentation and brain expansion
External fermentation, on the other hand, is when the breakdown of food occurs by bacteria in the environment or on the surface of the food. External fermentation provides similar benefits to internal fermentation. It improves the host’s intestinal health by contributing to its microflora, increases the absorption of nutrients, increases the bioavailability of nutrients by breaking down ANFs, and helps convert toxic substances into edible material. In addition, external fermentation improves the host’s immunity as the ingested probiotic bacteria colonize the intestine and prevent the colonization of pathogens in the area.
A diagrammatic representation of the External Fermentation Hypothesis.
According to the researchers, the colon has experienced a significant 74% reduction over the course of evolution, indicating a reduced need to break down plant-based foods. The external fermentation hypothesis states that this change could potentially be an adaptation following consumption of externally fermented food.
The feasibility of external fermentation has been discussed in more detail, and it has been hypothesized that early hominins may have transported and stored food by unintentionally initiating external fermentation. Over time, this practice may have evolved into a culturally reinforced phenomenon that contributed to brain expansion and cognitive development in hominins.
Explanatory advantages of the present hypothesis over others
Several explanatory advantages of the external fermentation hypothesis over previous hypotheses have been identified. Compared to dietary changes such as harvesting tubers, eating meat, and cooking, consuming externally fermented foods requires much lower cognitive abilities. Fermented foods offer all the benefits of cooked food without requiring special planning, social coordination, or attention. Fermentation seems more likely to be discovered than the fire required for cooking, for example. Additionally, while fermentation is a passive process, fires must be actively maintained. Fermentation is also a simpler alternative to other intensive food preservation techniques. The researchers suggest that hominins with lower cognitive abilities and smaller brains may have achieved fermentation more easily than other methods.
Current fermentation practices
Today’s fermentation technology is quite advanced and widespread. People all over the world manage to ferment all kinds of foods from a variety of sources, under different climatic conditions and time periods. Researchers have compiled 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, researchers propose several ways, including examining genetic changes related to metabolic, digestive, and immune processes affected by external fermentation, analyzing olfactory receptor genes for potential positive selection regarding fermented food detection, and investigating changes in fermentation. The human microbiome compared to its monkey relatives. They emphasize the need for empirical research, including microbiological studies, comparative analyses, and genetic and genomic studies, to support or refute their hypotheses.
Solution
Researchers in this review put forward the “external fermentation hypothesis,” arguing that early hominin adoption of fermentation technology was a key mechanism for the expansion of the human brain and the shrinking of the gut. They suggest that converting intestinal fermentation into an external application may be an important innovation that determines the metabolic conditions for selection for brain expansion. This review provides new insights into the evolution of human nutrition and gut and brain anatomy and invites interpretation and experimental testing to further validate the hypothesis.