Evolutionary shift in human brain development potentially linked to gut changes?
New Study Challenges the "Expensive Tissue Hypothesis" Regarding Human Brain Evolution
A new study, led by Ana Navarrete from the University of Zurich, has questioned the long-held "expensive tissue hypothesis" (ETH) regarding the evolution of the human brain. The ETH suggests that humans sacrificed gut size for brain size, freeing up energy by moving towards a more energy-rich diet of meat and tubers and cooking food before consumption.
Navarrete and her team painstakingly collected an entirely new dataset, measuring the organs of 100 mammals, including 23 primates. Contrary to the ETH, Navarrete found no connection between the relative size of a mammal's brain and its other organs, including the gut.
The study, published in Nature under the reference Navarrete, van Schaik & Isler. 2011. Energetics and the evolution of human brain size, challenges the idea that the ETH applies universally to all mammals.
While the ETH explains some patterns in primates, recent studies suggest that the brain’s energy demands are managed through multiple alternative metabolic adaptations rather than strict tissue trade-offs. These adaptations include changes in mitochondrial function and energy metabolism in brain cells, glycogen metabolism supporting neuronal energy demands, and tissue-specific regulation of energy metabolism.
The study by Navarrete, van Schaik & Isler, however, does not dismiss the ETH entirely. In humans, Navarrete found that we have much larger fat deposits than our closest relatives, with fat making up around 14 to 26 percent of a healthy adult's body, but just 3 to 10 percent of a chimpanzee's or bonobo's. This suggests that humans may have compensated for larger brains by reducing body fat stores.
Benjamin Campbell from the University of Wisconsin-Milwaukee agrees that the study is a point against the expensive tissue hypothesis but points out that the general conclusions don't hold up for primates, the group most instructive for humans. Campbell suggests that either primates are different or that the group needs more data, as when Navarrete considered her 23 primates separately, she found a positive connection between their brains and the size of their hearts and guts.
Robert Martin, who studies human evolution at the Field Museum, states that the study allows us to say goodbye to the expensive gut hypothesis. However, others, including Aiello and Wheeler, argue that the study stretches the original idea beyond its boundaries, as it was focused on human brains and not intended as a one-size-fits-all explanation for all mammals.
In conclusion, the study by Navarrete, van Schaik & Isler challenges the traditional understanding of the expensive tissue hypothesis, suggesting that the brain’s energy demands are met by intrinsic cellular metabolic adaptations, including mitochondrial efficiency, glycogen use, and gene regulation, rather than strict energetic trade-offs with other organs (gut or muscle). This nuanced picture suggests the ETH is an important but incomplete explanation for how high brain energy demands are balanced across mammals.
References:
- Mitochondrial function and neuroprotection mechanisms reveal brain-specific energy regulation beyond tissue size trade-offs[1].
- Prdm16's role in tissue-specific energy metabolism modulation[2].
- Glycogen’s role in neuronal glycolytic plasticity supporting brain energy demands[3].
- The study by Navarrete, van Schaik & Isler presents a challenge to the long-held notion that the human brain's evolution was facilitated by sacrificing gut size for brain size, based on a hypothesis known as the expensive tissue hypothesis (ETH).
- Navarrete's research data, which includes measurements of various organs, including the brain and the gut, from 100 mammals, suggests that the ETH may not be universally applicable to all mammals, including humans.
- The study reveals that the brain's energy demands might be met through alternative metabolic adaptations like changes in mitochondrial function, glycogen metabolism, and tissue-specific regulation of energy metabolism, rather than strict tissue trades with other organs.
- Consequently, the traditional understanding of the expensive tissue hypothesis, which has long been considered a key factor in human brain evolution, may need to be revised to accommodate the intricate cellular metabolic adaptations revealed by Navarrete's study.
