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Modern humans’ brains are shrinking – study shows

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A new study, published in Brain, Behavior, and Evolution, says that there has been a drop in encephalization levels in modern humans, with much of this decline explained by increases in obesity.

Hominin body size has risen during the last 4 million years, with the brain increasing at a rate that is disproportionate to the rest of the body. Mammals’ higher cognitive abilities can be attributed to an increase in encephalization. The brain mass of the anatomically modern (AM) Homo has reduced dramatically in the last 50,000 years, despite the millions of years of human evolution.

Study lead author, Jeff Morgan Stibel, says that it is unclear “why human brain size has been declining since at least through the Late Pleistocene.” Relatedly, we do not know of its “impact, if any, to human cognition.”

The study only included samples that had enough remains to establish independent estimations of both body and brain mass in our research.

There were thirty Holocene and twenty-five Late Pleistocene AMs. Hominin skeleton dimensions were employed to estimate cranial and postcranial sizes, as well as 16 earlier hominin fossils for a broader comparison.

For modern humans, autopsy data from 19 people who died between 1980 and 1982 was included.

Prehistoric Homo’s cognitive abilities were represented through encephalization. The correlation between brain size and cognitive function in humans has been proven to be robust, yet this could still be a restriction. However, due to the difficulties of working with prehistoric remains, it was not possible to take into account parameters such as the number of neurons, the size of the brain, or other variables associated with cognitive aptitude. Thus, it was only possible to measure cognitive aptitude by analyzing bodily traits.

Studies that explicitly test selection against traits linked with general cognitive ability, such as assessments of cognitive function and surveys on educational attainment, were employed to examine if there has been an evolutionary impact on human cognitive function.. Genome datasets from diverse research were utilized to identify characteristics linked with reproductive success that have been shown to influence evolutionary fitness in subsequent generations.

The study found that modern humans’ brain size has shrunk by 5.415 percent, and Homo encephalization has shrunk significantly over time as well. Human brain size has recently grown at a faster rate than our bodies have in the past few decades.

The majority of recent changes in human brain size appeared to be explained by changes in body size.

With the exception of modern humans, whose BMI has increased over the last 50,000 years, encephalization levels have remained rather steady. Stibel claims that recent brain shrinkage is an adaptive reaction to modern humans’ changing physiology, specifically their body mass. When obesity is taken into account, the brain and body mass of prehistoric AM Homo are comparable.

According to previous findings, brain mass and cognitive function have been closely linked throughout evolutionary history. The findings of various genome-wide association studies investigating evolutionary changes in cognitive ability suggest that both general cognitive function and educational attainment are under negative selective pressure, with evidence supporting a genetic decline in cognitive ability that is consistent with an evolutionary decline in brain size.

The genetic data, according to Stibel, has a paradox. Despite negative selective pressures, general intelligence and educational attainment have increased over the last century, with short-term changes in general intelligence largely driven by environmental factors such as health, education, and technology, which may offset or enhance long-term genetic trends. Genetic intelligence, on the other hand, is inherited; consequently, brain size and genetic intelligence do not predict general intelligence at the individual, group, or species level.

Given that environmental variables are more temporary than genetic predispositions, it is uncertain whether physical changes to the brain or genetic predispositions would have a long-term negative impact on human cognition. However, Stibel points to indicators that the Flynn effect — a phenomenon that describes the long-term favorable impact of environmental factors on human intelligence throughout the twentieth century – may be reversing.

Significant IQ losses have been reported in various parts of the world during the last 30 years, with the largest declines occurring in industrialized countries. As a result, improvements in the environment may not be sufficient to counterbalance the long-term effects of genetic and physical changes to the brain over an evolutionary timeline. Stibel wonders if natural selection can push species intelligence over a certain point of fitness.

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