Why Is the Human Brain Size Decreasing and What Does It Imply?
The phenomenon of decreasing human brain size observed over the last several thousand years is a topic of ongoing research and debate among scientists. Here are some key points that may help us understand this trend and its implications for human cognition and society.
Evolutionary Adaptations
Some researchers suggest that as humans have developed more complex societies and technologies, the need for certain cognitive functions has shifted. For example, a reliance on tools, language, and social structures may have reduced the evolutionary pressure to maintain larger brains. As society becomes more intricate, individuals might no longer need to have extensive knowledge about everything, thereby putting less of a workload on their brains, leading to a reduction in size. This view is notably echoed in the recent decrease in brain size observed in the last 14th century as population growth and the distribution of knowledge among people reduced the burden on each individual's brain.
Diet and Nutrition
Changes in diet over millennia, including the agricultural revolution, may have influenced brain size. A diet that provides sufficient energy and nutrients can support brain function, while deficiencies might affect brain development. This is evident in the significant changes observed in cranial volumes over the last 50,000 years, where modern human brains are about 13% smaller compared to those of our distant ancestors. Studies show that early hominins had larger brains due to factors favoring brain mass expansion, but over time, a reduction in brain size can be traced back to several centuries.
Social Structures and Brain Efficiency
With the rise of social organization and cooperation, individuals might rely more on social networks for information and problem-solving, potentially reducing the need for larger brains that are necessary for individual survival. Additionally, some studies suggest that brain size does not directly correlate with intelligence. The efficiency of neural connections and the organization of brain structures may be more important than sheer size, leading to a potential decrease in size without a loss in cognitive ability.
Genetic Factors and Population Dynamics
Genetic changes over generations could also play a role. As certain traits become more favorable in a changing environment, the human genome may have adapted in ways that include a reduction in brain size. For instance, the global mean cranial capacity for modern humans is reported to be around 1300 to 1400 cubic centimeters, compared to about 1500 cubic centimeters for early Homo sapiens.
Climate Change Influences Brain Size
A more recent study by J. Silva et al. indicates that the brains of modern humans are smaller than those of our ancestors, particularly since the last ice age. This shrinkage is attributed to the rapid population growth and the associated rise in the distribution of knowledge, spread out among people. However, other researchers argue that brain size changes are due to climate change. Our brains are very demanding organs; they consume 20% of the oxygen we breathe and use 400-500 calories per day. This high energy consumption and heat generation make the brain susceptible to heat stress. As climatic conditions have changed, brain sizes have followed, with cooler climates correlating with larger brains and warmer climates with smaller brains.
Implications for Memory and Intelligence
While the decrease in brain size might suggest a reduction in cognitive abilities, correlation does not always imply causation. Brain size correlates only slightly with IQ (
Before writing and drawing were mastered by humans, everything had to be remembered by rote and passed on from generation to generation verbally. This extensive need to memorize is an important factor that increases the number of neurons and the size of the hippocampus. With the invention of writing about 4000 years ago, there was a shift towards externalizing memory. While the medieval student at Bologna University had to memorize key texts like the seven volumes of Galen, the 16th-century writer Montaigne used mnemonic devices when quoting ancient authors. The invention of the printing press further externalized memory, making it easier to access and learn from written materials rather than remembering everything manually.
The loss of certain skills, such as mastering the art of mnemonics, cursive writing, and drawing, has contributed to the shrinking of the modern brain. With ever more reliance on electronic memory and the loss of certain motor skills combined with global warming, this trend is expected to continue.
Conclusion
Understanding the decrease in human brain size involves a complex interplay of various factors, including evolutionary adaptations, changes in diet and nutrition, social structures, genetic factors, and climate change. While this trend has been observed over time, it does not necessarily mean a decline in human intelligence. The modern human brain is just as capable of cognitive tasks, but it’s more efficient and adaptable.