The human brain in numbers: a linearly scaled-up primate brain

Area of Science:

• Neuroscience
• Comparative Anatomy
• Evolutionary Biology

Background:

• Historically, the human brain's size and cerebral cortex prominence were considered indicators of exceptional cognitive ability.
• Previous assumptions suggested humans possess more neurons and glial cells than expected for their brain size, justifying perceived cognitive superiority.
• The concept of encephalization quotient (EQ) often linked brain size to cognitive capacity across species.

Purpose of the Study:

• To re-evaluate the exceptionality of the human brain by examining its cellular composition and neuron count relative to other mammals.
• To determine if brain size is a reliable proxy for neuron number across different mammalian orders.
• To investigate the distribution of neurons within the human cerebral cortex compared to other mammals.

Main Methods:

• Development and application of a novel method to accurately determine the cellular composition (neurons and non-neuronal cells) of brains.
• Comparative analysis of brain cellularity in humans, other primates, rodents, and insectivores.
• Examination of cellular scaling rules specific to different mammalian orders.

Main Results:

• Different cellular scaling rules apply across mammalian orders, invalidating brain size as a direct proxy for neuron number.
• The human brain's cellular composition is not exceptional; it aligns with expectations for a primate brain of its size.
• The human cerebral cortex contains approximately 19% of all brain neurons, a proportion similar to other mammals.

Conclusions:

• The human brain's neuron count advantage stems from primate-specific economical scaling rules and its large size within that group, not exceptional cellularity.
• Cognitive abilities are better explained by absolute neuron numbers rather than brain size relative to body mass (encephalization).
• The findings challenge the notion of human brain exceptionality and necessitate a re-examination of established concepts in neuroscience and evolution.