The human brain in numbers: Comparative quantitative studies of the cellular composition of the nervous system of humans and other mammals to investigate the morphological bases of our cognitive advantage
The human brain is usually considered exceptional, an outlier among mammalian brains: the most cognitively able, the largest-than-expected from body size, purportedly containing 100 billion neurons and 10 times more glial cells, consuming an outstanding 20% of the total energy budget for the body despite representing only 2% of body mass, and endowed with an overdeveloped cerebral cortex that represents over 80% of brain mass, the largest of any animal in relative terms. Such uniqueness has seemingly been necessary to justify the superior cognitive abilities of humans over larger-brained mammals such as elephants and whales. However, the research program pursued in my lab offers a new perspective on the cerebral bases of our cognitive advantage over other animals, despite the "modest" absolute size of our brain. We aim to examine our novel proposition that, when numbers of brain neurons are examined directly, humans are, after all, not outliers in what regards brain size, brain development and maturation, relative cortical size, and even brain metabolism. Rather, the key feature that sets us apart from other animals might be sheer number of brain neurons, achieved by a combination of developmental and evolutionary mechanisms that, without being unique to humans, leads to our cognitive advantage over other animals.
Our main research goal in the Laboratory of Comparative Neuroanatomy at the Federal University of Rio de Janeiro is to unveil the origins of brain diversity in evolution and development and, through a comparative approach, to understand how the cellular composition of the human brain might make us human. To that end, we propose to examine what different brains are made of (that is, to perform a quantitative analysis of their cellular composition); to determine how brain size scales across species as a function of its numbers of neuronal and non-neuronal cells, and how these cells organize into different structures; to examine what developmental mechanisms allow the cellular composition of the brain to change in ontogeny and evolution; to identify what rules (if any) apply to all mammalian brains as a whole, and what rules differ among groups. Through these studies, executed in parallel in humans and other species, we aim to establish how the human brain compares to others, smaller and larger than ours, in its cellular composition, cortical folding, prefrontal grey and white matter expansion, cortico-cerebellar expansion and connectivity, synaptic density and metabolism; and how its cellular composition impacts on our cognitive abilities making them second to none.