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Developmental biology - Brain
'Brain Soup' Connects Number Of Neurons to Longevity
Lifespan and sexual maturity depend on the number of neurons in your brain...
New Vanderbilt University Brain Institute research finds the length of human life and that of other warm-blooded animals - including when sexual maturity is reached - has more to do with the brain than physiology. Specifically, animals with more neurons in their cerebral cortex, live longer whatever the size of their body.
"Whether you're looking at birds or primates or humans, the number of neurons found in the cortex of a species predicts around 75 percent of variation in longevity across species."
Suzana Herculano-Houzel PhD, Department of Psychology, Department of Biological Sciences, Vanderbilt Brain Institute, Vanderbilt University, Tennessee, USA, and study author.
Body size and metabolism by comparison, only predict 20 to 30 percent of longevity depending on the species. Birds live ten times longer than mammals of the same size. Humans are considered to be a "special" evolutionary oddity, having a long childhood and postmenopausal females. However, Herculano-Houzel's new research published in the Journal of Comparative Neurology, reveals humans take just as long to mature as predicted by their number of cortical neurons.
The Study
Herculano-Houzel examined more than 700 warm-blooded animal species from the AnAge database a comprehensive longevity record of animal species. She then compared AnAge Database records with her data on the number of neurons in the brains of different animal species. One finding: parrots and songbirds, including corvids (the crow family), live longer than primates of similar body mass, while primates live longer than non-primate mammals of similar body mass.
"In species with similar basal metabolic rates, parrots and songbirds live longer and take longer to reach sexual maturity than many mammalian species, especially non-primates," explains Herculano-Houzel. Her previous studies determined that parrots and songbirds have more cortical neurons than similar-sized primates, which have more cortical neurons than other mammals of comparable body size. Her new analysis confirmed her suspicion that longevity increases uniformly across warm-blooded species matching the absolute number of neurons in that animal's cerebral cortex.
"The more cortical neurons a species has, the longer it lives. It doesn't matter if it is a bird, a primate or some other mammal; how large it is - or how fast it burns energy."
Suzana Herculano-Houze PhD.
Are Humans Unique?
Anthropologists and researchers interested in evolution and human behavior have been working under the assumption that one of the ways the human species is unique is our uncommonly long childhood and adolescence, allowing for a longer period of learning and social interaction. However, if larger animals live longer, then gorillas should live longer than humans - but they don't. Humans outlive gorillas. Another favored hypothesis is that being cared for by grandmothers could have led humans to delay sexual maturity while increasing postmenopausal longevity through care giving.
"Now we can say that humans spend just as long in childhood and live exactly as long after reaching maturity as you would expect based on the number of neurons in their cerebral cortex," adds Herculano-Houzel. "The delay gives those species with more cortical neurons more time to learn from experience, as they interact with their environment." And if longer lives accompany more cortical neurons, those species will also enjoy a greater overlap between generations, with more opportunities to pass along what they learn. "Which means that grandma is still fundamental in the lives of those with plenty of cortical neurons. She's probably just not the reason why our species is long-lived," argues Herculano-Houzel.
Function Of The Cortex
"The data suggest that warm-blooded species accumulate damage at the same rate as they age. But what curtails life is damage to the cerebral cortex, not the rest of the body. The more cortical neurons you have, the longer you will have to keep your body functional". Contrary to the rest of the body, which replenishs old cells through continuous cell division, cortical neurons are thought to last a lifetime.
Herculano-Houzel: "The cortex is the part of your brain capable of making behavior complex and flexible, yes, but that extends well beyond cognition and doing mental math and logical reasoning. The cerebral cortex gives your body adaptability, as it adjusts and learns how to react to stresses and predict them. That affects physiological function: making sure your heart rate, your respiratory rate, and your metabolism are on track with what you're doing, with how you feel, and with what you expect to happen next. That, apparently, is a key factor impacting longevity," she believes.
Brain Soup
Herculano-Houzel pioneered the method for accurately measuring the number of neurons in brains. She creates "brain soup" by breaking down the cells in brain tissue, then applying fluorescent tags to the nuclei and counting them as they float in this "brain soup." In collaboration with Jon H. Kaas PhD, Distinguished, Centennial Professor of Psychology at Vanderbilt University, Herculano-Houzel has counted the neurons in different primate brains, including those of the great apes. Along with colleagues in Brazil, these "brain soups" produced the first accurate count of neurons in the human brain - average 86 billion - making it the largest primate brain - a gorilla having 33,400,000,000.
Aging starts once most species reach adolescence. There is no way to regain lost neurons and research shows humans can lose prefrontal cortex neurons. So Herculano-Houzel suggests taking good care of our mind to keep cortical neurons healthy and busy, in order to live long and well.
Abstract
Maximal longevity of endotherms has long been considered to increase with decreasing specific metabolic rate, and thus with increasing body mass. Using a dataset of over 700 species, here I show that maximal longevity, age at sexual maturity and post-maturity longevity across bird and mammalian species instead correlate primarily, and universally, with the number of cortical brain neurons. Correlations with metabolic rate and body mass are entirely explained by clade-specific relationships between these variables and numbers of cortical neurons across species. Importantly, humans reach sexual maturity and subsequently live just as long as expected for their number of cortical neurons, which eliminates the basis for earlier theories of protracted childhood and prolonged post-menopause longevity as derived human characteristics. Longevity might increase together with numbers of cortical neurons through their impact on three main factors: delay of sexual maturity, which postpones the onset of aging; lengthening of the period of viable physiological integration and adaptation, which increases post-maturity longevity; and improved cognitive capabilities that benefit survival of the self and of longer-lived progeny, and are conducive to prolonged learning and cultural transmission through increased generational overlap. Importantly, the findings indicate that theories of aging and neurodegenerative diseases should take absolute time lived besides relative “age” into consideration.
Author
Suzana Herculano-Houzel.
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Nov 2, 2018 Fetal Timeline Maternal Timeline News News Archive
 Human life span and sexual maturity can be predicted by that individual's number of cortical neurons - with 75% accuracy. Credit: The Aspinall Foundation, David Aspinall with female gorilla and her baby. Female gorillas are 40 - 60% smaller than males.
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