The number of neurons an animal has in its brain’s cerebral cortex is nature’s best indicator of how long that creature will live, and that also may be a clue as to why humans take longer to mature — and live longer lives — than just about any other being on the planet.
Previously, it was thought that body size served as a clue for how long a creature would live, but it’s more about the number of neurons, according to new research.
“We do know that the cortex goes well beyond just cognition and mental math, it also takes care of running your physiological functions,” says Professor Suzana Herculano-Houzel of Vanderbilt University.
A new research article into this area — “Longevity and sexual maturity vary across species with number of cortical neurons, and humans are no exception” — was published in October in the Journal of Comparative Neurology, and will be discussed at the Neuroscience 2018 conference, November 3-7 in San Diego.
The number of neurons predicts about 75 percent of the a species’ longevity, says Herculano-Houzel. Meanwhile, body size only predicts between 20 to 30 percent of longevity for a species.
“It’s only natural that our species should take a long time to have that cortex mature,” says Herculano-Houzel.
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.
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