Why Do Humans Grow So Slowly?

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Justine Alford

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1908 Why Do Humans Grow So Slowly?
Dan Foy, "20071225-2039-32" Flickr, CC BY 2.0

Human children are slow growers in comparison to our closest animal relatives. We spend around twice as long in childhood and adolescence as chimpanzees, gibbons and macaques, but scientists never knew why. Many believed that this may have something to with the fact that our brains are so energy hungry that less glucose is available to fuel the development of the rest of the body, but this has been tricky to prove. Now, a new study conducted by Northwestern University anthropologists has finally found strong evidence to support this theory. The study has been published in PNAS.

To find out how much glucose our brains guzzle from birth to adulthood, the researchers used previously gathered PET and MRI brain scan data and compared this with body growth rate. PET scans are used to measure glucose uptake, whereas MRI scans can determine brain volume.


As predicted, they found that body growth slows when the brain consumes a lot of glucose. Furthermore, the brain was found to be hungriest when children are around 4 years old, which coincided with a period when body growth was slowest. At this time, the brain consumes more than 40% of the body’s total energy expenditure. According to lead author Christopher Kuzawa, brain cell connections “max out” at this age because we learn so many new things that are critical to becoming successful humans. This means that there are less resources available for the rest of the body to use to facilitate growth of other body parts.

This could help to explain why it’s difficult to guess a young child’s age based on their size, Kuzawa says. Instead, we use other indicators such as speech and behavior.

Together, these data indicate that human brain development demands an unusual amount of energy and consequently childhood body growth must slow in order to compensate. This lends support to the “expensive tissue” hypothesis that was first proposed back in 1995. However, it was originally thought that smaller digestive systems during childhood were freeing up more energy for the brain to use.

The researchers would like to take this work forward by seeing if this trade-off of growth happens in closely related primates such as chimpanzees, but acknowledge that this data would be difficult to attain. 


[Via Northwestern UniversityScience and PNAS]