Why Do Human Children Stay Small For So Long?

By John Skoyles, University College London | December 2, 2014 11:40 am

child with orangutan

This article was originally published on The Conversation.

Why does it take so long for human children to grow up? A male chimp and male human, for example, both end up with the same body weight but they grow very differently: at year one the human weighs twice that of the chimp but at eight the chimp is twice that of the human. The chimp then gains its adult weight by 12 – six years before the human. A male gorilla is also a faster growing primate – a 330-pound male gorilla weighs 110 pounds by its fifth birthday and 265 pounds by its tenth.

Clues to the answer can be found in the young human brain’s need for energy. Radioactive tracers allow scientists to measure the glucose used in different areas of the brain but this procedure is only used rarely when it is justified by investigating neurological problems. However, the few cases we do have reveal how radically different the childhood brain is from that in adults or infants.

From about the age of four to puberty, the young brain guzzles glucose – the cerebral cortex, its largest part, uses nearly (or more than) double that used earlier or later in life. This creates a problem. A child’s body is a third of the size of an adult but its brain is nearly adult-sized. Calculated as a share, a child’s takes up half of all the energy used by a child.

Brain Energy Use and Body Size

Map child growth against what is known about brain energy consumption and they shadow in a negative way: one goes up, the other down. The period in which the brain’s need for glucose peaks happens just when body growth most slows. Why? In a recent study in the Proceedings of the National Academy of Sciences, I proposed that this prevents a potential conflict over blood glucose that might otherwise arise between brawn and brain.

A young child has at any moment a limited amount of glucose in its blood circulation (3.4g – the equivalent in weight to about three Smartie candies). Fortunately a child’s liver can quickly generate glucose, providing other organs do not compete against the brain for the glucose. But as French child exercise physiologist Paul Delamarche noted:

Even at rest, it would appear to be difficult for children to maintain blood glucose concentration at a steady level; an immaturity of their gluco-regulatory system would seem to be likely, therefore causing a delay in an adequate response to any stimulus to hypoglycemia like prolonged exercise.

Organs elsewhere in the body fuel themselves with energy sources that do not compete with the brain such as fatty acids. But skeletal muscle can compete when exertion is intense and sustained.

In adults, the liver quickly ramps up its generation of glucose so even active brawn does not usually compete against the brain. But conflict can arise even in adults, and it could pose a real threat to children. Luckily they do not let it happen: they stop exertion if it gets intense and sustained. Not that this makes children inactive – they do even more low and moderate exercise than adolescents and adults.

So putting a break on growth in childhood aids limiting skeletal muscle as a potential glucose competitor to the brain. And not only are their bodies smaller but they contain (as a percentage of their bodies) less skeletal muscle than in adults. And even that skeletal muscle, some research suggests, is of a type that uses less glucose than in active adults.

So human growth rate negatively shadows increased energy use in the child’s brain. An interesting fact – but does it tell us more? Neanderthals and other earlier species of humans developed brains as big as ours. Why did they not survive? Bad luck? Competition from our species? Or has an overlooked advantage arisen in our evolution that puts us apart? Neanderthals grew up faster than us, and this suggests, given the link between a child brain’s energy guzzling and slowed growth, a new story.

It’s the Connections That Count

Bigger brains may be smarter brains but they might be even smarter if their connections got to be better refined in brain development. Neuro-maturation involves an exuberance of synapses – the connectors between neurons. This initial excess lets the developing brain refine down connections, to “wire” itself in the most effective and efficient manner. Connectome research, which studies this process – both theoretically and empirically – links better efficiency of connectivity to improved cognitive ability.

Synapses are the primary energy consumers within the brain and it is their exuberance that causes the child’s brain to use so much extra energy. We cannot directly see how long this period lasted in earlier humans but we can indirectly from their pattern of growth. Since this was faster than in us we can infer that they lacked – in spite of having brains as large as ours – the extended period of connectivity refinement that we have. This means they also lacked our extraordinary capacity for complex cognitions.

This not only resulted in us displacing them but also the creation of civilization and the complex lives we each now live.The Conversation


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CATEGORIZED UNDER: Living World, Top Posts
  • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

    Immediately legislate mandatory Federally-imposed 24/7 superior vena cava hyperalimentation for all children. This is doubly important for females as they grow faster than boys,stunting their cerebral development by nutritional derailment into somatic tissue.

    We must also impose intense child physical development through mandatory sports participation lest any child achieve an intelligence rendering more deserving children disempowered by contrast.

  • Don’t Even Try It!

    Let the kids in school eat food that will benefit them! Moochelle’s recommended school lunch program is NO GOOD for an active child’s brain development!

  • Herne Webber

    The story is not quite as simple as “we replaced them.” Neanderthals’ genomes analysed over time show that they had increasing amounts of H. sapiens genetic introgression, and we know that modern Europeans and Asians have Neanderthal and Denisovan DNA from introgression in the opposite direction.

    When does introgression become two species simply becoming one, with the ‘inferior’ traits going away over time? We have Neanderthals’ immune system genes, plus whatever else, while maintaining the H. sapiens body growth vs. brain directionality. It is possible the Neanderthals were also moving toward the better refinement and slower growth, but the blending of our species simply changed their rate of advancement. Instead of their genes slowly moving toward a slower body growth and better growing brain via new mutations, they imported H. sapiens genes, along with a huge amount of the rest of what made early H. sapiens different from them, and ended up getting swamped in the process. Plus, who knows? It is possible that a handful of unique, positive brain alleles arose in the Neanderthal/Denisovan lines, and they are only now becoming fully exchanged among all modern humans (due to intercontinental blending). Bringing positive genes together around the world should slowly pull our species ever forward in brain development, as each groups’ contributions join the other groups’, until we’ve become something new compared to whom we are today.

    As for childrens’ diets, clearly a lot of kids are getting *way* too much of fat and/or sugar calories (and probably protein calories as well). If a kid is morbidly obese, you really do not need to worry if their brains are getting *enough* feeding!!! So to the person who usesas his or her avatar a depiction of our President in a gun’s crosshairs, you do not have Science behind you. More like your Satan. You just keep being evil, ’cause he likes that. :)

    • mjm

      Actually, the scientific zeitgeist is finally starting to come to terms with the fact that fat is not actually bad for us, but is instead an essential nutrient both for adults and especially for children. For most of the past 40 years, flawed interpretations of flawed studies have been quoted over and over again to create a false impression that fat is harmful for us and should be strictly limited in our diets.

      The problem is not fat, but carbohydrates. The body has built-in mechanisms for self-limiting fat intake (you feel “full”, and then if you keep eating, nauseated) but not for carbohydrates (you can keep eating them until your stomach is completely filled up, and then can keep eating more as soon as it empties). Fat makes up the largest fraction of the brain after water, and growing brains require a steady supply in order to develop healthily. Fat and cholesterol are the precursors for many of our essential hormones, and are also necessary for making new cell membranes as our bodies grow and repair themselves. Most importantly, scientific studies have demonstrated over and over again that people do not put on significant amounts of weight no matter how much fat and protein they eat, as long as they restrict their carbohydrate intake.

      • Herne Webber

        Agreed, which is why I explained that kids are getting “way too much” fat and (simple) carbs. They need “enough” of those things, because that’s healthy, not more than enough, because that’s not. Given the apparent differences between kids’ and adults’ dietary needs, I am surprised we didn’t go extinct! (Kidding.)

        • mjm

          I understand what you are saying, but my point is that the trend toward “low fat” lunches is actually harmful. All humans, including kids, tend to overeat less if our meals include adequate fat. This is because fat makes us feel “full” in a way that carbohydrates and protein do not. If we were to start giving our kids meals that contain enough high-quality fat, I think we would see much less obesity.


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