Humans are strange creatures. We are unarmored, largely hairless and precariously perched atop two legs. Bizarrely vulnerable and - in an evolutionary sense seemingly malformed - we have nonetheless dominated the planet.
While we may still become prey to larger animals and succumb to disease and natural disaster, we are remarkably immune to the forces that affect our wild relatives.
Why do humans grow so slowly compared to other animals?
This is in large part due to our outsized mental capacity. We have giant brains, which we have used to develop an array of tools and shortcuts. Collectively, we refer to them as civilisation - agriculture, buildings, weapons, electricity, running water, medical procedures.
But our brains come at a premium. We take an absolute age to reach maturity. Our extended development phase allows our grey matter to ripen to its full potential - both in terms of size and utility. That is: our brains continue to grow throughout childhood and early adulthood. And during that time, we cultivate the skills that have positioned us at the pinnacle of the animal kingdom.
Perhaps a quarter or more of a human lifespan is spent maturing - more or less. Depending on the metric, the human brain does not fully develop until the early to mid-twenties - or even the early thirties. And the body takes only slightly less time.
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These are energetically expensive processes. We require enormous amounts of food and protection during the early parts of this phase especially. Infants, toddlers and even teens require an absurd amount of coddling compared to other animals during the same periods in their lives.
Most animals, if they receive any parental care at all, are on their own in a matter of weeks or months. Some may get the benefit of a year or two of supervision before they are mature.
Even our nearest relatives, chimpanzees, are grown by age fifteen at the latest.
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The reasons for our extensive youth and the organizing principles of our social structure form something of a feedback loop. Self-sufficiency in humans only occurs after decades of growth. Nurturing our feeble, delicate young requires a stable system that can only be supported by the outputs of a large brain. That is: growing a large brain requires the efforts of mature individuals with large brains.
This system is recursive - of course the earliest humans did not even have the benefits of agriculture or modern medicine or even permanent housing. But they did have pair bonded parents and extended social structures that protected the generations who ultimately developed these further innovations.
There are early signs of our strangely slow growth patterns in the fossil record. The 1.8 million year old skull of a probable Homo erectus youth discovered in the Republic of Georgia in 2001 indicates that our near relatives developed slowly, too. Growth lines in the teeth of the youth, who died at around age 11, suggest that it showed similar patterns of development up to age five. Then, it matured more quickly.
H. erectus did not develop the sophisticated societies seen in H. sapiens but some of the early signs were there. The species used tools and was able to slaughter larger animals - allowing it to support the longer development of its young.
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Top image: Catherine Delahaye / Getty Images
