Why Don't Our Bones Keep Growing Forever?

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1:00Fish, amphibians, and reptiles actually almost all vertebrates get bigger throughout their lives. They generally keep on growing until they die. Then there are humans, who by the age of 20 or so, just stop getting bigger, or at least taller. Why do we stop growing part way through life? Hi, I'm Cameron, and this is MinuteEarth. We can think about the question of why humans stop growing, like a lot of other questions about the world around us in two ways. Like, on an individual level, what actually stops our bones from getting longer at some critical moment? But then also on a grander scale… Why do humans stop growing at a certain point while other animals don't? Let's start with the first one. Many bones have special layers of cells on each end known as growth plates. These cells are constantly dividing and turning into a brand new bone. That's what makes both you and, say, an alligator grow. And for that alligator, the growth continues throughout its life. But in humans, once puberty is complete, our bodies release hormones that harden our growth plates permanently, turning them into no more growth plates.

2:01And occasionally some medical problem, like a hormone imbalance, will block this normal process from happening. That's how the tallest people that have ever lived, like nine-foot-tolerabert wadlow, reach their towering heights. But assuming things work as they should, the hardening of your growth plates means you can't get any taller. Now, here's where, if you're thinking what I am thinking, you're contemplating that bigger question of why this whole growth-stopping process happens at all in us humans. And it has to do with the fact that humans, along with other mammals and birds, are energy-hungry critters. Our fast metabolism allow us to run, or swim, or fly long distances, but they require a lot of energy to sustain even when we are at rest. And burning all of that energy produces a lot of heat. And if you recall, as an animal grows, it's volume, and therefore the amount of heat it produces, increases much faster than its surface area does. Surface that's needed to vent that heat outward. Getting too big would burn us up, so it makes sense that there's a biological process that limits our growth. But other vertebrates, like alligators,

3:03have much slower metabolism that don't burn much energy most of the time. Even as they grow bigger, and their relative surface area decreases, they're generally able to vent the limited amount of heat their bodies produce. And since being big does confer a lot of benefits, they might as well just keep on getting bigger throughout life. And bigger might be better for us humans too. After all, it would be nice for everybody to be able to reach the highest fruit on the tree or to change light bulbs without a ladder or something. Unfortunately, we just can't take the heat.