Original photo by Peter Porrini/ Shutterstock
The human skeleton is sometimes called the scaffolding of the body, and the name is apt because, like scaffolding, our bones are less permanent than you might think. Human bones grow in a process known as modeling, and once a person reaches adulthood, the skeleton system refreshes itself in a process known as remodeling. During remodeling, certain cells in the body break down bone and funnel its minerals into the bloodstream, while other cells build healthy bone back up. Every year, the body replaces around 10% of bone via remodeling, which means we get an entirely new skeleton about every 10 years.
Oops, incorrect!
It's a fibThe human epidermis, the largest organ in the body, replaces itself at a much faster rate than every 10 years. In fact, skin doesn’t even last a month; the cells that make up the epidermis are replaced about every 27 days. (Hope you don’t mind dusting.)
Bone remodeling is just one of the ways in which our bodies are in a constant process of regeneration. Human hair is replaced every two to seven years (and around 100 hairs fall out of our heads every day), fingernails take about six months to replace, and our intestinal lining — constantly under assault from digestive acids — regenerates every week (or less). One of the most dramatic examples of regeneration is the production of red blood cells; the body creates upwards of 3 million of them every second, and totally refreshes these cells every four months.
Does all this rejuvenation mean that humans are essentially a walking, talking Theseus’ paradox? This philosophical question, first proposed by Greek philosopher Plutarch, ponders whether something that slowly replaces itself is still the original object or something new. However, there are a few things in the human body that remain the same. Parts of our heart stick with us forever, and we’re born with most of our brain’s neurons. Also, our teeth can’t regenerate once grown, and the core that makes up the lens of our eye forms during prenatal development and never changes. So while many things in our body do change, some stay the same — meaning that humans are always a mix of both the new and the old.
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The cells that heal and form bones are known as ______.
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The cells that heal and form bones are known as osteoblasts.
Scientists once thought dinosaur fossils belonged to giant humans.
Around 1677, English naturalist Robert Plot discovered a dinosaur fossil nearly a century and a half before science knew what a “dinosaur” was. Analyzing a femur that belonged to what we know today to be a megalosaurus, Plot suspected that it might be an elephant bone, before ultimately theorizing that it had belonged to a giant human. For more than a century, naturalists like Plot attempted to describe dinosaur fossils as belonging to giant versions of animals that existed on Earth, including humans. It wasn’t until 1824 that scientists identified Plot’s bones as belonging to a giant lizard they named megalosaurus. As a result, the first dinosaur was named before scientists even knew these creatures existed. Finally, in 1842, the word “dinosauria,” meaning “terrible lizard” in Greek, officially entered the lexicon, and humanity’s perception of Earth history changed with it.
Darren Orf lives in Portland, has a cat, and writes about all things science and climate. You can find his previous work at Popular Mechanics, Inverse, Gizmodo, and Paste, among others.
