There’s something sitting inside your skin. It’s spindly and startlingly white, and, by its weight, it’s stronger than steel.

That might sound spooky — that something so strange and so spectacularly strong is sitting inside you — but that thing within your skin is what allows you to stand, walk, and sprint. It protects your innards and provides your squishy organs with structure. Pieced together from a slew of separate parts, it’s been with you since you were a baby, plying you with the cells and substances that you need to survive.

We’re talking about your skeleton, of course, a collection of bones and connective tissues that act as your body’s scaffolding. But just because it’s out of sight doesn’t mean that the skeletal system should be out of mind. This is what you’ll want to know about your inner skeleton.

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How Many Bones Are in the Human Skeleton?

The average adult body boasts about 206 bones, but babies are born with around 300, which meld together as they mature and age.

Differences in Adult and Baby Bones

When we’re babies, many of our bones are made out of cartilage, a smooth, shock-absorbing tissue that also appears throughout the adult skeleton, though in comparatively small amounts. Being both firm and flexible, all that cartilage makes a baby’s bones much more malleable than an adult’s. This allows a baby to squeeze through tight spaces, both before birth and during delivery.

Once born, however, a baby’s bones begin to ossify, being surrounded and supplanted by harder, heavier bones. As ossification occurs, some baby bones also start smushing together, fusing into single, solid structures. Fastened together with strong, fibrous “sutures,” this skeletal fusion is why the 270 bones of a baby become the 206 bones of an adult.

Differences in Adult Bones

That being said, some adults are bonier than others. In the United States, 1 in 1,000 individuals is polydactyl, meaning that they were born with more than 10 fingers or 10 toes. While some people possess more ribs or more vertebrae than the average, other people possess fewer ribs, vertebrae, or digits, resulting in a remarkable range in the number of bones in adult bodies.

Sometimes, these differences are determined at a person’s birth, but not always. Around 185,000 amputations occur annually in the United States, for instance, meaning our bones are not only varied in number but variable over time.

How Much Does A Human Skeleton Weigh?

There is also an abundance of other differences in our skeletons beyond differences in the number of our bones. Take, for instance, the structure of our skeletons. Our skeletons are all a unique size and shape, based on our uniquely sized and shaped bodies. This means that there isn’t a single weight that all skeletons should be. On average, though, our skeletal systems make up around 20 percent of the weight of our bodies.

There are also all sorts of superficial scrapes and structural traits that distinguish our bones. For instance, there are ditches, depressions, smooth patches, and projections that differentiate one skeleton from another, along with many other types of unique markings.

Read More: 5 Vestigial Body Parts Found in Humans

What Are the Types of Bones in the Human Body?

Clearly, the 206 bones that comprise an average adult skeleton are all distinct, too, falling into one of four categories — long, short, flat, and irregular — according to their structure.

Long Bones

Shaped like a long stick with two bulky bulbs at either end, long bones are longer than they are wide. Examples of long bones include the bones of the leg and arm — like the femur and the fibula — as well as the bones of the fingers and toes.

Like the other three types of bones, long bones are composed of a combination of compact bone and spongy bone. While compact bone is harder, heavier, and denser (providing the bulk of the bone’s structure and strength), spongy bone is lighter and less dense (providing plenty of porous spaces for blood and bone marrow to sit).

Unlike the other three types of bones, however, long bones are primarily composed of compact bone and contain only small blobs of spongy bone in each of their bulky ends.

Short Bones

Short bones, on the other hand, are about as long as they are wide, like the bones of the wrist and the ankle. Unlike long bones, but like flat bones and irregular bones, short bones are primarily spongy, though they are also covered in a thin coating of compact bone.

Flat Bones

Flat bones are broadened and flattened structures, as in the cranium, the sternum (breast bone), and the scapulae (shoulder blades).

Irregular Bones

Irregular bones do not fall into any of the above categories in terms of their size and shape. Among them, the vertebrae and the mandible (jawbone) are particularly prominent.

What Are the Functions of Bones?

Functioning as a framework for your body, your skeleton allows you to take up space without falling or flopping around. When you’re standing still, you’re depending on your bones to prop you up. And when you’re moving around, you’re depending on your bones to solidify your movements, strengthening your shifting arm or your flailing fingers.

Similarly, your bones support the soft tissues inside your body, securing your muscles and tendons in the proper places. Shielding your other internal organs, your skull surrounds your brain, while your vertebrae protect your spinal cord and your ribs preserve your heart and lungs.

Beyond these basic, structural benefits, your bones are also a storage site for important minerals, including calcium and phosphorus, and bone marrow, the soft tissue that produces your body’s red blood cells, white blood cells, and platelets. In turn, these cells carry oxygen, combat infections, and control your blood as it circulates throughout your body.

Read More: 5 Skulls That Shook Up the Story of Human Evolution

What Is the Oldest Human Skeleton Ever Found?

Being behind such basic bodily functions, skeletons have helped humans from the start, supporting our ancestors as they fashioned their first tools, holding them as they tried their hand at agriculture, and carrying them as they created civilizations that crisscrossed the world. Our bones have been with us for all that time. And when the first Homo sapiens arose around 300,000 years ago, their skeletons were surprisingly similar to those of today.

A 300,000-Year-Old Skeleton in Morocco

In 2017, a team of archaeologists uncovered the fossilized bones of five 300,000-year-old Homo sapiens at the Jebel Irhoud archaeological site in Morocco. Representing the oldest skeletal remains of our species, the front of the skull and the teeth of the specimens are remarkably modern in their shape and structure, though the skull cases are slightly stretched in comparison to the rounder skulls of more modern humans.

But the full reach of the skeletal differences between the first Homo sapiens and more modern Homo sapiens cannot be seen in the Jebel Irhoud remains. That’s because the team found only fragments of skulls, arms, and hips at the Moroccan site rather than full bodies of bones.

A 12,000-Year-Old Skeleton in Mexico

The most complete H. sapiens skeleton from over 12,000 years ago was found in Hoyo Negro, an underwater cave on the coast of Mexico’s Yucatán Peninsula. The skeleton contains almost all of its bones and carries a completely intact cranium from 13,000 to 12,000 years ago. (The remains also include most of the ribs and the vertebrae, an almost-complete pelvis, and all the bones of both arms and one leg, as well as many hand bones.)

What Is the Largest Human Skeleton Ever Found?

Since the time of these ancient skeletons, H. sapiens skeletons have undergone slight shifts. The shape of the skull is rounder, and the size of the skull cavity is smaller. In the past 10,000 years, our skeletons have also become shorter and smaller, though this trend has started to reverse in the last couple of centuries, as the average height of the human skeleton has, again, started to increase.

In fact, the tallest skeleton in human history was associated with a man from approximately 100 years ago. Born in Alton, Illinois, in 1918, Robert Wadlow was the tallest indisputably H. sapiens individual and thus had the tallest indisputably H. sapiens skeleton, standing at a height of 8 feet, 11 inches.

While his bones were special, helping him shoot above his peers (and his parents) by the time he was 8 years old, they acted largely like any other bones, providing his body with the structure and support to survive.

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