Animals are amazing! Some animals can breathe underwater. Some have protective skeletons on the outside of their bodies. A few can even regrow lost limbs! Animals can do these things thanks to their bodies. Humans can't do any of these things, but wouldn't it be great if we could? Imagining humans with these features brings up some incredible questions.
Fish can live underwater because they have gills that take oxygen from water. What if humans had gills? We could swim all day without having to surface for a breath. Why haven't humans evolved gills? It would be so convenient! Or maybe not.
All mammals, including humans, are warm-blooded. This means they produce their own heat to keep themselves warm and regulate their body temperature. This takes a lot of oxygen. Fish don't need much oxygen. They are cold-blooded. Their body temperature changes depending on the surrounding temperature.
A human's lungs are much larger than a fish's gills because humans need more oxygen to create body heat. Gills aren't meant for this. If humans had gills large enough to provide the oxygen we need, they would be enormous. Our bodies would look completely different! Gills are better for smaller, cold-blooded creatures.
Another cool animal feature is the exoskeleton. That's a skeleton on the outside of the body. Insects, spiders, and crustaceans have them. It's not a rare feature at all. More than 80 percent of all animals on earth have exoskeletons. So why not humans? Built-in armor would certainly look cool! But it might not work as well for humans as it does for tarantulas or lobsters.
Like gills, exoskeletons are best suited to small creatures. An exoskeleton has to be light enough for a creature to carry. And it has to provide the creature with protection. A human, of course, is much larger than a shrimp. Even a thin exoskeleton would be far too heavy for us to carry around. This is one of the challenges the military faces when designing exoskeleton armor. It's hard to find materials that are super light but also strong.
An exoskeleton might not even offer a human much protection. Animals that have exoskeletons go through a process called molting. This is when they shed their old skeletons to grow new ones. During molting, an animal is left with no protection. Luckily their new, small exoskeletons grow pretty quickly. But it would take a human much longer to grow a new skeleton than it would take a small crab. All that time the person would be in danger—and unable to stand up. After all, during molting, we wouldn't have a skeleton to support our muscles.
Not all animal kingdom adaptations would be impractical for humans. Some animals, like starfish, have the ability to regenerate lost limbs. This really would be useful since losing a limb can be a major challenge.
Humans can already grow new blood cells, skin, and muscle. But no mammal can regrow an entire lost limb. Scientists aren't sure why. It may have something to do with scars. When a body part is injured, the body produces chemicals that help prevent infection. Those chemicals also cause scars. Starfish don't get scars—they just grow new limbs. But human cells are programmed to only grow in a controlled way. Cancer is a disease that occurs from cells that grow uncontrollably.
Scientists don't think regrowing limbs is completely impossible. In fact, they are making progress in learning to grow human body parts. With luck, this animal what-if might one day be real.
Animals can do many things that humans can't do. But our bodies let us do some incredible things that animals can't do. Because of the structure of our bodies, we can talk and communicate in ways that animals can't. We're the only animal that walks upright all the time. Our hands are more flexible and useful than other animals' hands. Thanks to the complexity of our brains, we can figure out how cells work. And best of all, we can imagine what-ifs! Animals definitely can't do that!