One of the most popular questions that students have in biology class is, “Are there really aliens?” Although most scientists will say that no aliens have been discovered yet, this is actually a much more difficult question to answer than you might think. What really is an alien? A little green man with antennae? A slimy mass that inches around?
An alien is any form of life that does not come from Earth. Unfortunately, that doesn’t make it much easier to be able to tell what an alien is. There are millions of undiscovered species of life here on Earth, and many of them look like they might come from other planets. In fact, scientists estimate that over 100 undiscovered species go extinct every day just in the world’s rain forests!
Even as we’re still just looking on Earth, how do we figure out what is alive and not alive? Is something alive because it moves? By that definition, air and water would be alive. The real answer is much more complicated, and not all scientists agree on what things are alive and which are not living.
The first way that we can tell something is alive is by whether it creates new molecules. Just as you are growing, making new blood, hair and skin all of the time, all living things make new molecules. Even single-celled organisms are renewing parts of their cell and growing so that they can reproduce. In fact, just think what would happen if organisms didn’t make new molecules all of the time – how would sperm and eggs ever form, how would one cell split into two cells, how would the organism ever grow? In other words, for an organism to use the food and water around it, it must somehow make new molecules.
When those new molecules aren’t needed any more, all organisms must have some way of getting rid of wastes. Humans do this in many ways: we breathe out carbon dioxide through our mouths, we often sweat out urea (which is also in our urine) and we defecate solid matter that our bodies can’t use. On a smaller level, however, each one of our cells is getting rid of wastes. When our cells let certain materials in, they also release other materials that aren’t needed any more. Every cell produces its own waste, from the simplest bacteria to the strangest deep sea creature (like the “dumbo” octopus to the right). Again, imagine what would happen if our cells were able to make new molecules, but not get rid of wastes: We’d get full of carbon dioxide and explode very quickly!
The “trash” that we make inside of our bodies has to be transported to places like the nose, bladder and anus. If it weren’t for the transportation of materials, bad and good, we wouldn’t be able to get oxygen and nutrients to all of our cells. This is another characteristic of all living things: the ability to move materials from one place to another. In humans and many other animals, we accomplish this by having blood that reaches every single cell in the body. In fact, our blood makes a full trip around our body about every 45 seconds. Single-celled organisms don’t have to transport molecules very far, but they still move molecules around their cells so that nutrients can get to parts of their cell that can make use of these nutrients.
In addition to moving nutrients around, all living things have to move energy around. Our stomachs digest our food and move that food to the small intestine, where most of the nutrients are passed directly to the blood. There is a tremendous amount of energy inside of these nutrients that helps our muscles move, our nerves send signals and our organs to expand and contract. If it weren’t for the fact that this energy could get to our muscles, we wouldn’t be able to move at all! All organisms have this same problem: Food comes in one place, but must be used in another place.
The final characteristic of life is the most complicated: Homeostasis. Put simply, homeostasis is the ability of an organism to maintain a balance between what’s going on outside and what’s going on inside itself. As humans, we keep our bodies at a pretty constant 98.6 degrees Fahrenheit (except when we’re sick and trying to create an unfavorable environment for bacteria or viruses). We maintain 98.6 F by sweating off heat when it’s hot outside of our bodies and by shivering and shaking to produce more heat when it’s cold. A dog does not have the ability to sweat, so it releases heat by panting; a reptile can’t produce its own heat, so reptiles need to sit in the sun or on warmed earth in order to digest their food. All organisms need to maintain a balance of more than just heat. Just think what would happen if you couldn’t keep all of the water inside of your body from evaporating! What would happen to a tree that couldn’t absorb water from its environment? All living things need to be in balance with their environment.
If we ever find an alien, there will certainly be a lot of questions to answer. For biologists, they will be quick to ask how it is that the alien accomplishes these five tasks that form the basis of all living things.