What’s the world’s smallest living creature? It depends on what you mean by “living.”
Micro-microbiology. I'm Bob Hirshon and this is Science Update.
Listener Misha Sallis Winers from Rockville, Maryland, asked what the smallest living thing is. We turned to microbiologist Mike Conrad of the University of North Carolina at Chapel Hill.
He says the smallest we know of is a parasitic bacterium called Mycoplasma genitalium. It’s about one five-thousandth of a millimeter in diameter.
And some viruses can be much smaller, even, say, ten times smaller—like, poliovirus is one of the smallest virues.
But many scientists don’t consider viruses to be true life forms, because they can’t do anything outside their hosts. If you’ve got a science question, big or small, call us at 1-800-WHY-ISIT. If we use it on the show, you’ll win a Science Update mug. I'm Bob Hirshon, for AAAS, the science society.
Making Sense of the Research
In order to answer this question, you need to know the definition of life. The challenge is that there is no universally accepted definition. That’s why there are several possible answers.
Scientists generally agree that living things have certain characteristics that distinguish them from non-living things. Among them are: living things are composed of one or more cells; they metabolize (produce and use energy); they can grow; they can respond to external stimuli; they can adapt to their environment; and they can reproduce.
Obviously, a human, a plant, or even a bacterium can do all of these things, while a rock can’t. But are viruses alive? They can certainly grow and reproduce, and they use genetic material found in other forms of life. They can adapt to their environment—for example, by developing resistance to certain drugs.
However, unlike bacteria, viruses lack the internal machinery that would allow them to metabolize and reproduce on their own. Instead, they hijack the host cell and use its metabolic processes to make more viruses. Outside of a host cell, a virus can’t do anything at all; it’s an inanimate bag of genetic material. So, although viruses have some characteristics of life when they’re inside their hosts, they’re not at all lifelike on their own. A step down even from viruses are viroids, which are just naked strands of genetic material—in other words, a virus without the bag. They’re known only to cause diseases in plants, and they can be as small as 10 nanometers (20 times smaller than Mycoplasma).
What about prions? They’re even smaller and simpler than viruses or viroids. They’re misshapen strands of protein that can somehow cause neighboring proteins to bend out of shape themselves. Prions cause BSE (mad cow disease), and the human Creutzfeldt-Jakob disease, which has been linked to BSE. So, in a sense, they can reproduce, in that they can make more proteins like themselves. They’re also made of a component of life (protein), they may adapt to their environment (for example, by jumping from cows to people), and by causing infections, they behave in a way that most inanimate objects can’t.
Yet prions don’t reproduce the way living things do, using genetic material; they’re more like zombies in horror movies that turn their human victims into zombies as well. They’re not made up of cells, and they don’t have any kind of metabolism. Because they lack genetic material and a cellular structure, prions are less often grouped in with living things than viruses.
Some scientists classify viruses, viroids, and prions in a separate category, sometimes called “proto-life.” This category covers anything that is not truly alive but not quite inanimate either. As time goes on, we may find still other kinds of proto-life that challenge our definition of life itself.
Now try and answer these questions:
- What is the smallest truly living thing?
- Why is this question difficult to answer?
- What is the case for viruses being alive? What is the case against classifying them as life forms? What about prions?
- Is it important for science to come up with a formal definition of life? Why or why not?
How Big is a...?, from Cells Alive!, is an interactive tool that allows students to compare the sizes of microbes and viruses.
In the National Geographic News article Is It Time To Revise the System of Scientific Naming? read about recent challenges to the centuries-old Linnean classification system for life.