Bacteria can get pretty small, with sizes ranging typically from .2 to 10 microns. Can organisms get even smaller? Answering that question isn’t just a fun trivia fact, but could help us in our search for extraterrestrial life.
Viruses can get even smaller than bacteria, with a single virus getting as small as just 20 nanometers, or ten times smaller than even the most itty bitty bacterium. Technically though we don’t really count viruses as being alive. They don’t eat or have a metabolism, but even if they did they still can’t replicate on their own. They need to hijack a host like a bacterium in order to make copies of themselves, so without a host a virus can’t multiply.
To really be alive an organism has to fit all the microscopic machinery that allows it to eat and self-replicate inside itself. Since these machines are on a molecular scale, they’re limited by fundamental laws of the universe. They can only get as small as molecules and atoms will let them. With that in mind, scientists have calculated that the absolute minimum size a free-living organism can be is theoretically around 200 nanometers in diameter.
So far, that prediction seems to be holding up, as the smallest living microbes we’ve found are about that size. This lower size limit doesn’t just apply to Earth microbes, it’s universal for all carbon based life. However, some scientists are hoping that limit is off.
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