The Science Behind Why We All Have Snot

little girl with snot — Almost all living creatures have muscus Tatevosian Yana/Shutterstock

Snot. It's not the most appetizing subject. It would probably disturb you to know that you swallow down loads of it in a day — even on your healthiest of days. That slimy gelatinous goo we call mucus doesn't just roll down the back of your throat or clog up your nose; it's actually found on all the wet surfaces of your body not covered in skin. That includes the lungs, sinuses, mouth, stomach, intestines, cervix and even eyes, to name a few.

So why do we have to put up with it? Because, as disgusting as it may be, mucus plays a hugely important role in keeping us healthy. And not just us humans. The same mucus helps protect other creatures, too.

Mucus is a bit of a mystery. Although mammals, fish and amphibians (and other creatures) all produce mucus, a new study has found many mucus genes don't share a common ancestor. This is unusual because genes with a similar function often evolve from a common ancestral gene. But in humans, for instance, genes that encode for mucus are members of several families that probably evolved independently.

During this study on the mucins (the key component that makes mucus slimy) in saliva across 49 different mammal species, a team from the University of Buffalo (UB) discovered that some non-mucin proteins in some mammals had evolved into mucins. (The study was published Aug. 26 in the journal Science Advances.)

"If these mucins keep evolving from non-mucins over and over again in different species at different times, it suggests that there is some sort of adaptive pressure that makes it beneficial," said UB Ph.D. student Petar Pajic, lead author of the study in a press release. "And then, at the other end of the spectrum, maybe if this mechanism goes 'off the rails' — happening too much or in the wrong tissue — then maybe it can lead to disease like certain cancers or mucosal illnesses."

"I don't think it was previously known that protein function can evolve this way, from a protein gaining repeated sequences. A protein that isn't a mucin becomes a mucin just by gaining repeats. This is an important way that evolution makes slime. It's an evolutionary trick, and we now document this happening over and over again," said Omer Gokcumen, Ph.D., UB associate professor of biological sciences and senior author of the study, in the same press release.

What is Mucus and Where Does It Come From?

Mucus is made up almost entirely of water, along with hundreds of compounds, including proteins, fats and salts. The key component in mucus and the ingredient that makes it slimy is a set of proteins called mucins.

baby seal with runny nose — This baby seal on South Georgia Island in the South Pacific has a runny nose.
Anne Dirkse

Mucus serves our bodies in different ways, by:

preventing tissues from drying out and cracking, which would expose them to infection
lubricating the eyes
protecting the stomach lining from acid
removing or trapping substances, preventing them from getting into the lungs or bloodstream
keeping the body's trillions of bacterial inhabitants under control

Our bodies are constantly producing mucus. In fact, the respiratory system alone cranks out more than a liter (33.8 fluid ounces) of it each day. A lot of it slides down the back of your throat, into your stomach, and eventually makes it way out of your body. When you're healthy, you're probably not aware of all the mucus rolling down the back of your throat.

But when you're sick, your mucus becomes thicker and stickier as your body ramps up production of it to quickly flush out any offending pathogens.

How Does Mucus Protect Other Creatures?

Humans aren't the only creatures to produce mucus. Other mammals, amphibians, fish, mollusks and certain invertebrates also produce mucus, and some do some amazing things with it:

The visco-elastic mucus that snails and slugs excrete acts as both an adhesive and a lubricant enabling them to scoot seemingly effortlessly over rough terrain.
All fish are covered in mucus, but parrotfish also puke out little mucus sleeping bags that they encase themselves in every night to protect them from parasites.
Mucus keeps the eyes and nasal tissues of sea lions moist and they are said to launch snot rockets across "great distances," according to the book "Believe It or Snot."
Female cave-dwelling birds called swiftlets use their saliva to build gooey nests to stick to steep cave walls. The nests are a delicacy in China and are boiled down (without eggs) to form a gelatinous soup called bird's nest soup.