There’s one human invention so fundamental to our  lives that every known modern culture can make it.

You could argue it’s one of the  top three most important things   we’ve ever made as a species,  behind stone tools and fire. And it has really stuck with us.

You probably have some of this  tacky tech in your craft drawer. So let’s talk about glue! [♪ INTRO] Glue, or more formally, an adhesive,  is a substance that holds two or   more things together all on its  own, with no machinery required. Welding, soldering, or bolts?

Not glue. You need other tools to make those work. But sticky gunk?

Now that’s what I call glue. And glue’s smear-ability and mold-ability, or  plasticity, are some of its greatest strengths. Soft, liquid glue can be drizzled and molded   onto odd-shaped objects and then  left to dry to hold them together.

Okay, so that might not be  super mind-blowing today. But at one point, this was  cutting-edge technology. The earliest glues came from plants.

If you’ve leaned on a tree  and your hand came up sticky,   you’ve encountered the original  glue ingredient: tree resin. And if you were living in the Stone Age  and you wanted to refine some of this   sticky stuff for yourself, here’s what you’d do. Gather up lots of tree bark,  probably birch or pine.

Then, roll it up, throw it  in the embers of a fire,   and cover it with ash to try to keep  the temperature down and the air out. You don’t want to burn it all away, after all. After 20 or 30 minutes, you might get  about one gram of cooked-down resin,   called tar, for every 100 grams of bark you bake.

Scrape the tar off of the bark, and use it while  it’s hot to glue some rocks and sticks together! That’s hypothesized to be how we got the  oldest known example of a tool built with glue. Roughly 200,000 years ago, someone stuck two  rock flakes together with birch tar, and in 2001,   researchers announced that they’d found those  rocks under the ribs of an ancient elephant.

Together, they look like a blade and a handle. Now, let’s put that into perspective. For more than 2 million years before that,  all our tools were just individual rocks,   and we have no direct evidence that those  rocks were connected to anything else.

You had big hammerstones to smash stuff,  or sharp flakes of rock to cut stuff,   but you’d be holding onto  the rock itself in your hand. More complex stone tools were actually  sharpened and shaped into points,   but can you imagine using  a knife without a handle? It's very difficult.

If you want to throw your sharp rock really far  to kill your prey, or you want some leverage on   your knife while you cut up that meal, it helps  to attach that stone tool to something else.

Enter: Glue! Archaeologists are generally really excited to  find tar on stone tools because it represents   a massive technological and cognitive leap  by humans and our extinct hominin relatives. It takes creativity to realize that a rock  on a stick is better than a rock by itself,   and serious problem-solving  skills to figure out how to   put them together with the gunk  from some slow-roasted tree bark. But that leap allowed early humans to create  tools with handles and hafts, like arrows,   spears, and domestic tools, that would make  essential tasks like hunting much easier.

Those earlier humans may have discovered resin   glues after burning bark in campfires  and finding all that tar left behind. And it doesn’t take a huge leap to then  scrape it off and stick it to stuff. But it wasn’t just us that made this connection.

That 200,000-year-old glue I mentioned  earlier was Neanderthal-made. Over a couple tens of thousands of years,   we think Neanderthals managed to develop  a pretty elaborate glue-making process. There’s one site from about 65,000 years ago  where they’d made a dedicated mound-shaped   hearth that could pull resin from birch and other  plants more efficiently than just a pile of ash.

But our own species wasn’t  out of the glue-making game. One of the earliest examples of this comes  from around 80,000 to 60,000 years ago,   when humans at a site in South Africa called   Sibudu Cave were using an adhesive  made from conifer resin and ocher. While resin-based glues served us well for tens of  thousands of years, they couldn’t do everything.

Around 8,000 years ago, humans  started to make glue from the   tough bits left over from butchering meat  and fish, and even leftover plant scraps. When preparing fresh meat for dinner,   the parts of the animal including connective  tissue like tendons, ligaments, and cartilage,   and other leftovers like the hide and  bladder, are not usually on the menu. But they are far from useless, since  they’re full of a molecule called collagen.

When you prepare it right, collagen  makes a great base for glue. All you need to do is heat it in water  to between 60 and 79 degrees Celsius   long enough for the coiled-up  collagen molecules to unravel. These collagen molecules are made of 3 chains,   twisted together like a rope and held  together with magnet-like hydrogen bonds.

When heated, the rope unravels, and that’s your   chance to smear your collagen-based glue  on whatever you want to stick together. Then, when the glue cools, the  chains want to join together again,   because of that magnet-like adhesive force. But they also hold on to whatever  you’ve applied the glue to.

You can create different glues depending  on what animal parts you used to make it,   and whether it was mixed with other  sticky substances, like plant resin. Cultures around the world developed collagen  glues at different times, for different reasons. True to glue’s combat-related past,  collagen glue was used to make   layered wooden shields in ancient Rome.

But it also had artistic applications, like  holding together terracotta sculptures in   ancient China, and it was an important  part of plaster in ancient Egypt. Later, it would be used for gilding  artworks in 18th century Italy. Collagen-based glues were  popular through the 19th century,   which means glue-making got the  industrial revolution treatment.

The world’s first glue factory  opened in Holland in the 1700s,   and the rest is horse jokes and history. Most of us don’t use collagen-based glues today,  unless you’re into woodworking or violin-making. For the rest of us, glue probably brings  to mind basic, white Elmer’s glue.

Or, even, superglue. But before you ask, no, these common household  glues generally don’t use animal products. They’re mostly made of plastic.

And like the original resin-based  glues of the Stone Age, the first   plastic-based glues were invented to make weapons. The most common craft glues use a chemical  called poly-vinyl acetate, or PVA. PVA was discovered in 1912  by chemist Fritz Klatte.

He worked for a chemical company  that made lye and calcium carbide,   a chemical used in the steel industry. And the process of making both  of these things created a lot   of chemical byproducts, which  nobody really had a use for. Klatte figured out how to turn those byproducts  into long chain-like molecules called polymers,   like PVA and another familiar  plastic, poly-vinyl chloride, or PVC.

And it turns out, both of those are really useful. Mixing PVA and water creates that familiar  white liquid glue, which hardens and turns   clear when the water either evaporates or  seeps into whatever you’re gluing together. One of the first uses of PVA glue was as a lacquer  coating for German planes during World War I.

Apparently it didn’t have a great  reputation for that, though. Better to stick to arts and crafts. If you want something a little  stronger, you might reach for superglue.

Superglue, also known as cyanoacrylate,  didn’t come on the scene until the 1940s. During World War II, chemist Harry  Coover was working to develop a super   clear plastic that could be used in gun scopes. And the researchers had happened  upon this material that was clear,   but also ridiculously sticky.

Any contact with moisture, even just from the air,   would cause the molecules to start linking  together into longer polymers and harden. It wasn’t great for the lenses like he’d  hoped, so he moved on to other materials. Coover returned to his accidental invention after  the war, in 1951, while working on airplanes,   and realized that there was an upside to the  sticky situation his wartime research had created.

Cyanoacrylates are different from PVA  glues because of when the polymers form. The polymers in craft glue  come pre-made in the bottle. But cyanoacrylates only form their all-important   polymers after you squeeze  the glue out of the bottle.

That gives the molecules the chance  to stick to everything – each other,   your DIY project, the table  you’re working on, and your hands. Coover filed for his patent in 1954, and the first  version of Super Glue hit the shelves in 1958. The molecules each contain  electron-heavy nitrogen,   which has a magnet-like pull toward  the hydrogen atoms of other molecules.

There’s also a pattern of double and  single bonds that are primed to form   long chains with a little nudge  from moisture in the environment. When dry, super glue dissolves in a mild  solution like nail polish remover or lemon juice. So keep that in mind if you ever have  any superglue-based disasters at home!

And just like the glues of old, cyanoacrylates  reached the battlefield eventually. Not in the weapons, but in medical bags. During the Vietnam War, U.

S. medics  used a spray-on version of superglue   to temporarily stop major bleeding, which gave  them more time to get the wounded to hospitals. It wasn’t technically approved for medical use at  the time, but field medics had other priorities. Coover applied for approval of a medical  cyanoacrylate in 1964 from the U.

S. Food and Drug Administration,   which had to be different from the  original version for a few reasons For one thing, when superglue dries, it  releases heat, which can lead to burns. And the polymer can degrade into  toxic compounds like formaldehyde,   which you don’t want on an open wound.

Cyanoacrylate is also really firm when it dries,  and when it comes to medical adhesive, you want   something that moves with the skin as it heals,  instead of something that’s brittle or sharp. Other countries were playing around with  medical cyanoacrylates by the 1970s,   but they didn’t hit the market for a little while. And the FDA only approved the first  one for use in the U.

S. in 1998. And now, surgeons use substances that are  similar to superglue to fix blood vessels,   seal ulcers, and repair bleeding organs. Dental surgeons use it, too.

It turns out that glue is useful  in medicine for the same reasons   that our ancestors wanted it  tens of thousands of years ago: it’s sticky, it’s flexible,   so it can hold together odd-shaped stuff  without the use of outside machinery. But there are still challenges  in making medical glues better. Our tissues are wet, like, all the time,   which makes it hard to use a glue that needs  to dry in order to stick stuff to them.

Luckily, we can draw inspiration  from the animal kingdom. Specifically, shellfish. Marine mussels are fantastic  at sticking to rocks and metal,   and they can hang on through crashing  waves and submersion in saltwater.

So scientists are studying their natural glue,   called “mussel foot proteins,”  to see what we can learn. And they’ve honed in on a part of  the mussel foot protein called DOPA. They’re still figuring out exactly how it works,   but they do know that DOPA is key to mussels’  ability to stick to so many surfaces underwater.

And since it works in the ocean,  which is the wettest of the wet,   that could make it useful  for medical applications. There have been a few trials  of DOPA-based glues in mice. And one team of researchers created  a DOPA-based glue that could help   during fetal surgery – like, surgery  on a fetus inside a pregnant person.

Fetal surgery can treat a bunch of  critical conditions early enough   that they don’t impact fetal development,  and result in healthier babies at birth. Which is amazing, but also risky  because the surgeon needs to get   into the amniotic sac without damaging it. So this research team worked on a DOPA-based  patch that they could apply to the amniotic sac,   then insert the surgical tools through  that patch for a water-tight seal.

They also hope it might prevent the damage from  surgery from spreading beyond the incision. But it’s still a work in progress. That research was done in  rabbit models, and for now,   there are no clinical trials for  mussel-inspired glue in progress.

But hey, maybe someday! It’s clear that the human desire to stick stuff to   other stuff has brought us  a long way from birch tar. We started this journey putting rocks on sticks to  build weapons, which we used to cut things apart.

So it’s kind of awesome that the  future frontiers of glue-making   are all about putting things back together again. [♪ OUTRO]