Science Friday - The 4,000-Year History of Humans and Silk

Episode Date: April 29, 2024

Silk is one of the most luxurious fabrics for clothing and bedding. Unlike cotton or linen, silk is made most commonly by insects—often the Bombyx mori, a domesticated moth that feeds on the leaves of mulberry trees. Humans have a 4,000-year history with the textile and the creatures that make it, as documented in the new book Silk: A World History.Since silk has an unconventional origin as a secretion rather than a plant product, it has unique biological qualities that make it strong and enduring. And because it’s a natural protein fiber, it’s biodegradable, so scientists think it could have a future as a sustainable alternative to plastics and electronic parts.Guest host Arielle Duhaime-Ross speaks with Dr. Aarathi Prasad, biologist and author of Silk: A World History. They discuss the ways humans have changed silk-creating creatures through domestication, future applications of the textile, and Prasad’s experience growing silkworms of her own.Read an excerpt from Silk: A World History at sciencefriday.com.Transcripts for each segment will be available after the show airs on sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

Silk is one of the strongest fibers found in nature. It is said that if a spider was the size of a human, the web that we made would be strong enough to stop a jetliner. It's Monday, April 29th, and you're listening to Science Friday. I'm SciFRI producer Kathleen Davis. Silk is synonymous with luxury. And unlike other textiles, it's made most commonly by insects. Insects with whom humans have a 4,000. year history. Since silk has the unique ability to be both super strong and biodegradable,

researchers are looking at using it as an alternative for plastics. Guest host Ariel Duem Ross speaks with Dr. Arity Prasad, biologist and author of a new book, Silk, A World History. Welcome to Science Friday. Thank you for having me. I'm very happy to be here. So you wrote an entire book about the history of silk and its properties. What drove you to focus on a single type of fiber? I actually came to silk because I was interested in regenerative medicine. So the idea that you can use a substance from natural world and put it into your body and encourage the body to heal itself. And I discovered that there were many technologies being developed using silk as a base. And then I started looking at what kind of silk was used for that purpose and then

realized that it wasn't the only silk. And they were all the same silk. And they were all. these other animals, but also places from which silk originated. And then I realize it probably is the oldest material that's been in continuous use and has the surprising future. And you also have a personal history with silk as well, right? Yes, so I have an Indian mother and my Indian aunties always wore silk because in the east it's a, it is a luxurious fabric, but it's something that's very special and it's worn for, you know, special occasions, religious festivals. And I was always fascinated by the way it felt and looked. And as a child, I saw it being woven on, on handlooms in India. And I didn't really know where it came from. I mean, I just wore the fabric. And this gave me

pause to think about what it was made by and how it was made and why. Right. So what exactly makes silk so unique? I think silk's unique because it's a natural animal protein, but it has incredible properties. If you look at silk as a thread, it's a thousand times finer than a human hair. And yet the properties it has of strength, that's stress breaking point. And elasticity is really quite incredible. And that has something to do with the protein structure, yes? It has everything to do the protein structure and also the protein folding. The protein structure, the protein structure gives silk such strength because it's very, very, very boring. It's repetitive. The amino acid, the building blocks of the protein is a regular structure that goes on and on and

on. And that allows it to fold into a form that gives it a lot of strength. It has a unique protein structure because of the purpose for which the animals that created make it. And that tends to be for protection. And that's protection against predators, but also against pathogens, bacteria and fungi. So the most famous silk, the silk that we have at home, as you said, as our bed sheets or our shirts or dressing gowns come from one type of animal, one species of moth predominantly, and that's called the Bombix mori. And as you said, that's about 4,000 years old. And I say only about 4,000 years because you imagine insects having a much longer history evolutionarily. How do we figure out when humans first started using silk? And how exactly did that start

happening? The silk moth that makes the silk that's most of us own was domesticated in China. And although genetic studies haven't been able to entirely validate when that happened, just because in order to do that, you need to find enough of its wild ancestor to compare it. But if you take what genetics we do have together with archaeological finds, so that's ancient cocoons, or it could be carvings found in tombs in China that represent the silk moth, there are many of those, because they're a symbol of metamorphosis, then we think it's somewhere between 4,100 and 5,000 years old. this is only between maybe 4,400 to 5,000 years old,

is because it was bred from an ancestor, from a wild ancestor, intentionally. So it was domesticated by Neolithic farmers in China. And the reason they did that was because its ancestor was a very fast-flying, dark, well-camifledged moth, and it made a beautiful, fine white silk, probably a better silk than the Bombixmori does. But it wasn't very cooperative. And it was considered a pest because it ate mulberry trees. So mulberry leaves.

So they domesticated. And in the process of domestication, what happens to an animal is they generally get a little bit more boring and docile because they are bred for our purposes. So the bomb bixmori became very small, very pale. And it's completely dependent on humans for everything. It's the world's only truly domesticated insect. It's born blind. It can't really fly. And it needs humans to feed it and for it to be able to mate.

Wow. Okay. And what exactly did we do with silk in the beginning? Did we always make clothing with it? Yes, it's very hard to imagine why we would domesticate something if it wasn't already useful. So I would say that the wild ancestor of the domesticated silk moth would have been used exactly in the way that the farmers later used it for the production of textiles, very special text. because the ancestor of the domesticated silk moth actually made a very beautiful, a silk that was even finer than the moth that they bred. So there must have been some kind of trade-off then? The trade-off was the moths that were bred in human hands became bigger. So the females became bigger, and that meant that they carried more eggs so they could produce more moths for human purposes.

And even though the silk was a little less fine, the kremen. cocoons were bigger, so they produced more of it. Got it. Okay, so bigger cocoons means more silk. Exactly. Okay, and in a single cocoon, how much silk are we able to get out of that? If you unraveled a bombetsmory silk cocoon completely, it would be somewhere like one to two kilometers of silks, a really very, very long, continuous strand that's created by the caterpillar as it starts going into metamorphosis to become its adult.

form. That's remarkable. And I think you actually raised silk moss at home as part of your research for this book, right? How did that go? And what did you learn? I learned that they're very smelly and they poop a lot. My daughter. Love that. My daughter was mortified. But they were so adorable. They're very, very cute. They start off very tiny and then they eat copious amounts of mulberry leaves. That's their favorite food. People always tell me, oh, when I was little, I had silk worms and I fed them lettuce, and they turned the silk turned green. And that's completely possible. But it's not their best food and the food that they eat in their normal habitat. And so the silk they produce is the best silk if they feed on the white mulberry. And they eat so much of it and they grow till about three

inches in length and they become very juicy little insects. And then they stop. They stop eating and they stop moving. And that's a very dangerous thing for a juicy little insect to do. And so it has to protect itself because this is in a mode of development called complete metamorphosis. So this little caterpillar is about to turn into something that it looks nothing like, which is the adult moth. And so it exudes a silk through glands running along its body and it comes out of its delivery glands. and it's liquid. And when it hits the air, it becomes solid.

And then it wraps itself in this thread progressively over hours. And then the thread hardens. So it actually becomes, it doesn't feel soft at all. The cocoon, it feels very hard. There's a little sort of floss like handy floss type silk around it. But it's generally very hard. And then after some time, it vomits an enzyme that breaks down the cocoon and it escapes as the moth.

That is so cool. I got to ask, though, if you can do this at home, if you can raise silkworms at home, what makes silk so luxurious? Why is it so expensive? Well, it's not the easiest thing to make and to make it to a great quality and the way that the Chinese developed it over millennia of skill, of not just unreeling the cocoons, but in weaving it and in dyeing it. There are many different ways that you can weave it. It takes a lot of work. Very, very labor intensive as well. Got it.

Okay, okay. So there's more to this than just raising silkworms as well. We've been focusing most of this conversation on the silk moth, but there are different kinds of silk, right? Like different animals that make silk, not just silkworms. There are not just silkworms, but as well as the Chinese domesticated silkworm, there are many wild silkworms across the world. So there was an archaeologist and textile expert at Harvard, Dr. Irene Good,

and she said that, you know, if you find a fabric or a thread in an archaeological site in China, you probably tested to see if it was silk. But if you found it in Mexico, Madagascar or India or Cameroon, you probably wouldn't. But in all of these places and across the world, there were wild moths. And the people who lived there were making textiles out of it. them. In Madagascar and in Nigeria, there was a cloth made of a wild moth. And by the way, while the domesticated moth is tiny and pale, the wild moths are, you know, as big as an adult hand and colorful. And they make huge cocoons. And the reason why the African communities who

use this thread to make a textile, they used it to make death shrouds, because it was very well known to have anti-wrought properties. And that's the thing about the cocoon that all silkworms make. It protects the insect, both from being eaten, but also from pathogens like fungi and bacteria. So these moths are spread across the world. But there were also some surprising animals that I hadn't really thought of. I knew about spider silk. Spider-silk is the holy grail of silk because it's incredibly strong. It is said that if a spider was the size of a human, the web that we made would be strong enough to stop a jetliner. So that's some incredible strength. And they make many different types of silk, some to wrap their eggs, some to make the web, some to drop from,

and they all have to have different properties. And they're produced from single glands to have these incredible properties. And scientists say, you know, what a spider can do on a diet, a bugs, technology still hasn't been able to catch up with. So that silk is really rather incredible. And for hundreds of years, people have trying to find ways of extracting that from the spiders, either by making contraptions that they drew it out from the body of the spider. And the spiders really weren't happy about that, but it didn't kill them. It probably did hurt to genetically modifying other animals to be able to make spider silk. Right. I think that has happened with goats, right? That has happened with goats. The scientists,

Randy Lewis at Utah, who did it with goats, first started doing it with e-coli. These are bacteria that replicate very quickly. So every time they replicate, the idea was, if you put spider silk genes into them, they will also replicate it very quickly. Then he tried yeast, the kind of yeast that we make beer out of, I think. And then he used the spiders, the sort of goat, the spider goats, the goats that when you milk them, the silk comes out in the milk. But again, it comes out as a liquid form and somehow you have to take it back to a solid form, but we don't know how the spiders do that. On top of that, you're not putting an entire genome of spider into a goat. You're selecting a part of it. And now the labs trying to use the silk one because he says,

well, they've been doing it for thousands of years. But again, if you use a tiny part of spider DNA and put it into moth, you'll get part moth silk and part spider silk. So we still haven't cracked that nut. Wow, that'll be incredible if we actually managed to do this. We don't have a ton of time left, but I do want to ask you this. One thing that really intrigued me from your book was the section on future applications for silk, like replacing plastics. How would that work? So we've become very good at making plastics, and plastics are polymers, and the polymers that we make are sourced from fossil fuels. These are hydrocarbons. Silk is also a polymer. but it's a natural biodegradable one.

And if we have the technology to make plastics from hydrocarbons, then it's sort of a no-brainer that we can also use that technology, but using the material of silk that's biodegradable. Are we talking about a one-for-one replacement here? Yeah, I mean, I think silks can very easily substitute plastics, and the scourge of plastic waste is terrible. But what I find more exciting is the idea that silk can, substitute for electronics because I think that's far more pernicious problem for us and the world at

the moment. Oh, tell me more about that. Well, so at Tufts University in Boston, Fiorenza Omenato, he started life as an engineer. He now runs Silk Lab at that university. And he's been looking at making human electronic interfaces using silk as a base. So they've used of all sorts of medical and surgical purposes, but they're also able to make sensors that you can put into the body because the body won't reject it to report back on vital signs, for example. You can put sensors on food to prevent food waste because the silk would also be edible. So there are many ways in which they're trying to create electronics with silk as a base so that it can interface with biology in a way that we haven't been able to do with plastics. Wow. All right. Okay. So I,

I got to ask, after spending years thinking and writing about silk, does it still feel luxurious to you? Do you want it to stay that way? Or is the future one where silk is a lot more accessible and, you know, democratized? Definitely accessible. I mean, I didn't mention the giant mollusk that lives in the Mediterranean Sea, but that's been used to make silk for millennia as well. And the point, what I've realized is the organisms that make silk are distributed all over the world. And that, if you're making sustainable innovations for the world, that production needs to be local. And that's why it's so exciting. It's not just local. If you are maintaining a population of animals like wild animals, they absolutely need the trees and the forests that they live in in order to produce the product that we need.

And that means we also have to protect their natural environments. That's all the time we have for now. Dr. R.T. Prasad, biologist and author of Silk, a world history based in London, England. Thank you so much for being here. Thank you so much for having me. And if you want to read an excerpt from this book and learn more about the future applications of Silk,

you can go to our website, sciencefriety.com slash silk. And that's all the time that we have for now. A lot of folks help make the show happen, including Annie Nero, Emma Gomez, Charles Bergquist, Danielle Johnson, and many more. next time we'll talk about how to take a video of what's happening near a black hole.

But for now, I'm Zyvry producer Kathleen Davis. We'll catch you then.