Daniel and Kelly’s Extraordinary Universe - Nettie Stevens and the Discovery of Sex Chromosomes

Episode Date: March 25, 2025

Daniel and Kelly explore the oft-forgotten scientific contributions of Dr. Nettie Stevens, who discovered that chromosomes are responsible for determining an organism's sex.See omnystudio.com/listener for privacy information.

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so you can work, create, and boost productivity all on one device. December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys. Then everything changed. There's been a bombing at the TWA. Terminal, just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged. Terrorism.

Listen to the new season of Law and Order Criminal Justice System on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Wait a minute, Sam. Maybe her boyfriend's just looking for extra credit. Well, Dakota, luckily, it's back-to-school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend's been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now, he's insisting we get to know each other, but I just want her gone.

Hold up. Isn't that against school policy? That seems inappropriate. Maybe find out how it ends by listening to the OK Storytime podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. The U.S. Open is here, and on my podcast, good game with Sarah Spain. I'm breaking down the players, the predictions, the pressure, and of course, the honey deuses, the signal. signature cocktail of the U.S. Open. The U.S. Open has gotten to be a very wonderfully experiential sporting event. To hear this and more, listen to Good Game with Sarah Spain, an Iheart women's sports production in partnership with deep blue sports and entertainment

on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Brought to you by Novartis, founding partner of IHeart Women's Sports Network. Hello there, friends. Heads up, as I'm sure you can tell from the title of today's show, we're talking about the birds and the bees again, so keep that in mind as you decide whether or not you want to listen to this with your kids. Also, today, when we use the word sex, we're specifically referring to gametic sex,

which is to say we're talking about whether or not a person produces sperm or eggs. Okay, let's get started. About 2,300 years ago, Aristotle postulated that babies come from a mixture of male semen and female blood that mixed together in the uterus. To explain how this mixture of fluids turned into either a boy or a girl child, Aristotle pointed to heat. You see, he argued, all embryos start by developing into boys, but if not enough heat is present, then that development will stop, and you'll get a girl instead. 500 years later, and we still hadn't made loads of progress. Gallen, a famous Greek physician,

and philosopher, had a much better sense of human anatomy, but was still focused on this heat thing. Ovaries, he argued, were actually just testes that hadn't experienced enough heat to make it through the journey to the outside of the body. By the early 1900s, we had learned a lot. Gregor Mandel had done his experiments in pea plants, so we knew that traits could be inherited from parents. And we were starting to get an inkling of where the blueprints for that inheritance were being stored. We had seen chromosomes inside of cells, but had not yet tied those chromosomes to the inheritance of specific traits. And this is where Nettie Stevens comes in. Netty, a rare Ph.D. holding woman scientist at the start of the 20th century, peered into the cells of insects and made an

exciting discovery. One pair of chromosomes were different sizes. If you got the smaller chromosome, you were a male. If you got the bigger of the pair, you were a female. Not only had she discovered what went on to be called the X and the Y chromosomes, but this was the first time a particular trait had been tied to a particular chromosome. Today, we talk more about Nettie Stevens' life and how she came to make this amazing discovery. Welcome to Daniel and Kelly's Extraordinary Universe. Hi, I'm Daniel. I'm a particle physicist, and I'm very excited to get into the messy questions of biology. I'm Kelly Weiner-Smith. I'm a biologist, and wow, these questions really can get very messy.

There's a lot we haven't figured out here, but I'm excited to talk about the early studying of how we determine if you end up making sperm or eggs. Messy questions, messy answers, messy methods. That's what biology is all about, getting gooey with it. I like the getting gooey with it part. I don't know about the messy methods, but I'll let that slide. I didn't mean sloppy. I mean, that sometimes you literally get gooey. I mean, you come home and goo all over yourself, right? You're standing in a dumpster full of goo.

Yeah, no, there have been a couple instances where I've been like, how did I get fish guts in my hair? And I discovered it later in the day. And Zach was just absolutely appalled. And, yeah, because you marrying a biologist is really gross stuff. I think it's underappreciated out there by the general public. how much the methods and the day-to-day work decides where you end up in science. Because, you know, like, you're interested in the questions of biology, but I'm imagining

you're not answering the deep questions of biology every day. Mostly, you're having fun working with fish guts or whatever, and so you got to enjoy that bit. And, you know, I spend most of my time writing programs on the computer, and I enjoy that bit. So it's like, you know, what kind of messiness do you like determines what kind of science you do, probably more than your inherent curiosity about the universe. Absolutely. And I don't necessarily know that we tell students that enough. Like when I introduced students to working with me when I was a grad student, I mentioned like a bunch of undergrads. And I'd be like, look, straight up, a bunch of your time is going to be spent in a room that smells awful, like a combination of dead fish and formalin or like staring and trying to count tiny little items under a microscope. And if you care about the question, you will love every second of that. But if you don't care about the question, you should find a different field because you're going to be miserable. And some of them drop. out not that long after but it's good to find out early do you like that kind of stuff or do you not and sometimes you can care about the question but just not really enjoy the day-to-day work of it you know

my first experiences in research were like plasma physics and i was like hey i'm gonna figure out fusion and save the world and i definitely still care about that but i find vacuum systems and plasma machines annoying and i did not have a good time that summer and thank you to those physicists out there who mentored me that summer but i was deeply bored You really got to try this stuff out when you're in undergrad as much as possible. Like, I thought I was going to love molecular work because it would be like an episode of like CSI, you know, where in there and they're like fancy labs and they're pipetting to find amazing answers. And I hate pipetting.

I'm miserable at it. My hands hurt. They get tight. I overthink everything. It doesn't matter how great the music I'm listening to is while I'm doing it. Like I cannot get through it. And so I just, I stopped doing molecular work. Wait, but do you actually musical montage your way to an answer sometimes?

You're like, I'm going to jam through this. Let's put on the music and dot, dot, there we are. I love when they do that in TV shows. Someone told me like, look, you're overthinking it. If you just flow, it'll be fine. So pick some music you can flow to. And I lost like $1,000 because I messed up a bunch of kids because I have to do more than just flow. I'm not a person who flows, I guess.

But, you know, this is a good intro, actually, perhaps even accidental. Because the woman that we're talking about today, there were maybe 50 or more species of insects where she went through and she like crushed cells, stained cells, and watched different stages as they divided to produce sperm. And I read one of her papers and it had something like 250 plates where she had like very carefully drawn what was happening with chromosomes and these various stages. That must have taken many, many hours and been incredibly tedious work and counting to track all the chromosomes and how they were matching up with one another. And, you know, the patience that it must have taken to even just like extract the gametes out of like a tiny

cricket so that you can look at them like that takes a lot of skill so yeah she really persisted through a lot of what would have been very boring work to many of us maybe she loved it i don't know but i looked at the plates and i immediately was like whoa i would have been way too bored to do this maybe she was jamming to the hip music of the day the whole time right that's sort of like a lost question in history, what music did famous scientists listen to while they did all their important work? Yeah. Did we have records in the early 1900s? We must have, right? Yeah, I think we did for sure. Jamming to some record? Nice. I like the idea that that's what was happening. Too bad she couldn't listen to our podcast. Somebody out there write a book about the history of music listened to by

scientists. I don't know if that sounds career advice. I don't know how big the audiences for that book. I'm going to read that book, so I don't care. Somebody else there write it. Okay, great. Sounds good. Today we're not talking about music, but we are talking about pioneering scientists. We made incredible discoveries that influence the way we think about big, important questions in our life. In this case, big, important, messy biological questions. Yes, and we're specifically focusing on scientists that most people have never heard of, despite the fact that they made absolutely profound discoveries about the way that life works. And so we post, to our audience, the question, what was Nettie Stevens' big biological discovery? And if you want

to answer our questions, contact us at Questions at Daniel and Kelly.org and we'll add you to the list and we'll send you our question before an episode and you can give us your best guess. All right, let's hear what our audience came up with this time. Nettie Stevens discovered that as long as radioactivity is negligent, a cellular mechanism will be able to achieve stasis for a nominal period or duration of time. I'll be honest. I've never heard of Nettie Stevens. Well, this one's short. I have no idea. Did Nettie Stevens invent the Nettie pot that allows you to irrigate the biological war zone

happening in your sinus cavity as long as you hold your head at the proper angle? Otherwise, you end up with a salty, slimy soup for, lunch? My completely uninspired guess is that she discovered that the Loch Ness Monster is real.

All right. So to be clear, despite the profound confidence that we're hearing in that very first explanation, that is not correct. Is that totally manufactured or is that somebody else? No, no, no, that's totally manufactured. He wrote and made some sort of joke about

like, you know, the more confident you sound, something like that. That sounds like a chat GPT answer, right? Total confidence and total nonsense. Sometimes chat GPT gets it right, but yes, often it's hallucinating, which this listener was perhaps also doing. She's not associated with the Nettie Pot. And then we had two listeners who had never heard of her.

Well, it sounds like there's a lot of folks out there who need to know more about Nettie Stevens and what she contributed to our understanding of the biological world. So let's go. Let's find out. Let's hear all about Nettie Stevens. So, Kelly, tell us, who was Nettie Stevens? And I'll do my best to sound confident so that even if I'm wrong, everyone will believe it. Well, I'm pretty sure you're not an AI.

I mean, I've known you long enough. I think I would have figured it out by now. I don't know. AI is getting pretty good. All right. So Nettie Maria Stevenson. She was born in 1861 in Vermont. And she was born to like a middle class family.

And unfortunately, her mom died when she was pretty young. But she was lucky that she had a father who really wanted to invest. in his daughter's education, and she also had a sister. And so he sent them to school. They went to Westford Academy, and she studied to be a teacher. Is this something that was unusual at the time? Like, did most women go to school? Did most folks living in middle class Vermont go to school?

Or was it unusual for them to go? It was pretty unusual. So training to be a teacher wasn't super unusual. Training to be a scientist definitely was. But it was still fairly unusual. And so she leveraged this early education as a teacher so that through various points in her life, she could teach for a while to save up money

so that she could afford to follow her dream, which was to become a scientist. And so she would teach for a while, save up money. And at one point after saving her money in 1896, she started at a new school that, you know, maybe you hadn't heard of called Leland Stanford Jr. University. It ends up becoming Stanford. And she gets a bachelor's and a master's there.

Wow. And how many women are at? attending Stanford in the late 1800s. Not many. I don't know the exact number, but there were not many. And as you'll see, like at one point when she gets recognized for her amazing contributions, she's on a list of the top 1,000 men in science. Oh, what?

And she was one of 18 women on that list. So to sort of give you some sense of how many famous women scientists there were at the time, there were 18 women who broke into this list of 1,000. and this was towards the end of her career. And did she also get married and have a family, or did she have to choose between those paths? So I wasn't able to find a lot of personal information about Nettie. I haven't been able to find a biography that people wrote about her,

the Encyclopedia Britannica, Wikipedia, everywhere you go, they pretty much say the same things. I found a couple scientific papers talking about her early life and contributions, and they all just sort of like list off the same facts. Sounds like somebody needs to do a deep dive and write a book about Nettie Stevens. We're like book projects out of the Wazoo on this episode today. Absolutely.

And she was connecting with a lot of major players in the field at the time. And so I think there'd be a lot of interesting information to go on. And maybe like the letters of her advisor who ended up being a famous scientist. We'll talk about him like might have mentioned enough where you could get some more personal details.

But anyway, there weren't a lot of personal details. But no, she did not end up getting married or having kids and she ended up being buried with her dad and her sister. So science was her life as far as I was able to tell. Married to biology. Maybe she had some great hobbies and had some really cute dogs, but we don't really know. Well, it sounds like she had a really fulfilling and satisfying life. So go Nettie.

Yes. No, absolutely. So early in her career, she identifies two new species of ciliates, but she ends up wanting to study more sort of genetic-based stuff. What's a ciliates, sorry? These are tiny little eukaryotes, and ciliates are like little hairs that they have on the outside of them. And so these hairs sort of are moving frantically to get them from place to place. and there are teeny tiny little things and she managed to find two new species.

And how exciting is that to find two new species of ciliates? Is that the kind of thing we do every day because there's the zillions of them like beetles? Or is it like a big breakthrough? Or what do you learn when you name two new species of ciliates? There's a lot of ciliates. To name a new species, especially today, requires a lot of like precise measurements

and studying the ecology of the animal and coming up with an argument about why it's different than anything that's been identified before. So it's really good practice for very careful measurements of different sort of like organ parts and getting really good at sort of like drawing things and tracking sizes and having a good eye for what's different between things. And so that was probably a really great skill for her. But I don't think anyone's ever won a Nobel for identifying a new ciliate, unfortunately.

It would be pretty silly to win a Nobel Prize for ciliates. I don't know about, oh, silly, it's. Ah, ha, ha. Maybe I am an AI. I'm not getting the jokes. Well, you know, I often am criticizing biology as like just being botany because I feel like the interesting bit of science is not let's go out and describe what we see in the world, but it's the part where we harmonize that we put it in context, we understand the differences, the distinctions, what those trends mean, right? Not just like, hey, here's a list of all the different silly bits in the world and we call them siliates. You know, and we do this in particle physics also, right?

We have all these particles. We don't understand them, but we want to, right? we're asking those questions like, why do we have an electron, a muon, and a tau? So when somebody makes a new species, is it always interesting just having seen it, or is it only because it raises these questions about evolution and the context and the history? I think it's both. And actually, I feel like right now, especially in my field, it's underappreciated when you describe a new species because there is such an emphasis on like, how does this fit into bigger theories and like that's the kind of stuff that get to an NSF grant. And to be honest, I feel like we are

racing way ahead on the theory and not stopping to do enough cataloging so that you actually have the data you need to test these theories. We should have a whole episode on that, but we're getting off topic. I would argue that both of those things, the cataloging and the big theories, you can't do one well without the other. They're both critical. You need botany and philosophy to come together in harmony. All right, but let's get back on track with Nettie. So she's discovered two new silly kinds of ciliates. What happens to her next? She saves up some money again, and then she ends up going to get her Ph.D. at Bryn-Mar. And she's saving up money because she's paying to do this. So she's like paying for her education. I think she probably had to pay for things like her

housing. I don't know what kind of fellowship they offered her. But I do know that when she got there, she was an absolute rock star. And she got the Bryn-Mar president's European fellowship. So she got to go to Naples to work with this famous guy doing genetics work at the time. So she was like an absolute rock star. And she wraps up her Ph.D. And this is when stuff really starts going. So she starts getting interested in whether or not chromosomes are what determines if you end up making sperm or eggs. And she gets a big grant to answer this question. And when we come back from the break, we're going to talk about what the prevailing theories at the time are for what gives you boys and what gives you girls. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys.

Then, at 6.33 p.m., everything changed. There's been a bombing at the TWA terminal. Apparently the explosion actually impelled metal, glass. The injured were being loaded into ambulances, just a chaotic, chaotic scene. In its wake, a new kind of. enemy emerged. And it was here to stay. Terrorism. Law and order criminal justice system is back. In season two, we're turning our focus to a threat that hides in plain sight. That's harder to predict and even harder to stop. Listen to the new season of Law and Order Criminal Justice System

on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Oh, wait a minute, Sam. Maybe her boyfriend's just looking for extra credit. Well, Dakota, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend has been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now, he's insisting we get to know each other, but I just want her gone.

Now, hold up. Isn't that against school policy? That sounds totally inappropriate. Well, according to this person, this is her boyfriend's former professor, and they're the same age. And it's even more likely that they're cheating. He insists there's nothing between them. I mean, do you believe him? Well, he's certainly trying to get this person to believe him

because he now wants them both to meet. So, do we find out if this person's boyfriend really cheated with his professor or not? To hear the explosive finale, listen to the OK Storytime podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast. Have you ever wished for a change but weren't sure how to make it? Maybe you felt stuck in a job, a place, or even a relationship. I'm Emily Tish Sussman, and on she pivots, I dive into the, inspiring pivots of women who have taken big leaps in their lives and careers.

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Now on the IHeartRadio app, Apple Podcasts, or wherever you get your podcasts. The U.S. Open is here. And on my podcast, Good Game with Sarah Spain, I'm breaking down the players from rising stars to legends chasing history. The predictions will we see a first time winner and the pressure. Billy Jean King says pressure is a privilege, you know. Plus, the stories and events off the court and, of course, the honey deuses, the signature cocktail of the U.S. Open. The U.S. Open has gotten to be a very fan. wonderfully experiential sporting event.

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on the IHeart radio app, Apple Podcasts, or wherever you get your podcasts. Presented by Capital One, founding partner of IHeart Women's Sports. All right, we're back. So let's take a bit of a historical perspective to figure out where we are after Nettie Stevens has finished her Ph.D.

So she finished her Ph.D. in 1903. So over 200 years earlier, Anton von Leyenhook had looked at his sperm underneath a microscope and the sperm of dogs also. So we knew sperms existed for over 200 years. And Lewin Hook is the guy who discovered the cell originally, right? He's like first dude to look in detail at microscopic biology. Von Lewin Hook was not the first one to see cells. That was Robert Hook.

Von Luenhook saw little organisms in pond water. He called them animicules. And he also did a lot of exploring for little animicules in on and from. his body. So he was the first one to observe sperm. I love the word animicules. That's awesome. It is. And it's also cute. And I like cute. Exactly. Like little tiny animals, how nice. Yeah. So was he surprised to discover like wiggly things in his semen? Like, is this a big shock to him? Or was he looking for that? Yeah. I think he was a little bit surprised. And there was some question about whether or not that was like contamination and the like fluid around the sperm is what was actually

important for making babies. And so I think he was surprised and he wrote it up, sent it to the Royal Society and turned out, yeah, that those sperm are important in making babies. All right, cool. And then about 150 years later, we have the first, like,

description of mammalian eggs. So for a while, actually, we used these phrases, male sperm and female sperm. And the idea was that out of the ovaries came sort of the female equivalent of sperm. And it took a while before someone actually observed eggs.

I think they had dissected a rabbit to see eggs for the first time. But so by about 1825, we have seen eggs. We have seen sperm. And how do you know what an egg is? How do you recognize it's like, oh, this is obviously the counterpart to the sperm? Like, isn't there just another cell in the body? Yeah, that's a good question because it would be another 50 years before we saw the fusion of sperm and egg to come together to produce like an embryo. But I think that it was a series of experiments that were done on rabbits where they were sort of dissecting.

at different times and they observed like the same kind of cell sort of moving out of the ovaries and into the uterus. And I think that that's what they, when they were like, okay, we don't have female semen. We've got this little thing that like makes a journey from the ovary into the uterus. And I think that they had seen bird eggs, which sort of gave them like this egg sort of idea. I believe that that's the history of how we observed eggs. And this sort of the broader context, like people are wondering how sex works and how you end up with boys or girls. And there's already this prevailing concept that it could be determined by something like microscopic. You know, where are we in like relation to germ theory in this idea that like

there are tiny invisible bits in biology that can have big macroscopic impact? So I think the prevailing theory is that like, yes, these things can come together and they can be important in the production of offspring. But how does that end up as a boy or a girl is still an open question. And so one of the theories from like 1748, there was this book by a French anatomist whose last name was Kuto. And the book was called The Art of Having Boys, because obviously that's what you'd want to be doing. And the idea was that sex organs come in pairs. So males have two testes, females have two ovaries. And so it must be that one testy is important for making males and one makes females. And they hang at different heights.

And so you should be able to figure out which one makes boys and which one makes girls. And so... This is also very logical and also off base. Well, this book actually went through four editions, which is more editions than my book has gone through. So this was apparently better than anything I've done in my life. So he actually proposed that if you want boys, you could figure out which testes makes boys and which one makes girls and you could remove the one that makes girls to make sure you make boys. But for the more timid, he notes that since there's... one ovary that makes girls and one ovary that makes boys, you could instead just have the woman

lay on one side. So gravity will send the sperm to the ovary to get the child you want. Wow. So none of this works. Don't trust best selling authors. That's a more general longstanding lesson. Yeah. So people understand it's something microscopic, but we want to understand the more detailed mechanism and that's what people are using microscopes and dissecting stuff. Yes, right. And while we're on the topic of what ends up giving you boy and girl babies, as classically defined at the time, nutrition was another popular idea. And so the idea was that if you are underfed, you'll produce boys. And I think the argument there is that sperm, they're like smaller and they're chasing to get to the bigger eggs. And so if you're not eating enough, you produce these

smaller sperm things. And it's confusing. Man, if we had social media back then, there were been lots of crazy influencers telling people how to have. boys or girls. Oh my gosh, so many. But, you know, to be fair, so like, in the animal kingdom, whether or not you get an individual who produces eggs, an individual who produces sperm, or an individual who produces both, so hermaphrodites of different types, is actually very complicated. And so this idea for nutrition wasn't completely crazy. So like they had taken a bunch of sheep and they malnourished some and they overfed others. And they ended up with 60% female offspring and 40% males.

And then there were some what they called lower vertebrates where they did these experiments and the effects were even more pronounced. And so there are some species where nutrition sort of plays a role in whether or not you're more likely to get offspring that makes sperm or offspring that make eggs. And so they had made some observations that were consistent with this idea. But it just turns out that's not how most things work. And of course, that's not how humans work. All right.

So Lowen Hook sees the sperm. People discover eggs. Kuto writes a nonsense book about the art of having boys. It's understood to be very complicated. How did Nettie get involved? What was her interest? And what did she contribute?

Taking a very slight step back. In like 1865, Gregor Mendel did his pee experiments that you might have learned about in high school, but it was a long time ago, for me at least. So essentially he did all these crosses of pee experiments and he discovered that the traits in the like baby pea plants were being inherited from the parents. But it wasn't clear what,

material was passing that information from one generation to another. So he made these amazing contributions to science, which were promptly forgotten about. And nobody looked at it. Like, I think literally, so a long time ago when books would be printed, sometimes you'd have to separate pages with like a little knife because of the way they were printed. And so Darwin had Mendel's book on his bookshelf, but the story, and maybe this is apocryphal, but I've been told it like five or six times. So I'm going to go ahead and share it, was that he had.

hadn't actually opened up the pages in his book where he would have learned about this inheritance stuff. So he could have learned about it, but he didn't. But anyway, okay, Gregor Mendel does this inheritance stuff. Everybody forgets about it. Around 1870, we first see genetic material. So it's called nuclean. We don't really know what it does, but we've now seen genetic material.

Meaning that we've gone now inside cell. So not just like, here's a sperm, but we're like digging into the cell and finding the bits that carry that information. Yes. Yep, exactly. But then by the early 20th century, so around 1900, Mendel is rediscovered by a bunch of scientists. And we're like, oh my gosh, okay, traits are inherited from parents. We have good evidence for that. But what are the blueprints that are transmitting this information? And so now you can combine the fact that you've seen this stuff inside of cells. And that feels like maybe that's the stuff that transmits the information. And so now people are starting to dig into that. And so this is the scientific background of where Nettie Stevens jumps in. So in 1891, Herman von Hanking, he's studying chromosomes. He's noticing that they almost always pair up. But he notices that there's this one thing that doesn't pair up, and he calls it the X element. But he doesn't know what it does. And this is before the 1900s when Gregor Mandel had been sort of rediscovered and his works had been rediscovered. So people at that time weren't really

trying to connect the stuff that was in the cell to traits that got inherited across generations. So he's just like, oh, there's this thing and it kind of acts different than the other stuff and he calls it the X element. About 10 years later, Clarence McClung notices the same thing and he changes from the like super X many awesome sounding X element

to the accessory chromosome, which is a boring, which is so boring. It still has X in the title, right? Excessory chromosome. I think we got to talk about your spelling. But in 1902, he notices this accessory chromosome and he says, okay, look, it doesn't seem like all of the sperm cells are getting it. And so maybe this is important in determining who becomes a male and who becomes a female,

because that's the most obvious difference between the organisms that he's studying as male and female traits. And he postulates incorrectly that the males are the one who gets this. extra accessory chromosome it increases the metabolism of the organism who gets it and that's how you get males. So he's right on the edge of discovering this but he gets it like basically exactly backwards.

That's right. But you know I think so often when you study the story of how a scientific discovery comes along what you want is like one person who out of the blue came up with the idea because that makes for a better story. But no, this is just like almost all science is like a community sort of moving towards an answer and there's multiple players sort of along the journey.

So McClung is getting really close, but he gets it in reverse. I think that's almost always the case. Like if you go back and understand like Einstein's work, you see how many pieces he has pulled together from other folks and you see other people publishing the same idea like weeks after him done independently. Like it's almost always the case that science flows as a wave.

And there's somebody surfing at the very edge of it. And we like to write those dramatic stories. It was like single genius in the dark with a candle, you know, and a blanket to ward off the Scottish cold. But more often it's a big community effort. Yeah, I think that wave is often made of many, many colleagues who are holding that person up and pushing them forward. It's not just one person. And that ends up being the case here, too. So Nettie Stevens, after her Ph.D., she got this big grant to study how those chromosomes might determine the sex of individuals.

And in 1905, she publishes her seminal study, studies in sperm, metogenesis. And what she did was she looked at a bunch of different kinds of insects. So she looked at a species of termites, a species of sand crickets, proton bugs. And crucially, she looks at mealworm beetles. And what she does for each of them. And this is what we were talking about earlier. I said there were 200 plates where it was meticulously documenting where the chromosomes were going over time and what was happening. And she was counting all the chromosomes. So she notes accessory chromosomes in some of the species that she's looking at. But when she gets to the mealworm beetle. She's like, okay, when you pair up the chromosomes, there is a pair where you have a big

chromosome and a little chromosome. And I think that the individuals who get the little chromosome become the males and the ones that get the big chromosome become the females. And part of why I think that is because when you look inside the adult cells, forget the sperm, let's look at adults. In those adults, the males have that little chromosome and the females have that big chromosome. So that must be the thing that determines sex. And getting that big X, that makes you a female. McClung said it made you a male, but that makes you a female. And is this what Nettie was studying? Is this what she was like out to figure out? How did she get interested in this question? This was what she set out to discover. I think, you know, she was a person who was good at

looking at tiny little things and making measurements and observations on tiny little things. And she was getting interested in genetics because this field was burgeoning and becoming exciting while she was working on her PhD. So like, while she was working on her PhD, this was the time when Mendel's rules were being rediscovered, concurrent with our discovery of some genetic material. So trying to tie those things together became something she was interested in. And actually, her Ph.D. advisor ended up becoming a gigantic name. He's considered one of the fathers of modern genetics. So he ended up ultimately establishing like Drosophila as a major organism used for lab work. And he connected a bunch of different chromosomes to different traits.

And so I think it was sort of like in the air. And she came from a lab that was sort of interesting. in these sorts of questions. Wow, so this must have been a huge discovery for her, like to see such a clear answer to this question that's on everybody's minds. Yeah, okay. That's totally true. It should have been a super big deal. It happened concurrent, though, in the same year that she published this,

Edmund Beecher Wilson, who was another geneticist who was farther along in his career, he was looking at a different insect species. He was looking at the accessory chromosomes. And he also said, hey, accessory chromosomes, these are what makes males and females, and the female is the one that gets the X. So he came up with the same discovery in the same year. His paper got published a few months before hers. Oh no. I know, but they had been talking to each other. And so he references in an updated version of his paper, he references that like, hey, Nettie Stevens found the same thing.

And actually, she found this little Y thing. And so I believe they had been looking at insects that had slightly different systems for sex determination. So in some cases, males had the Y, and in some cases, males were made by simply not getting anything. So the females got the extra X and the males got nothing. They were just sort of missing that information. And so they had both come up with this at the same time. They were both, it sounds to me, like respectfully communicating their results to one

another, and they just happened to submit and his came out a little bit before hers. But she's largely credited with being much clearer, and she went ahead. and took that extra step and looked in the cells of the adults and was like, okay, this thing that we're seeing in the sperm plays out in the adults. The adult males have the Y and the adult females have the X. But I don't actually feel like it's worth trying to decide like whose words were more firm than another. Like I know we want one person to be out ahead of the other, but like they both did really good work. Yeah. Around the same time. And this is how science works. And, you know, joint credit seems reasonable to me. To me, the issue of these

priority disputes is not who sent in their paper first or who took longer in review and got it published first. It's just like, was the work independent? Because if you read somebody else's paper and then publish based on that, then you shouldn't get credit for their discovery because you're building on top of it. But if you figured it out in your own dink Scottish laboratory, then you figured it out on your own. And if it's in parallel, you should both get credit, it doesn't matter if you published six months earlier or six months later. It's an independent piece of work. I agree. But I mean, in the way that human stories always go, you know, people were talking to each other. And Wilson had been at Bryn Marr right before she got there.

And her PhD advisor kept collaborating with Wilson. And so like, this is a question they all wanted to have answered. So they were all kind of working on it concurrently. And so there was a lot of discussion between them. Nettie made this really nice, really clear observation. Wilson also made some great clear observations. So, I mean, to me, it would be great. if Nettie Stevens and Wilson were both known for the discovery of sex chromosomes, but you know who got the credit? I'm guessing it's not Nettie Stevens because I'd never heard of her and neither had the listeners. So then is it Wilson?

No. It's Thomas Hunt Morgan. And after the break, we'll talk about why. December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys. Then, at 6.33 p.m., everything changed. There's been a bombing at the TWA terminal. Apparently, the explosion actually impelled metal, glass.

The injured were being loaded into ambulances, just a chaos. chaotic, chaotic scene. In its wake, a new kind of enemy emerged, and it was here to stay. Terrorism. Law and Order Criminal Justice System is back. In season two, we're turning our focus to a threat that hides in plain sight. That's harder to predict and even harder to stop. Listen to the new season of Law and Order Criminal Justice System on the IHeart Radio app,

Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Oh, wait a minute, Sam. Maybe her boyfriend's just looking for extra credit. Well, Dakota, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend has been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now, he's insisting we get to know each other, but I just want her gone.

Now, hold up. Isn't that against school policy? That sounds totally inappropriate. Well, according to this person, this is her. boyfriend's former professor and they're the same age. And it's even more likely that they're cheating. He insists there's nothing between them. I mean, do you believe him?

Well, he's certainly trying to get this person to believe him because he now wants them both to meet. So, do we find out if this person's boyfriend really cheated with his professor or not? To hear the explosive finale, listen to the OK Storytime podcast on the Iheart radio app, Apple Podcasts, or wherever you get your podcast. Have you ever wished for a change but weren't sure how to make it? Maybe you felt stuck in a job, a place, or even a relationship. I'm Emily Tish Sussman, and on she pivots, I dive into the inspiring pivots of women who have taken big leaps in their lives and careers.

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Presented by Capital One, founding partner of IHeart Women's Sports. All right, we are back from the break, and Kelly just dropped a big bomb on us, bringing in a dark horse. Kelly, you can't just bring in a new character in Act 3 without. laying the pipe in act one i mean don't you have you taken screenwriting 101 who is this

thomas hunt morgan fellow and why does he get the credit for wilson and stephen's independent work well i'm going to try to not have my feelings hurt too much because i had mentioned thomas hunt morgan earlier oh no a couple of times but you know we were throwing a lot of names around so maybe it didn't stick but he was her phd advisor so morgan is stevens phd advisor yes oh nice no i totally take it back you absolutely did lay the ground work, but like an excellent mystery novel, I overlooked the important clues. Oh, man, maybe on the next Agatha Christie. Exactly. So is this the standard case of PhD advisor gets credit for grad student work? Is there gender stuff at play here? There's definitely

sexism at play here. First of all, it's kind of ironic because Morgan at the time that Wilson and Stevens published their result, he was like not super sure about this sex chromosome thing. He was like, well, maybe that's not how it works. He still needed to be convinced at the time these results came out. But in 1906, there was a conference, and both Morgan and Wilson were invited to give talks about their theories of how sex is determined. So how you end up with individuals that make sperm and how you end up with individuals that make eggs.

But Stevens was not invited. And is Wilson already like a professor and Stevens is a grad student? Is that how it works? Wilson is already a professor. Stevens has finished her PhD. but she's in a part of her career where she's having trouble getting the professorship that she deserves. Right. So actually it will take five, maybe seven years for her to finally get offered a professorship.

So she gets offered a postdoc at Carnegie Institute, Washington, and then she gets to go back to Bryn Marr as a research associate, but she doesn't get a faculty position. A couple years later, they offer her that faculty position, but before she can accept it, she dies of breast cancer. Oh, gosh. All right. So before we dig into that tragedy, let's go back to the conference you were talking about

where her advisor Morgan and her sometimes friendly rival, Wilson, both get offered big talks to give their theories of sex determination, but she doesn't. And she's also the most junior person of the three, right? Right. And dying early is actually part of the story here. Oh, wow. Wilson and Morgan both go on to become even more well-known scientists. Morgan, in particular, ends up doing amazing work on fruit.

fruit flies attributing particular genes to particular traits in these fruit flies. And when Stevens dies, Morgan writes her obituary. Oh, wow. He writes her obituary in science, and he doesn't get all of it right. Oh, gosh. This is like a Morgan character assassination here. I feel like we need to have somebody from the Morgan estate on to rebut some of this, like equal time.

I mean, okay, so let's take a step back here. He was the advisor for a woman scientist, which was. very progressive at the time, I think. Maybe he wasn't being aggressive here, but let's dig in a little bit. So in the obituary, he says that Stevens confirmed McClung's hypothesis. And you might remember that McClung was the guy who was like, X makes males. And he got it exactly wrong. So she didn't confirm his hypothesis. She flipped it on its head. She flipped it on his head and then did a bunch of extra work to make a much more sound argument. And so here her contribution are getting sort of obscured. So she's already getting cut out of the conversation. Her contributions

are not being attributed correctly in her obituary. But Morgan must have known, right? I mean, obviously, he's like a big man in his field. He was her advisor. He's right there. He understands all this stuff. He must have known what he was doing. Is there any way to see this other than being an intentional obscuring of her name and deflating of her credit? I think that's probably what happened. Maybe we should get somebody who's like a Morgan expert on and maybe they can say like, oh, this was during a period where he was drinking too much and he was just sloppy and something like that. I don't know. Maybe there's a nicer way to look at it. But I had a PhD student and they died and I was writing their obituary. And especially if they were like one of the first women in their field,

I feel like I would be very careful about how I wrote that obituary. But so he either was sloppy or he was, you know, being dishonest. But there is a long history of PhD advisors taking credit for graduate student work, and especially when it's a male advisor and a female student. And we have lots of examples of like a man winning the Nobel Prize for work done by their female graduate student. So it's definitely not just biology. And at the time, I think it was very common, unfortunately. Right. And here's another piece of evidence that sort of builds on, you know, the argument that you just made. Morgan eventually writes a textbook called the Mechanisms of Genetics. And he talks about the work that Stevens and Wilson did, but he doesn't.

He doesn't mention, I don't think he mentions either of their names, but he definitely doesn't mention her name. Oh, man. And the way that he wrote it was right before he talked about the contributions that he made to the field. And when you read it, you could be forgiven for thinking that he was on that, you know, whole section just describing his contribution. Oh, man. Not a fan of Morgan over here. I'm feeling a little grumpy about Morgan, too. And in 1933, he goes on to get a Nobel Prize, not for the discovery of sex determination.

but just for his general work in genetics. And so over time, she's just sort of been forgotten. And so a lot of people forget about both Wilson and Stevens because Morgan became such a hot shot. And a lot of people read this textbook. And I think a lot of them just inferred, oh, okay, Morgan was the one who figured that out too. And so over time, Stevens has been forgotten.

And so has Wilson relative to Morgan. And Morgan has gotten a bunch of the credit. I think a lot of people don't appreciate how much campaigning and politicking. there is in getting a Nobel Prize. Like, it's not just you do a bunch of good science and then you're recognized. It's not some, like, pure meritocracy. You know, there's a lot of behind-the-scenes campaigning.

Like, this person should get the credit, and this person should get the credit, and this is really important. Like, the folks who get the Nobel Prize are the ones who bubble up to the top of that campaign. And every year, there's lots of arguments and discussion, and so wrangling your way into the textbooks and getting the story told a certain way

is definitely a good way to lay the groundwork for later getting the Nobel Prize. So I don't know that that's what Morgan was doing. And Nobel Prizes were definitely a newer thing back then. But, you know, writing yourself into the history of science is not a new game. Yeah, unfortunately. And I think often women aren't playing the game or aren't in a position where they can play the game the same way. I saw this happening like in person.

When we discovered that Higgs-Boson, we knew that somebody was going to get the Nobel Prize for it. And all of a sudden, there were lots of theorists giving talks about their contribution to Higgs-Boson. theory and the role they played in bringing it to the discovery because everybody knew it could be shared between three people. And Higgs was getting it, but who else was going to get it? Oh, man. So there's a lot of campaigning, a lot of talks on that topic of the history of those discoveries. And it was very clear what was happening. I think one day we should talk about your research in particular. But were you one of those people who could have campaigned? Were you around during the Higgs time? I mean, I was on one of the teams. But one question in the air was,

were they going to give it to theorists who came up with the idea or to experimentalists who discovered it or both? In the end, they gave it just to theorists, which is still sort of controversial. And if they had given it to experimentalists, probably they would have only given it to the heads of the experiments because the experiments have 5,000 people on it. And you can't give them all the Nobel Prize. That might be one reason why they only gave it to the theorists, because how do you decide which experimentalists to give it to? sort of a frock question. But yeah, I contributed. I was on those papers.

I was there. Oh, that's exciting, though. I don't think I deserve a Nobel Prize from my contribution. I'm not going to be campaigning for that. But it was fascinating to see the human side of it, to see people actively out there arguing that they deserve the Nobel Prize. And I know that that was influential. Okay.

Yeah. That is an interesting insight. I don't think I had on my radar. Yeah. Okay. That's sort of like the history there. But one of the things that I wanted to talk about briefly is how different this story could have been

if it wasn't the case that why chromosomes are a different shape. And so maybe you would have ended up with the same thing because there were some males who just weren't getting anything. You know, they weren't getting like a paired chromosome at all and that's how they became males. But why chromosomes, it turns out over time,

they tend to degrade. And so the idea is that most chromosomes often swap material at the beginning of meiosis. And this process of swapping allows you to get rid of genes that have kind of gotten messed up or they've gotten some bad mutations. But if you have this part of a chromosome that's now controlling, you know, for example, say it has the code for making testes and for a beard and for a vast deference, you don't want to be swapping that and risk getting those male-related genes onto the female-related

chromosome. So they stop doing that recombination. But that means that anything else that's also on that chromosome that doesn't have anything to do with mailness is now not getting sort of like rejuvenated or removed by this crossing over process. And so over time, those genes as they get messed up, we think they just sort of like, they fall off because you're not using them anymore. And every once in a while, chromosomes sort of lose bits. And sometimes that's a problem. Sometimes it's not.

But over time, the Y chromosome tends to degenerate. And you also see this in birds. It's a different chromosome. the males have Z and Z, and the females have Z and W. And that W chromosome is the one that has started to degenerate. So it's also started to lose things. So this observation, like it seems to me that the reason we started with sex was because of this weird way that these chromosomes sort of change shape over time. And if you hadn't had that clue that these things are different shapes, it would have been much harder to make this observation.

And I don't know, that just seemed interesting to me while I was going through this research. Yeah, because it's sort of macroscopy. topic. I mean, you're zooming in, you're using the microscope, but you don't need to look at the actual genome and decode it and, like, see the structure of DNA to see that there's something different between the X and the Y, even just under a microscope. And the other thing that's kind of amazing is that they were able to figure anything out, because the way that whether or not you get an individual that produces sperm or eggs or produces both, if they're hermaphrodites, like the various mechanisms through which those things come about are super varied. Like reptiles,

it does depend on temperature. And then in the birds, they've got like the opposite, like we just talked about. The females are the ones that have the different kinds of chromosomes. And you get some animals that are sequential hermaphrodites. They've got all the genetic code that you need to make either decision and they can switch between what sets of code they're using. And while I was reading about sex determination stuff, there were papers that had titles like sex chromosome evolution.

So many exceptions to the rules. It's like there are just so many. many different ways that this all works out. It's kind of amazing we were able to get a foothold, but we did. Yeah, that's incredible. Everything is messy and it's incredible you can tell any story. What do we know about the history of this? You're talking about how over time these degrades. What is the history of the evolution of sex in biology? Like, do we know when things went from asexual to having two genders? So I think that there were a number of different transitions and I am absolutely not an expert in this.

This would be a whole topic on its own, I think. But from my reading, what I think I was picking up was that often when you end up with a transition and you end up with offspring that either just have eggs or just make sperm, that usually starts from a hermaphroditic ancestor. So you have the code to do both. And then in some individuals, the set of codes for male traits is turned off.

And for some individuals, the set of codes for female traits gets turned off. And you end up producing, individuals that can no longer produce both sperm and eggs, they produce only sperm or eggs. And why that would be beneficial to lose that ability to do both is maybe a little bit unclear. I think one of the leading hypotheses is that there's often not a benefit to reproducing with yourself. So hermaphrodites can often mix their own sperm and eggs to produce offspring. But in an evolutionary sense, the extra genetic diversity that you get by mating with individuals other than yourself has a huge payoff in terms of being able to stay ahead of parasitic infections and

stuff. Yeah. And so I think there's selective pressure for forcing an individual to mate with other individuals, whereas if you're hermaphroditic, maybe you're like, oh, it's easier to just mate with me. We've all been there. I'm going to stay in. It's Friday night and I'm tired. That's right. That's right. And that's where the show got not appropriate for kids, maybe. Well, I think about that because we talk about earliest common ancestor and how all life on earth probably has a single ancestor. But, you know, we have gender along the line at some point and we didn't have it early on, which means that at some point it must have, you know, sprung up. And so it's fascinating to me to think about the history of this and where it diverges. And maybe one day we'll have the whole tree of life all the way back through time and we'll know the answers to all these questions.

Bacteria started with like conjugation where they'd just be like passing over small chunks of genes. And so there's been a lot of diversity along the way. And without going into too long of a diatribe about definitions, because to be honest, I'd like to check my definitions before saying them publicly, you know, gender, I think, is often now thought of as being different than just, do you make sperm or do you make eggs? It's more about the strategy that you employ as you go through life. And so that's a slightly different thing, just to get our terms correct.

And so today we were specifically talking about comedic sex. So do you end up with the genetic code to make sperm, or do you end up with the genetic code to make eggs? All right. So finish the story of Nettie Stevens for us. Tell us about how she tragically passes away. Yeah. So she finally gets offered that professorship at Bryn Mard, which she quite clearly earned

because two years before she got this offer, you know, as we mentioned, she was listed in the 1,000 top men of science. So she was, again, one of 18 women. So she was clearly globally known for the work that she was doing. And she had won an award for her work on sex chromosomes. So I think everybody realized this. that she was making big contributions. But by the time she was ready to start that professorship, she died at age 50 of breast

cancer. Oh. And so who know? I mean, she still had like a prolific publication record up until that point. I think she had published something like 40 papers. And that was like, you know, what? She got her Ph.D. in 1903. Wow.

And so in maybe a 15-year period, if you include when she was working on her master, she managed to really knock out a bunch of papers, which to me is incredible, especially when you consider the tide that she was working against at the time. This is like a Mary Curie type story. Could the work she was doing somehow have contributed to her breast cancer? Were she using like really dangerous chemicals and this kind of stuff in the lab to do her studies? Or is this just random and bad luck? Oh, that's interesting.

I didn't come across anyone who was suggesting that the work is what ended up killing her. She was using a lot of different kinds of stains. And so stains make it easier to see the chromosomes as they form and as they separate. I don't know if any of those stains were also carcinogenic, so we're also likely to cause cancer. It's possible, but I don't know. As long as we're just listing possibilities, you know, maybe Morgan somehow poisoned her. Let's further besmirch his character anyway.

That's right. I wonder if we're going to get angry emails. But, you know, we'll invite them on the show and they can defend Morgan if they'd like. That would be interesting. Exactly. If you are Thomas Morgan the fourth or whatever and you were grumpy at hearing your great, great-grandfather's character besmirched right in and we will clear the air but smirched is a great word that's not used nearly as often as it should be it feels sort of very 1900s i thought it was

era appropriate absolutely era appropriate so anyway reading about nettie made me like totally appreciate i have had difficult sort of steps in my journey i did end up leaving like a master's lab because of sexual harassment and so like it you know hasn't been a hundred percent easy but relative to what she had to go through, then I've had an easy path. Anyway, it's just kind of a bummer to hear about a woman who was doing amazing work. And, you know, I think it would be reasonable to say that she and Wilson should both jointly be known for this incredible discovery. But, you know, we all think of it as Morgan's, which is just a bummer on a lot of levels. So there you go. Here we are, pushing gently back against Morgan's politicking to get himself in the top men of

science. There we go. Well, he got a Nobel Prize, so I think he won and we're a little too late. But hopefully y'all will remember the name Nettie Stevens, and we'll go ahead and share this story with other people, and Nettie will get the credit she deserves. Congrats Nettie on all of your discoveries. Way to go, Nettie. Daniel and Kelly's Extraordinary Universe is produced by IHeart Radio. We would love to hear from you.

We really would. We want to know what questions you have about this extraordinary universe. We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you. We really mean it. We answer every message. Email us at questions at danielandkelly.org. Or you can find us on social media.

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Boost productivity, all on one device. December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys. Then, everything changed. There's been a bombing at the TWA terminal. Just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, terrorism. Listen to the new season of Law and Order Criminal Justice System

On the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Wait a minute, Sam. Maybe her boyfriend's just looking for extra credit. Well, Dakota, luckily, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend's been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now he's insisting we get to know each other, but I just want or gone. Hold up. Isn't that against school policy? That seems inappropriate.

Maybe find out how it ends by listening to the OK Storytime podcast and the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. The U.S. Open is here. And on my podcast, Good Game with Sarah Spain. I'm breaking down the players, the predictions, the pressure. And of course, the honey deuses, the signature cocktail of the U.S. Open. The U.S. Open has gotten to be a very wonderfully experiential sporting event. To hear this and more, listen to Good Game with Sarah Spain, an IHeart women's sports production in partnership with deep blue sports and entertainment on the IHeart radio app, Apple Podcasts, or wherever you get your podcasts. Brought to you by Novartis, founding partner of IHeart Women's Sports Network.

This is an IHeart podcast.