Daniel and Kelly’s Extraordinary Universe - Listener Questions #38

Episode Date: May 12, 2026

Daniel and Kelly answer questions about aliens watching Earth, about lone species, and about the size limits of intelligence.See omnystudio.com/listener for privacy information.

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Listen to Superhuman on the I Heart Radio app, Apple Podcasts, or wherever you get your podcasts. Life is full of hurdles, so how do you keep going? On Hurtle with Emily Abadi, we're talking with the most inspiring women in sports and wellness from professional athletes, coaches, and Olympic champions about the challenges that shape them and the mindset that keeps them moving forward. At our level, at this scale, being able to fail in front of the entire world, like, I can do anything. I can do anything. Listen to Hurtle with Emily Abadi on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Presented by Capital One, founding partner of IHeart Women's Sports. If telescopes looked back from afar, what signs of Earth would glow? Would life in air and

seas be something it could know? As sole survivors in the genus Homo, we've taken the throne. But are there other genera where a species is similar? alone? From the tiniest speck to giants tall, how small or big can thinkers be at all? Hmm. What do I wait? What do I say next? What was that sound? Blah, blah, blah, blah, blah, blah. Whatever questions keep you up at night, Daniel and Kelly's answers will make it right. Welcome to Daniel and Kelly's extraordinary universe listener questions number 38. I study parasites and space, and it's an exciting week to be someone who studies space. Do you know why, Daniel?

Hi, I'm Daniel. I study aliens and physics. And yes, I'm aware of the Artemis launch because I do not live underground. Okay, but so here's the thing, right? Okay, so, you know, of course I was watching the Artemis launch, and I was watching it when my daughter got off the bus and got home from school. And she came home and she's like, what are you watching? And I was like, I'm watching the humans that are going back to the moon. And she goes, what? And so I don't, you know, I haven't. She didn't know.

No. And so I don't think that the kids are like talking about this. Like I don't think that like this generation is very excited about the fact that we're sending people back to the moon. What's your sense? That's a great question. And, you know, I just realize now that I don't think I've discussed it with my kids at all. I just sort of watched it myself. and I have no idea if they know anything about it.

Hold on, let's find out. Okay. Isn't she supposed to be at school right now? Hi, Hazel. Did you know that we just sent astronauts back to the moon? Yeah, Artemis too, right? You know about Artemis? Wait, am I on the podcast?

Yes, live. And do you think it's cool or whatever? I think it's interesting, but I probably wouldn't spend very much ten thinking about it. Cool. All right, thank you. Have a good day. Okay, thanks. Bye.

Oh, that's a good disappointing answer. A little bit. Yeah, for all the money that was spent on it. Yeah, I mean, Ada was excited. And then we, so, you know, we watched, she sat with me. We watched the launch. And then we sat for a while and we were watching the, like, live feed. And she was like, well, when do we get to see, like, the astronauts?

And I was like, I guess they're not going to show you the inside. So, like, you know, they were doing, they've got to go around Earth a couple times before they leave, you know, the orbits of Earth to head to the moon. And I guess they decided they weren't going to show the astronauts on the inside because they were doing multiple tasks or whatever. But Ada was really bummed. She's like, well, I want to see what they look like. What it's like for them.

And imagine herself in that ship probably. Yeah, yeah. And so not only had she not heard about it, but then she really didn't get to see the people at all. And the next day she didn't ask, is there any news about the astronauts? And I know they're not landing, but I think the kids are all.

already sort of not super excited about it. Well, let me ask you what aspect of it is very cool for you, which part is getting you excited? Well, so I'm hoping that we will start doing more consistent research. Like we'll set up a research station and then we will have like long term research programs on the moon. So that we can learn about like life and low gravity and space and radiation and all the kind of stuff you guys recommended in your book that we need to do to be serious about settling space. Yeah. I'm hoping we do all of that. that stuff now. But, you know, that would be really expensive. And so that would require us to do something very different than what we did last time, which was just go to beat the Soviets and then

go home because we had done it. And so, you know, if we're just going to show the Chinese that we can do it again, just to beat them before they try to do it, and then we stop, then it will be sort of anticlimactic and like, oh, all right, we spent a bunch of money to prove we could do it again. And now we're going to stop. But I don't know. What about you? Are you excited? And if so, why. Well, that's just so, like, thoughtful and future thinking, is there a part of you that just feels like a visceral excitement, like, big rocket go boom, you know, or like, people are in space, you know, there's just the immediacy of that. Isn't that cool? That is cool. I'm glad the big rocket didn't go boom. That's what I was, I was worried about that. I wanted big rocket not go boom.

But, no, I mean, I thought that was cool. But I also think it's cool just when we get a satellite into space without the rocket going boom. Like, it's very easy to get me excited about space stuff. But yeah, yeah, I think it's cool that we've got humans going to the moon again. I'm always excited about, like, things go into space. That's very cool. Every Falcon launch or whatever, I think it's pretty fun. This mission in particular, I'm like not sure what we're learning or what we're doing.

We haven't done before. So it's also exciting in the, like, sense that things are starting again. We're exploring. We're working towards something in the future. So a lot of the excitement for me also comes. from the promise of what this might mean. If we go to the moon, touch our feet down on the moon, and we don't stay this time,

I will be bummed that we didn't spend that money on things like an exploratory mission to Europa or something like that. Like we could have used robots to explore other stuff and gotten more data. And like, because, you know, robots can do more than humans can. And so if we spent all this money to get humans to the moon and then we don't figure out a way to stay there,

I'll be bummed. Anyway, hopefully this time we'll stay and we'll do more research and get the data. But anyway, we'll have to wait and see. But I'm glad the rocket did not go boom. Yep. And good luck to all the astronauts. Yes, right. And, oh, yeah.

And, you know, I am excited that we have a woman in space. We have a black man in space. And we have a Canadian in space. And so those are all exciting things to me. So I am excited about those firsts. I mean, there have been Canadians in space before, but, you know, not to the moon, you know. So, hooray. And I'm excited to be answering questions from our listeners today because one of the exciting things about space is the sense of exploration, pushing back on what we don't know.

And that's exactly what we're trying to do here on the podcast, is to dive deep into your curiosity and push back what you don't understand to broaden your mental picture of the universe. Friends, if Daniel wasn't here, we would never get to the point. So thank goodness we have Daniel to keep us on track. And so let's go ahead and get to Jonas's question. about space telescopes. Hi, Daniel. Hi, Kelly. My question came to mind some time ago,

back when the James Webb Telescope was a big topic in the media and in podcasts like yours. If it were pointed at Earth from the distance of the nearest Earth-like exoplanet, what could it actually detect? How would Earth's appearance be shaped by its physical properties?

And could any biological signatures such as signs of life in the atmosphere realistically be inferred? Many thanks for exploring this question. This is a fantastic question that I don't know the answer to. It's a very cool question because he's putting himself in the minds of aliens and wondering if there are aliens out there that are similar to us, could they see us, right? It's a great question because obviously we haven't seen any aliens yet. And so he's wondering if the reverse is true.

Maybe aliens will discover us before we discover them. but it's also a good way to understand what we could possibly see with our technology. Yes, and so of course you would love someone who is thinking like an alien or thinking about aliens. And so let's go ahead and talk about what telescopes can see. Yeah, I think there's a lot of misconception about what telescopes actually do. And people imagine that really powerful telescopes could maybe like show you what's happening on the surface of an exoplanet or something. But telescopes are really good at seeing distant, dim, huge things. They're not very good at seeing small things that are far away, which is what Jonas is

hoping that they could see. And the reason is a little bit of physics, which is important to understand telescopes, which is the diffraction limit. And so you're saying Earth would be a small thing. So what would be a big thing? Like the sun? Or are we talking about like entire solar systems or entire galaxies? What counts as big? Yeah, galaxies are big. And that's why when we point James Webb telescope into the distant cosmos, the kind of things we're seeing. are entire galaxies because they're big, right? They take up enough of the sky that we can resolve them. We can see them.

Back to diffraction. I got you off track. That's what I do. You get us back on. Okay. Essentially, there's a minimum pixel size in the sky that telescopes can resolve. And the reason is that light at this scale acts like a wave. If light was just made of tiny little geometrical particles, little balls with perfect resolution,

then you wouldn't have this effect. But when light enters the telescope, it goes through an aperture, right? This is the opening in the telescope that gathers the light. And any time a wave goes through a hole or through any sort of opening, you can model it like a series of sources across that opening. This is sort of like the granddaddy version of interference, right? If you have, for example, two sources of light, then in some places those two sources of light will line up to be brighter. and in other places they will light up to cancel out, right? Because waves go up and down and up and down.

And if the up from one wave hits the down from the other wave, they cancel out. And if the up hits the up, then they add to each other, right? So that's interference. So if you have two sources of light, then you get like an interference pattern across a screen, right? Well, when light comes to an opening, you can model that as a bunch of sources across the opening. Okay. And so some will interfere? And so exactly, you get interference between,

all of those. And so you can't have an opening, you can't have an aperture, you can't gather light without getting some interference. It's impossible to avoid interference. This is the process we call diffraction. Anytime light passes through an opening, you get interference among itself because the light that comes out of the opening is just like as if you had a series of sources across that opening. Okay. And so any optical system, even if you have perfect optics with no aberration or anything like that, you're going to get interference because you have an aperture. And that causes fuzziness, right? And so essentially, you can't resolve things that are really, really small. And the size to the effective resolution, the size of a pixel on the sky depends on the size of your aperture

and the wavelength of light that you're gathering. And so for various wavelengths and various sizes of your telescope, you can resolve a pixel in the sky of various sizes. So the human pupil has an aperture that's like a few millimeters across. And so its diffraction limited resolution for like visible light is like 10 or 20 arc seconds. And Hubble is like two and a half meters across. And so its diffraction limited resolution is like 0.1 arc seconds, right? So much, much smaller. So the pixels you get with Hubble are much smaller in the sky than the pixels you get from the naked eye

because it has a bigger aperture. And James Webb is like six and a half meters wide, so it's even better. But still, these pixels are too big to see, like, the surface of an exoplanet. You're never going to, like, spy on an alien having dinner on an exoplanet because the size of that alien dinner in our sky is much, much smaller than anything we can resolve. So what these telescopes are good at is accumulating a bunch of photons from, dim things that are huge, big enough to see in our sky, but too dim to make out with a naked eye. So you can just point the telescope at them for a long time. And then you can gather enough photons so that it then appears.

A great example is like Andromeda. Indromeda is really big. It's a huge galaxy. It's actually quite large in the sky. It's bigger than a full moon. But you can't see it with a naked eye because it's too dim. But you point a telescope at it and gather it for a while, you can get a really nice picture of Andromeda, because it's big, much bigger than diffraction limited resolution than just too dim for your eye to make out so your telescope can accumulate enough photons to get an image of it. Okay.

So the good news is that the telescopes save us from ourselves. We can't be creepy and spy on the aliens in great detail. That's not good news. No, we want to be creepy and spy on the aliens. Oh, okay, okay. But what can we learn about the aliens? Can we learn if they are there or not? Like, can we detect life or not?

So currently with like JWST, a distant Earth from a nearby exoplanet would just be a pixel, right? Okay. You'd just see like one pixel. Now with future telescopes, maybe we could see more, right? But to have a resolution like spy on the surface of the planet, you'd need an aperture like the size of the sun. Wow. All right. We're talking about a telescope.

size of the sun. And that's possible. You can actually use the sun and its gravity to bend light and focus it into a collector so you could use the whole solar system as a telescope. That's like really an idea we could pull off one day. And you could use that to look at aliens having dinner. But currently, that whole alien planet would be a pixel. That doesn't mean that you couldn't learn anything about it, right? It would be a super interesting pixel because if you think, oh, it's just one pixel is just one piece of information, but there's a lot of information in two different dimensions there. There's how the pixel varies with time and also how the brightness of the pixel varies versus wavelength, right? One pixel means contributions from lots of different

frequencies of light. Is it red? Is it blue? Are there greens in there? We can tell a lot from how the light varies with time and how the light varies with frequency. Okay. What can we tell? So Earth emits in two major ways. Number one is it glows just because it has a temperature, right? Like Earth is pretty cool and everything in the universe that has a temperature glows. And so you can tell the temperature of Earth by the frequency at which it glows. Like you take that pixel and you pass it through a prism and you can see which colors of light come out. And from the various frequencies of light, you can see the black body radiation of Earth.

And you can use that to deduce the surface temperature of Earth. from a distance. Cool. Right? That's very cool. And so you can tell, oh, the earth is approximately what we call room temperature. Or maybe it's not. Maybe it's covered in magma.

It's super hot. Or it's an ice ball, right? So you can learn something about the temperature of the earth. The earth also reflects a lot of sunlight. So it's two major emissions. There's black body radiation from the temperature of the earth. And there's reflection of sunlight. The reflection mostly happens when you have oceans and when you have clouds.

So you can tell. hey, is it cloudy on that planet? Or is it a water planet? Right? So you can get a sense for the cloud cover. You can get a sense for the water cover of this planet, just from this one pixel, right? And the reason we've developed all these techniques is because nobody's satisfied just seeing a pixel. They want to know more.

And this is what scientists do, right? They learn how to extract as much information about the universe as possible from the day to they got. They're not just going to say, like, well, let's just wait 30 years and build that sun-sized telescope. like, no, we're here today. We want answer now, right? I love the ingenuity of scientists to extract this information from whatever tiny data they have. Totally, yeah, because you're not getting funding for that Sun Telescope, probably.

Kelly, the wet blanket is here to say. But so, okay, so if you know temperature and that there's water, you can maybe guess if there could be some sort of alien. Maybe it's alien bacteria, but now you've got, like, a hunch about whether or not those aliens, dinner? You can do even better than that. You can learn what the atmosphere is made out of and if there are biomarkers in the atmosphere, right? So again, we just still have one pixel, but we can do clever things like wait for sunrise on the planet. During sunrise, that alien star shines through the atmosphere of the planet and then those photons come to Earth. And because those photons have gone through the atmosphere and atmospheres tend to absorb at certain frequencies based on their

composition, we can tell what's in the atmosphere. So there will be absorption lines in that spectrum. There will be gaps that tell you what molecules are in the atmosphere because those molecules ate some photons. And by seeing which ones they gobbled, we can tell what's there. So like, is there water in the atmosphere? Is there ozone in the atmosphere? Is there methane? Is there CO2? Wow. Some of these things are very strong biomarkers because they're in chemical disequilibrium. Like, for example, oxygen is produced by photosynthesis, methane typically produced by biology, though there are geological sources.

But if you have them both in the atmosphere, they tend to react and disappear. So if you see oxygen and methane in the atmosphere of an exoplanet, that means that there is a constant replenishment of those things. is something on that planet making oxygen, making methane. Neither of these things are smoking gun proof of alien life, but they're like, you know, good, strong hints. Something out there is pumping out oxygen. So that's the kind of thing you can see.

Again, just from this pixel, right? Super cool. I was reading a really cool paper yesterday about how you can watch the transit, the eclipse, to learn more about the atmosphere. Remember that sometimes the way you discover these exoplanets is by seeing them cross in front of their sun and dim the light of the sun just a little bit, right?

But the cool thing is that you can look at that dimming in various frequencies, right? You can say, well, does red light dim? Does blue light dim? Does green light dim? And that's another way to tell what's in the atmosphere because the width of the eclipse and the depth of the eclipse

will depend on what's in the atmosphere. So, for example, if you're watching Earth, pass in front of the sun and you're looking at a frequency that nitrogen likes to absorb, then the earth is going to seem wider because now you're including the atmosphere. And in other frequencies where nitrogen is transparent and the earth is opaque, the earth is going to seem smaller, right? So it's like, is the eclipse including the atmosphere or not? Is a frequency dependent answer?

So by looking at different frequencies, you can help determine, oh, what's this atmosphere mostly made out of? really clever stuff. All right, yeah, you've got me. I'm amazed. All of this from a pixel, that's pretty impressive. We're not even done yet, right? Now we've got to think about time variation because as time goes on, the Earth is spinning.

So the sun reflecting off the Earth will change based on where are the clouds, where are the oceans? And somebody actually did a study answering this exact question. They said, if we were in the next star system and we looked at Earth and we watched the variation of the reflection of sunlight off of the earth, could we make a map of the earth? And so you can look this up, it's amazing. It roughly maps out what the earth's continents look like. I mean, it's very, very pixelated.

It's basically just like bands. But it shows you, oh, there's a bunch of land, then there's a really big ocean, then there's another little bit of land, then there's a smaller ocean. So you see the Pacific, you see the Atlantic, you see the Americas, you see Eurasia and Africa.

Like, again, not in great resolution, but much more than zero information. Super duper cool. It's very cool. That's amazing. Yeah, so cool. And that would be with the kind of stuff that James Webb Space Telescope is seeing. You can get all that information.

Exactly. Wow. But James Webb Space Telescope is not the best way to do this, right? What you want to do when you're looking for exoplanets is you want to block the light from the sun because these planets are like two billion times fainter than the star. So it's like you're looking at a firefly next to a street lamp, you know, across the country. So it's very, very hard to do. And the right way to do that is to block the light from the street lamp.

You can't just put like a circular shade to block the starlight because there's this weird interference effect where you end up having a bright dot at the center of that circle. We talked about this once when we were talking about the history of discoveries, a really cool story about the Poisson spot. Anyway, they have a new generation of space telescopes they're building. that have a complicated, shaped star shield to avoid this sort of interference effect. And those are going to be awesome at seeing these exoplanets. Really, we need a dedicated device with sensitivity at the right frequency and with one of these star shields. So, you know, in the next couple of decades, we're going to be seeing a lot more information about these exoplanets.

Yay! All right. Well, that was an amazing answer. Let's go ahead and see what Jonas has to say. Hi, Daniel. Hi, Kelly. Thank you so much for having my... question on the podcast. Your answer hit exactly what I was going for. Intuitively, I kind of knew

that the telescope would see very little, almost nothing really. But somehow that's where my mind went. Not what does it see, but what can we actually read from a tiny spot on the screen? And the answer blew me away. Just one pixel, and yet we can read so much from it. Hooray, science! And okay. Deep down, of course, I would. was wondering as well, how realistic is it that we actually find signs of life with the James Webb telescope. Thanks for the great discussion. Experience Harry Styles live in London, England at Wembley Stadium. This is Harry Styles. I.R. Radio wants to send you and a mate across the pond with flights from Virgin Atlantic, hotel from TripCentral.ca, tickets, and $1,000 cash.

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Apple Podcasts, or wherever you get your podcasts. Presented by Capital One, founding partner of IHart Women's Sports. All right, we're back and we're answering questions from listeners. And here's a question from a a regular listener who's got a really wonderful, charming accent. Hi, Kelly and Daniel. This is Jane from Redcar, England. I've got a question for you. Homo sapiens is the only species in the genus Homo, unless Bigfoot is real. Are there any other species that are similarly alone? What animal genus are the most species in it? And what influences whether you end up with one are a thousand? And does it have a lot? And does it have a lot of? And does it have anything to do with parasites. Still loving all that you do. Thank you so much for answering my

question. Have an extraordinary 2026. All right, Kelly, tell us whether we're alone and whether we should feel bad about murdering all of our cousins. Well, you know, we didn't just murder them. We slept with them and we had their children. And then, well, we don't, we're not sure we ate them. and, you know, check out a past listener questions episode for more information. And, you know, in fact, we had, on November 25th, 2025, we had an episode called How Long Have We Been Human, where we had Scott Solomon on. And so it's worth noting that, yes, the only species of homo that's around right now is us, but there were a lot of other homo species in the past, maybe not a lot, but there were others.

And they've just kind of died out for a variety of reasons. Have you ever wondered what it would be like today if we had several species and they were all like intelligent, but they had like, you know, different characteristics, different kinds of intelligence and different kinds of strength? Would that be wonderful and amazing or like divisive and political? I want to believe it would be wonderful and amazing, but I am a bit of a cynic and I'm a little bit scared of how our species would handle that. What do you think? Well, I think the fact that we killed them off before written history already tells us the answer. Yeah. We're not very tolerant of differences. Yeah. No. Okay. Anyway, that's a true downer right there. But anyway, okay. So Jane wanted to know about other examples. And so there's a couple fun ones here. So the platypus is another species that is the only member of its genus. Daniel doesn't like me saying Latin names. So I'm going to try to, I'll avoid them. Red pandas are the only species in their genus. And in fact, they're also the only.

species in their family. But give us a reminder, species, genus, family. I mean, I know as a professional biologist, this is just like back of the hand stuff for you. Why are you laughing? What are you joking about here? You know in my notes that I wrote this all down

because I either have to say the mnemonic out loud and the only one I memorized is dirty. And so I will not be saying it on the show. So I wrote it. So yeah, you got kingdom, then phylum, then class. order, family, genus, then species. And I've always wondered, like, why these divisions, are these arbitrary dotted lines humans have drawn, or could you have 19 or four?

Like, why this number of categories? Yeah, that is a good question. We were going to get into that a little bit later. Let's get into that now, though. And so, all right. Yeah, so to some extent, it probably is arbitrary. And so, like, the classification system was started by Linnaeus. in like, I think the 1600s.

And this was long before we knew about, you know, evolution, long before the theory of natural selection came along. And so this was before we had, like, you know, evolutionary trees and phylogenetic trees. And so to some extent, it must have been arbitrary because we didn't have these, like, connections in our mind. And we didn't know, you know, about one species coming from another. But there's some reason to believe that they're useful in, like, thinking about evolutionary questions and that they are real.

But so I read a paper that, at least for the genus level, they were trying to decide, like, is what we're seeing in terms of categories of genera real? And so what they did was they looked at real rates at which we think species are being made and real rates at which we think species are going extinct. And then they sort of ran some models and they said, okay, if we're right about calculating some of this stuff, then how often should you see genera popping up? How many species should you see in each genus? But do we even have a definition for genus?

Like I know definition of a species, it exists, even if it's controversial and fuzzy. Is the definition of a genus have some basis in biology, or is it just like, I'm putting this here and that there? Yeah, so different types of taxonomists sometimes have different... And then we have taxonomy of taxonomists now. Yes, yeah, yeah. So a paper I was reading was lamenting that different kinds of taxonomists sometimes have different criteria for when they identify something as a genus or not. Wonderful. And so they were arguing that we needed to try to decide, is it possible to have a category of genus that is the same for everybody?

And if we don't have that, how big of a problem is it? And so, yes, to some extent, some of the stuff that we're talking about today is going to be arbitrary because this is humans' best attempt at categorizing nature that does not care about us and our attempts to categorize things. And if you look back at this stuff like 200 years from now, you might find that genuses have been split or genuses have been clumped together because we've decided these things are actually like more closely related. We just hadn't looked at the genetics before. We hadn't had a chance to get around to it. I see. So in some sense, to answer the question depends on arbitrary choices made by taxonomists hundreds of years ago. But in another sense, there must be something about like the platypus, which makes it different from other species.

It's not just that it got randomly categorized into its own genus. It's like more different from all the other species than most species, right? There's like a larger phenotypical distance in some sense. Yeah, right. So you definitely shouldn't come away from this conversation thinking this is like totally arbitrary and random. You know, like there's many, many people working on this stuff. We now have a lot of genetic data. A lot of people have spent time looking at, you know, the internal organs of these species, the external features of these species, the, you know, genetic information from these species.

We have a lot of data that we're bringing to bear on this. And in a month or two, we're going to have Scott Egan on the show talking about how we define species. So we'll have a lot more information on that coming soon. But like people have collected a ton of data, thought very, very. carefully about this stuff. And a lot of the relationships between organisms, we are like nearly certain are true. But the question is just when do you say, okay, we're stopping here and we're putting a name on what's happening right here. You know what I mean? Like it's definitely, these relationships are flowing and we sort of know in the directions in which they're flowing.

The question is just when do you stop and say, what's happening right here gets a name? Right. Okay. And so like the planet poses a famous example. because it really isn't anything else sort of near it in the phenotypical tree of life, right? It really is sort of kind of on its own. Yeah, it looks real weird. And the red pandas, I'm confused about that because there are other pandas. Panda is an unfortunate choice of a name. Actually, I can't remember if the, like what most of us think of as pandas, like the big black and white bears, if they were named first or if red pandas were named first.

But they're not, you know, same way starfish aren't really fish. both of those pandas are not closely related. I love that the Latin name here means fire-colored cat. Yes. What? Yes. Yes. And it's a good name, but we let French zoologist George Cuvier name it.

But the locals at the time, who of course already knew about it long before Europeans went in and we're like, this is new. They called it wa or chitwa based on its vocalization. So that's probably what we should have called it. But anyway, we call it the Red Panda. And this thing is not just alone in its own genus, right? Right. It's alone in its family, which is one category up. So family genus species.

Pretty cool. Wow, pretty. It really killed all of its cousins, huh? It did have some. I think it had some past relatives that died. I can't imagine the sweet little red panda killing off its relatives the way we did, or the way we may have. But a fire cat could have done that. That's true.

I met a red panda once and it had arthritis and moved very slowly. So they don't seem vicious to me. But she had a very long, nice life in a zoo. Ardvarks are another member that's alone in their genus, but this one's also alone in its family, like the red pandas, and it's alone in its order, which is one level above family. So order family genus species, Ardvarks are alone in there. They had other relatives, but they died off. So there's some examples. but actually, as a more general case, I found a couple different estimates here, somewhere between 30 to 40% based on the numbers that I found, of genera of animals have only one species in them.

So there's a lot of examples. I just picked some cute ones. Here's a Latin question for you. Genera is plural of genus? Yes. Yeah, apparently I looked this up because I didn't want to get it wrong. Apparently genus is acceptable, but genera is what is plural of genus. preferred. I see. Well, I like genesis because it sounds like geniuses.

Yes. Yeah. And then maybe people will like accidentally transfer that idea to us. And they'll be like, oh, Daniel and Kelly are smart because Kelly said genius genius. I tripped on it. That's ironic. All right. So one third of all animal genera have one species in them. What does that mean? What does that tell us about the history of evolution or anything? A lot of dead stuff out there. It could mean a lot of things. So it could mean that there's a lot of new, like, lineages out there. So things just haven't had a chance to split yet. It could mean that there's a lot of extinction out there. And there were groups out there that had a lot of relatives, and then a lot of their relatives died and just one survived. Like us, you know, there were a bunch of other homo species, but only we survived.

And that's sort of an interesting question is what is it that determines who the survivors are? Yeah. But, yeah, it could be either of those things or. some other things. Does that suggest like a history of bottlenecks where, you know, diversity is crucial and only one species survives because they're different? I don't know if that necessarily suggests a bottleneck. I don't, gosh, that's the first time I've heard the word bottleneck applied to groups of

species instead of like a population. Yeah, I don't know. Yeah, I guess maybe you've got something catastrophic that happens and it knocks out a bunch of different species but one. Is that what you're saying? Yeah. Yeah, that would be worth thinking more about. Yeah, I don't know.

So I don't know if it's usually like the same catastrophe that wipes out a bunch of different species but one. Like it could be, you know, habitat loss wipes out this homo species and parasite wipes out another homo species or something. And it's not like the same thing. But that's a good question. I don't know the answer. Well, in the case of some of these animals, do we have like fossils of other critters in the same genus that are no longer around? So we know that it used to be broader? Yes. Yeah, for a lot of these we do.

Oh, cool. Yeah, yeah. For a lot of the examples that we went through earlier, we do. So they were not alone in the past. So another question, that bottleneck question was great. I'm going to be, that's like going to be a keeping me up at night question. Uh-oh. Okay, so then Jane's other question was, what about genera with lots of species in them? And there are some mega-diverse genera.

Really? Yes. And so for plants, there are some mega-diverse genera. So, for example, salonum, this is the genus that includes like potatoes, tomatoes, tomatoes, eggplants, thank goodness for salonum. It is a delicious genus. I think it's a genius genus. Yeah. Me as well. And I think the genus, the, gosh, the genius, genus homo has done amazing things with salonum.

If only I could have said that fast. Anyway, all right. Another very diverse genus is a begonia. You may have heard of begonias. They're kind of flower. There's a lot of them. For animals, because that is specifically what Jane was asking about. There are two beetle genera that are very diverse.

Of course, the Beatles. The Beatles. One that you might have heard of is the Emerald Ashboreer, which is causing a bunch of trouble in the United States right now. And Stennis, these are semi-aquatic rove beetles. So just another kind of. beetle. Just another kind of beetle. Wow. Just another kind of miss amazing biodiversity and evolutionary genius. Just like just another kind of beetle. I know the coliopter and people are shaking their fists at me. I'm sorry. I love beetles. They are wonderful. They are wonderful. In terms of vertebrates,

the pristamantis, there's over 600 species of these. These are absolutely super cute little frogs. And then there are some genera of geckos that also have a lot of species. And Anolis lizards have a lot of species too. So when do you get a genus that has a lot of species? And Daniel, don't look at the outline. Look at me. What do you think the answer is? When do you get a genus that has a lot of species?

It's a Kelly answer. This is D-K-E-U. What do you think the answer is? It depends. We don't know. Yes. That's right. Exactly.

The answer is it depends. And so I found a paper that said for birds, it seems to be that you get a genus that has a ton of diversity when the birds disperse annually and are feeding generalists. So when they like eat everything and they are willing to disperse to different places. So maybe they're just like, you know, filling a lot of different niches that way.

But that kind of ecological stuff doesn't seem to matter as much for mollusks. For things like irises, it tends to be more abiotic stuff. So by abiotic stuff, I mean, stuff that's not living, like, you know, temperature, water availability, stuff like that. Geographic range seems to matter more for things like Australian mammals. So as far as I could tell, there's no, like, rule of thumb that works for everything when you're trying to figure out what genus should have more species. It seems like it depends on what kind of group of organisms you're talking about. And of course, you'll note, I didn't even bother talking about bacteria because, yes, they probably represent more diversity than anything else. but who can wrap your head around those guys?

Not me. I know. So Jane's last question was the most important one. Right. Does it have to do with parasites? Jane is just pandering now. And thank you, Jane. I appreciate it.

And I hope you have an extraordinary 2026 as well. But unfortunately, as far as I could tell, it doesn't have to do with parasites. But you know what? I'm guessing that's because no one's looked yet. And they should look harder. I'm guessing that the diversity of parasites in terms of like what genuses of parasites have the most diversity. I didn't see papers on that. There's a research topic.

That's, yeah, there you go. And that probably has a lot to do with like, you know, how are their host diversifying? Like if a host genus has a lot of diversity, probably parasite genuses you find in them also have a lot of diversity because they probably split with the hosts. And so maybe that's where you can find some like predictable trends and, you know, good old predictable parasites, as I always say. Well, here I have a parasite question for you. Is there an example of a parasite losing its host species like the host goes extinct, but the parasite adapts and survives? Yeah. So I don't have an example off the top of my head, but I know that's called host switching.

So it is a thing that happens. That's amazing. Yeah. Oh, oh, oh, I have a sad example. Oh. Guinea worm is a really awful parasite that goes from copepods to humans. So copepods are tiny little aquatic crustaceans, and they're like so small that you might not see them, but you might accidentally drink them in your water if you're living in, for example, certain places in Africa. And if you drink them, they, I believe they, I wasn't prepared to talk about the life cycle.

I'll do it the best I can. I think they find mates in your body, and then the pregnant female, the male dies. Who cares? pregnant female I'm just joking No, I'm just joking I'm joking I like dudes

And so the pregnant female moves down towards Like your ankle Or some other part Like your foot And she sticks part of her rear end out of your body It causes a horrible

Feeling That makes you want to stick your foot Or your leg in cool water And so you tend to put your foot In the water supply When you do that She releases her eggs Into the water

And then they get consumed by those little critters in the water we were talking about, and the cycle starts again. And so the way that we've tried to deal with this parasite is that you take something like a matchstick and you slowly wind the parasite around the matchstick. And every day you pull out just a little bit of the parasite and then you clamp it onto the matchstick and you do it again the next day. This is a horribly painful thing and it can get infected and it's just, you know, miserable. This is something that actually our species, you can read descriptions of this going back, thousands of years because our species has been dealing with this for a really long time.

And it's very distinctive. But if you break it and by pulling it out too fast, it actually is secreting something to control our immune system. And if you break it, your immune system will have a massive reaction to it, which is worse than you feel when you're slowly trying to pull it out. So the goal is to just pull it all out in one piece without killing it. All right, but I was not asking, what is the most horrific parasite? Please describe it in detail.

I was asking about host switching. I'm getting there. there. Okay, so the Carter Foundation and a number of other different organizations have been going to all the different places where you find this parasite and they, anywhere at the time they find a person who's infected, they bring them to a facility and they say, okay, we'll like support you for the duration of your infection and any time your foot hurts and like, you know, every day you'll come to us, you're going to put your foot in cool bleach water so it'll kill the eggs or like they've got some procedure where they like make sure that

the eggs don't get back into the water. They help them, you know, wind out the parasite safely and they are stopping the life cycle. Okay. And so they've got it down to like zero cases in a lot of the countries where you used to find this parasite. And it was down to like one country where you were still finding it. And it was 14 cases in one year. And that was it.

And then they started finding it in the dogs. Oh, my gosh. So there was massive selection pressure for the parasite to be able to jump hosts because the humans were figuring out a solution around it. And maybe before, sometimes it was infecting dogs and we just didn't know. But now they're starting to find it in dogs. And the dogs run wild and they go in and out of the water all the time. And so anyway, this could be a case of host switching.

It's possible, actually, this is a case where we didn't realize it was in the dogs before and maybe I haven't actually answered your question. Anyway, we could do a whole episode on this if people are interested. This is like a really important case of humans trying to eradicate a parasite. Long answer. Host switching exists. That might be an example. Guinea worm is the worst.

Interesting. That's really the worst. Oh, my gosh. I'm so sorry, Jane, for accidentally including that description in the answer to your otherwise innocuous question. Okay. All right. Here we go, Jane. Thanks for asking about parasites at the end there. Thank you so much, Kelly, for answering my question. I was surprised how quickly you got to murder and cannibalism, though, and delighted that you managed to shoehorn host switching into the answer, even though it didn't appear to be relevant. Happy to pander to your interests any day. Next time, aliens? I was stunned to hear that we're not special in being home or alone, to coin a phrase, and almost a third of species are the only example in their genre.

I guess thinking about beetles makes us seem more unusual. Although I wonder if it was just that the beetle and frog taxonomists were more restrained when taxonomising. if that's the word. I'm glad that you find my accent charming, Daniel. I'll have to think of more questions to give you more chances to hear it. I do love the way Kelly says arbitrary, though.

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You stacked the deck, I think. And so let's go ahead and hear Brandon's alien question. Daniel, is it your birthday? It's Alien Day. All right. Hey, Daniel and Kelly. Like Daniel, I'm fascinated by the idea of aliens and how they could differ from us, especially in regards to size. We are as big or small as our environment allows us to be.

And pop culture has shown us a lot of fun depictions of the scale of different aliens. I guess the most egregious would be Dr. Seuss's who's living on a planet the size of a speck. I know Dr. Seuss isn't known for his realism. I think physics is going to call BS in that case. But it does get me thinking about what are the upper limits and lower limits of scale that physics will allow for intelligent life. Thanks so much for taking my question. Oh, Daniel, if you were going to meet an alien, would you rather meet a big alien or a little alien? Or a medium alien, like a Goldilocks alien.

I think I'd really like to meet a super duper tiny microscopic alien because they might have a very different view of the universe and experience phenomena, different scales, and have different ways of intuiting how it works and thinking about it. I think that might be super insightful. I'd love to meet quantum-sized aliens. But wouldn't you be worried about like stepping on them? I'd be worried about being infected by them. Or like inhaling it accidentally. Like, I'm so sorry, ZorBlaks. I read a great science fiction book once about aliens that come to Earth and basically just live on people.

Yeah, it's called The Woman Who Thought She Was a Planet. It's fantastic. Van Dena Singh. She's actually a physicist who's now a science fiction author also. Great stuff. Cool. All right. Okay.

So let's actually answer Brayette. Brandon's question, I've done what I do and got us off track again. So how small could they get? Yeah, I love that Brandon is thinking about this. What are the limits of physics? And it's important to do this because we want to understand what's realistic about aliens. You can't just fantasize it by them being super tiny or being the size of a galaxy because they are limited by physics. But also, we are limited in our imagination of what kind of aliens might be out there. So today I'll give you a sense for what I think could be the small, or the biggest aliens and what physics limits that. But, you know, there could be lots of other ways of being alive and having brains and bodies that we're not thinking about today. So these are not absolute limits. Okay.

So on the smaller end, I think we should assume that the teeny tini-tini-blocks of life have to be atoms. Like, we could dig deeper into corks, but, you know, that makes it very, very complicated. Corks can never be alone. So now we're talking about weird new particle physics. it makes weird new structures of quarks. But if we just stick to atoms, that already gives us a pretty tight lower bound. Because atoms are a certain size, right?

You really can't build something out of atoms that's smaller than like a tenth of a nanometer. I mean, that's pretty tiny. I guess I didn't imagine that we were even going to be working on the nanometer scale. But your imagination is vaster or tinier than mine. And I think that's what sets the scale for life on. earth, right? Like the smallest known life we have on earth or like bacteria or phages, if you think about even smaller. But those things are like, you know, a micrometer. It's harder to get much smaller than that when your building blocks are like order nanometer because you have

to have like stable chemistry. You have to store information. You have to have a metabolism. You know, all this stuff requires a lot of complexity. You can't build life out of like three atoms, right? You need complicated stuff to support even bacteria. And so that means that the smallest kind of life you can have is like a micrometer. And so it's fun to think about like Dr. Seuss and the Hoos, you know, a dust speck on our planet is like one to a hundred micrometers. And so if life has to be at least as big as a micrometer, then it's really hard to imagine having intelligent life and complex society on a dust spec, because that life would have to be much smaller than the dust spec, much smaller than a micrometer, and already we're hitting

the minimum size for simple life, bacteria, not to mention like intelligent life and complex society and all sorts of information processing in their brains. Okay, and as soon as you're getting into intelligence, you require more equipment and more size. Is that what your... Yeah, intelligence requires like memory and more... information processing, communication ability between the members of the species. You know, for example, the human brain has like 10 to the 11 neurons in it. So you're probably not getting intelligent life out of like six or seven building blocks. And even our AIs, right, if you want to build synthetic

life, those things have millions and millions of or billions of parameters, which means you have a very large number of artificial neurons in that neural network. So a lot of complexities required for real intelligence. So if you have a bunch of neurons altogether and you have a minimum size for like the basic building block, that gives you an estimate of like maybe a millimeter is the minimum size for really complex behavior. And, you know, here on Earth, we have critters that are like a millimeter, you know, insects, for example, order a millimeter.

And those don't individually show a whole lot of intelligence. There's some learning there. Fruitflies, for example, have like, you know, 100,000 or 150,000 neurons. in them. And they don't show a whole lot of really complex, intelligent behavior. They can learn, they can dance, they can sing to each other. It's fascinating. Actually, they just mapped the entire connectum of a fruit fly brain for the first time. Really super cool. So now they can like try to understand what is a fruit fly thinking and has that brain work and all sorts of stuff. Really cool bit of science done by a good friend of mine at Princeton.

Cool. So that's a good reason to think that like you got to be at least a mill and meter probably in order to be intelligent. And even still at that scale, you got a lot of physics to worry about, not just like what's the smallest building block. You can make these things out of, but like how do small systems operate? Because small systems have issues that big systems don't have, mostly having to do with noise. Oh, like what? Well, small things wiggle a lot. If you're made out of a smaller number of atoms, you're closer to like the vibration of those atoms. Okay.

Small things just have more thermal noise. Thermal noise just means related to the temperature. Because remember, the warmer you are, the more the stuff inside of you is jiggling. That's what temperature is. And so the smaller you are, the closer you are to that jiggling. If you're big, you may not have huge concrete blocks, then that jiggling all averages out and doesn't really affect you. But if you're a really small system, then you're going to be affected by that thermal noise. And it can drown out any signals.

This is one reason why, for example, our brains are the size they are. You might wonder, like, boy, childbirth is dangerous. Lots of women in history have died in childbirth. That's bad. Why didn't we select for denser brains, smaller neurons that could have the same intelligence at a smaller scale? And the answer is that if you make our neurons much smaller, then you're closer to the thermal noise. And so you lose intelligence. So there's a thought that we're sort of at the sweet spot.

right the smallest brain you could have to have this much intelligence and so you know you really can't get too small and still have complex systems that don't get drowned in noise interesting i didn't know yeah and so i would say that the smallest life you could have when building out of atoms is probably microbial life is it possible for microbes to be intelligent maybe possibly networks of microbes but it's tough. Insect level intelligence, millimeter scale intelligence, also possible, but tough.

So I think, you know, the smallest size critter you could expect to be intelligent at the human level is probably about the size of a human. Really? So, I mean, so we've ruled out insect size,

but why couldn't you have, like, a brilliant dog? And, like, yes, we have good boys and good girls who are very smart and do good jobs, But like, oh, man, Kelly, I'm doing this at the physics level, which means, you know, factors of two or five, whatever. So I'm gripping dogs in with humans, essentially. Oh, okay, okay.

You know, are we talking meter size or centimeter size or millimeter size? So I think a meter-sized alien is probably the smallest you could imagine having human-sized intelligence. Okay, perfect. All right. Let's move on to upper limits. So when you get bigger in three dimensions, then you suffer from the square cube law. This is a very basic fact in physics that as you get bigger, your volume grows with your size cubed, right? Which means your mass grows with your size cubed.

But your strength, the muscles and bones, depend on the cross-sectional area, and that tends to grow with a size squared. Just imagine a cube. If you make it twice as long on each side, then it's going to have eight times as much stuff in it, right? but each side is only going to be four times as big as the original cube. So it gives you a sense for like how the internals grow faster than the sides. And this is one reason why, for example, there's a limit to the size of a building you can build. Because if you build a building larger, then it gets really high volume, but the side doesn't get much bigger. And so now like pressure on the side of a building grows because you have like the same amount of force on a

smaller area proportionally. And this is also an issue for animals. You try to scale up an animal, then its volume grows faster than its strength, than the strength of its bones and its muscles, which depend on, like, the cross-sectional area there, not the volume. And so that's why, for example, elephants are not as proportionally strong as ants are relative to their weight. You know what I'm wondering?

So, you know, we've got like we've got elephants and they're really big. But why don't we, and you know, we used to have T-Rex and Allosaurus and all of those other like really big land animals. Yeah. But we have fewer of them now. Maybe we need to get Steve Broussadi on to tell us why we have fewer massive animals now. Because there's, you know, there's always been this constraint. Oh, it's probably because we killed them. Dang it.

Oh, man. We're coming back to that. We didn't kill all the dinosaurs, though. The asteroid did that for us. Well, no, but there was like giant landslots. You know, there were a bunch of homo species, and then there were, like, giant landsloths, and then there were all these giant birds, and then humans went around killing the giant landslars. No, that's a good point. That's good. Yeah, yeah, yeah. Anyway, we just got to have Steve Rusani on a lot anyway, because he's a lot of fun. All right, go ahead. Sorry. But, yeah, there are constraints there. Getting bigger is harder because you need really big muscles. You need really big bones. The bones proportionally have to grow faster than everything else in order to keep up with the volume.

Also, it makes dissipating heat hard because you have less surface area per volume. And all of this depends on the gravity, right? The more gravity you have on the planet, the faster this constraint comes into play. The less gravity you have, the slower, right? You can have bigger animals on a lower gravity planet. And in the same way, if you're in the ocean, you can escape some of this because the water helps support some of the weight. You have better cooling.

That's why, for example, the biggest critters ever are in the ocean. And the biggest critters ever are a lot of. live right now. Isn't that pretty cool that we're alive at the same time? And we didn't kill them all. Yay for us. Yet. Yeah. That's another fact I learned from Steve Brousotti. Anyway, sorry, go ahead. Very exciting. But there's also a limit to how big you can get even if you're in the water, because the bigger you get, the slower you can think. Signals move across your brain at a certain speed. It's like 100 meters per second, not light speed. And so coordination across your body becomes difficult, right? Now, your brain knows about how long it takes signals to travel from your toes or

from your nose, and it coordinates that, right? Somebody touches you at the same time on your toe and on your nose. Your brain knows that the signals should arrive at different times, and it'll figure that out, it'll reverse engineer that. But if you're like a kilometer-sized being, then that coordination is going to take a lot of time, seconds or even minutes. It's hard to imagine having like a unified consciousness in that picture. And your thoughts are going to just be slower. You know, if you have a crater the size of the solar system, you know, made out of dark matter or something really creative, it's going to think super duper slow.

It's going to be really hard to communicate with those aliens, you know? And do you feel, you feel certain about that? There's, like, no way around it. Are you telling us, like, a truth of the universe? Or is this a, like, this is how you'd set up your sci-fi fiction book because you're pretty darn sure about it? this is all based on a lot of assumptions about how biology works, which is inspired by how things work here on Earth. And aliens could circumvent a lot of this.

Like our nerve signals travel at 100 meters per second. You could definitely do that faster, right? The ultimate limit is light speed, which is a lot faster than 100 meters per second. So you could get bigger if you had faster signals, right? That's for sure. If you had different chemistry, if you had different information storage, you could do all sorts of things different. I mean, if things were like not biological at all, we're talking about machine intelligence or made out of crystals or plasma or something, then everything could be very, very different.

But I'm trying to extrapolate from Earth-like biology. But, you know, that's always limited as n equals one. Yep. Okay, cool. But the good news is that all of this suggests that intelligence on Earth is kind of in the sweet spot. We're big enough to have complex brains that avoid the noise, but we're small enough to be able to communicate across our bodies pretty quickly. and have like coordinated actions and intelligence. So yeah, humans are pretty awesome.

Oh, man. I love that we decided that we're the best. Go us. In this episode, we went from humans are the worst to humans are the best and back again. Well, you know, we cover a broad range of emotions and a broad range of topics on this show. We've got it all. All right. Let's send this answer to Brandon and see if we scratched his itch.

Thank you for taking the time to put such thought into answering my silly question. Of course, pop culture will take liberties to imagine intelligent life forms ranging from Marvel's galactus to Dr. Seuss's Hooville. And Daniel is always reminding us that our human and earthly context limits our ability to imagine just how truly alien aliens could be. But it's fascinating to know that scientifically, the most likely scenario is that any beings we encounter someday will roughly be on scale with humans. Thanks again. And now I will not pull a horton the elephant and start searching. for random glovers to seek out microscopic intelligent life. Well, thank you so much for sending your questions and your curiosity to us.

We absolutely adore getting to interact with you all. We really do. Your curiosity powers this podcast, and it also powers all of science. So keep thinking, keep wondering, keep demanding that the universe makes sense to you. And keep writing us at questions at Danielankelly.org. Until next time. See you later. Thanks everybody for listening.

Please go and do us a favor and rate the show on whatever podcast app you're using. It really helps people find us. Daniel and Kelly's extraordinary universe is edited by the amazing Matt Kesselman. He really is a wizard. You can also find us online on Blue Sky, Instagram, and X, D&K Universe. Come engage with us. You can email us at Questions at Daniel. And Kelly.org. We really do want to hear from you.

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