Listener Questions #38

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28:09All right, we're back and we're answering questions from listeners. And here's a question from 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 is the most species in it? And what influences whether you end up with one or a thousand? And does it have anything to do with parasites? Still loving all that you do. Thank you so much for answering my question.

28:51Have 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 we probably ate them. Well, 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.

29:31But 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.

30:02What do you think? Well, I think the fact that we killed them off before written history already tells us the answer. 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 of 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 i either have to say the mnemonic out loud and the only one I memorized is dirty.

31:00And 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 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.

31:47And 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 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.

32:26And then they sort of ran some models and they said, OK, 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 a 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.

33:17And 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.

33:59We 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, the answer to 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. And so you definitely shouldn't come away from this conversation thinking this is like totally arbitrary and random.

34:37You 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 the internal organs of these species, the external features of these species, the 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 carefully about this stuff. And a lot of the relationships between organisms, we are like nearly certain are true.

35:16But the question is just when do you say, OK, we're stopping here and we're putting a name on what's happening right here? You know what I mean? And 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. OK. So like the platypus is 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.

35:51Panda is an unfortunate choice of a name. Actually, I can't remember 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 – 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 Georges Cuvier name it. But the locals at the time, who, of course, already knew about it long before Europeans went in and were like, this is new. They called it wah or chitwah based on its vocalization.

36:34So 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. 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 but uh or the way we may have but a fire cat could have done that that's yeah 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 aardvarks 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 aardvarks 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 percent, based on the numbers that I found, of genera of animals have only one species in them.

37:47So there's a lot of examples. I just picked some cute ones. Wait, 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, genuses is acceptable, but genera is what is preferred. I see. Well, I like genuses 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 saw I tripped on it. That's ironic. All right. So one third of all animal genera have one species in them.

38:27What 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 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. 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?

39:03But 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.

39:37Yeah, 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 a 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. For a lot of the examples that we went through earlier, we do.

40:10So they were not alone in the past. So another question, that bottleneck question was great. That's like going to be a keeping me up at night question. 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, Solanum. This is the genus that includes like potatoes, tomatoes, eggplants. Thank goodness for Solanum. It is a delicious genus. I think it's a genius genus. Yeah. Me as well. And I think the genus, gosh, the genius genus homo has done amazing things with Solanum.

41:00If only I could have said that fast. Anyway, all right. Another very diverse genus is 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 beetles. The beetles. Agryllus. These are jewel beetles. One that you might have heard of is the emerald ash borer, 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.

41:34Wow. Just another kind. Dismiss amazing biodiversity and evolutionary genius. Just like just another kind of beetle. I know the Coleopteran 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 like genera of geckos that also have a lot of species. And Enolis 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?

42:15When do you get a genus that has a lot of species? It's a Kelly answer. This is DKU. What do you think the answer is? It depends slash we don't know. 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're 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.

42:54For things like irises, it tends to be more abiotic stuff. And 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.

43:29But 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.

44:12That's, yeah, there you go. And that probably has a lot to do with like, you know, how are their hosts 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.

44:51That's amazing. Yeah. Oh, oh, oh, I have a sad example. 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.

45:28I'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.

46:06And 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 like 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 you pull by pulling it out too fast, it actually it's 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.

46:41So 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. I'm getting there. OK, 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 the time they find a person who's infected, they bring them to a facility and they say, OK, we'll like support you for the duration of your infection. And anytime your foot hurts and like every day you'll come to us, you're going to put your foot in cool bleach water so it'll kill the eggs.

47:18Or like they've got some procedure where they like make sure that the eggs don't get back into the water. They help them wind out the parasite safely and they are stopping the life cycle. OK. 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. 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.

47:53And 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.

48:20Guinea 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.

49:07I 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 a 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. Keep being extraordinary. Thank you.

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52:26All right. And we are back. And man, Daniel, today is your day, man, because this is another aliens question. 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.

53:09I 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 at different scales and have different ways of intuiting how it works and thinking about it, I think that might be super insightful.

53:51I'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, Zorblacks.

54:08I 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. Vandana 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 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 about them being super tiny or being the size of a galaxy because they are limited by physics.

54:48But also, we are limited in our imagination of what kinds of aliens might be out there. So today, I'll give you a sense for what I think could be the smallest 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 tiniest building blocks of life have to be atoms. Like we could dig deeper into quarks, but that makes it very, very complicated. Quarks can never be alone.

55:24So now we're talking about weird new particle physics that 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 are like bacteria or phages, if you think about even smaller.

56:04But those things are like a micrometer. It's harder to get much smaller than that when you're building blocks or order nanometer, because you have to have stable chemistry, you have to store information, you have to have a metabolism. All this stuff requires a lot of complexity. You can't build life out of 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 Who's, you know, a dust speck on our planet is like one to a hundred micrometers.

56:43And 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 speck because that life would have to be much smaller than the dust speck, much smaller than a micrometer, and already we're hitting the minimum size for simple life, bacteria, not to mention like intelligent life in 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 you're - Yeah. Intelligence requires like memory and more information processing, communication ability between the members of the species.

57:24You 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 or billions of parameters, which means you have a very large number of artificial neurons in that neural network. So a lot of complexity is required for real intelligence. So if you have a bunch of neurons all together 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.

58:03And, 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. Fruit flies, for example, have like 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 connectome of a fruit fly brain for the first time. Really super cool. So now they can try to understand what is a fruit fly thinking and how does that brain work and all sorts of stuff.

58:42really cool bit of science done by a good friend of mine at Princeton. Cool. So that's a good reason to think that you got to be at least a millimeter 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 the vibration of those atoms.

59:19Okay. 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're made out of huge concrete blocks, then that jiggling all averages out. and it 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.

59:53You 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 really can't get too small and still have complex systems that don't get drowned in noise. Interesting.

1:00:31I didn't know that. 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.

1:01:14Oh, man, Kelly, I'm doing this at the physics level, which means factors of two or five, whatever. So I'm gripping dogs in with humans, essentially. Oh, okay, okay. 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.

1:01:53but 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 that gives you a sense for 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.

1:02:32And so now pressure on the side of a building grows because you have 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 the cross-sectional area there, not the volume. 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.

1:03:17Yeah. But we have fewer of them now. Maybe we need to get Steve Brussati on to tell us why we have fewer massive animals now. Because, 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. But we didn't kill all the dinosaurs, though. The asteroid did that for us. Well, no, but there was like giant landsloths. 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 landsloths. But none of those were as big as Brontosaurus.

1:03:47No, that's a good point. That's good. Yeah, yeah, yeah. Anyway, we just got to have Steve Broussani 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.

1:04:18The 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 alive right now. I know. 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 Broussotti. 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.

1:04:57No. 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, to 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.

1:05:33It's hard to imagine having a unified consciousness in that picture. And your thoughts are going to just be slower. If you have a crater the size of the solar system 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 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.

1:06:11And aliens could circumvent a lot of this. Like our nerve signals travel at a hundred 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.

1:06:43But I'm trying to extrapolate from Earth-like biology. But that's always limit is 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.

1:07:23We'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 Whoville. 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.

1:07:58Thanks again. And now I will not pull a Horton the elephant and start searching for random clovers 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 make sense to you. And keep writing us at questions at danielandkelly.org. Until next time. See you later.

1:08:38Thanks 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 XDNKUniverse. Come engage with us. You can email us at questions at danielandkelly.org. We really do want to hear from you. And you can find our website, www.danielandkelly.org, where you'll also find an invitation to join our Discord, where everybody comes and talks about the amazing universe.

1:09:17And we also have the most amazing moderators. This is an iHeart Podcast. Thanks for joining us.

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