Rationally speaking, is a presentation of New York City skeptics dedicated to promoting critical thinking, skeptical inquiry and science education. For more information, please visit us at NYC Skeptic's Doug. Welcome to, rationally speaking, the podcast, where we explore the borderlands between reason and nonsense. I'm your host, Julia Galef, and I'm pleased to introduce this episode guest Susanna Herculaneum's Uzzell. She is a professor of biomedical science at the Federal University of Rio de Janeiro in Brazil.

And soon in a month or so will be professor of Psychology and Biological Sciences at Vanderbilt University. She's written eight books now on science for the general public. And we're going to be talking about her most recent book and her first book in the English language, The Human Advantage, which addresses the question. It covers her own original research. And the questions she's focused on is what makes humans special. Essentially, what is it about the human brain relative to the brains of other species that gives us the huge cognitive advantage, the many cognitive abilities we have that other animals don't have?

What are what is it about our brain that that gives us that? Suzana, welcome to the show. Thanks, Julie. It's a pleasure to speak to you. So I thought we could start by putting this question in a historical context. I know generations of scientists, of people throughout the ages have wondered what it is that makes humans special. What are some of the previous approaches people have taken to this question? What are some of the previous theories people have had about what makes humans special?

Well, I think the original and simplest idea was ever since people realized that it is the brain that produces all our mental capacities, that whoever had the biggest brain should also have the most abilities. Right. Which kind of makes sense because brains are made of neurons. Neurons are the units that process information. So in theory, it sounds intuitive enough that whoever has the biggest brain should also have the most neurons and cognitive ability should just come along with that.

Like having bigger muscles makes you stronger. Yes, exactly.

But the problem was that it was quickly realized, I believe, by the end of the 19th century that humans do not have the largest brain around. I mean, an elephant brain, which I've held in my hands, is about the size of my forearm. That's three times as big as a human brain. And many dolphins and whales actually have brains that are much larger than than the human brain and even larger than that, an elephant brain. So once you realize that we pale in comparison to those animals in terms of brain size, it seems pretty obvious that, well, either our brains are not made the same way, but that took a long time for us to come up or else you just think, well, it's not absolute brain size, maybe it's something else.

Maybe the human brain is special in its relative size compared to our body, because larger animals usually have larger brains, but gorillas have larger bodies than we do up to three times as large as a human body. So you would expect gorillas to have bigger brains to go along with those bigger bodies. And yet it's the other way around. The human brain is about three times as large as a gorilla brain. So that's where this idea came from, that the human brain, even though it is not the largest brain around, it still is the largest compared to what you would expect the human brain to be for the size of our body.

Right. Right. And I can I can see intuitively how that hypothesis seemed like a good corrective to the original hypothesis, that it was just about absolute size, although I still don't quite grasp how that was supposed to work like I know.

So the idea is that larger bodies require larger amounts of brain to take care of that body. So if you have enough brain to to take care of that body and an extra amount of rain, then you're better off than other animals who have just enough.

See that? That's the original idea. I do not think that's right for a number of reasons, beginning with the fact that the the number of neurons that actually are directly related to taking care of the body is amazingly small. And that does not increase very rapidly as the size of the the the body of an animal increases. So I've come to think that it's really just the absolute number of neurons that you have in your cerebral cortex, especially in those parts of cerebral cortex that are that go beyond just receiving sensory information and organizing muscle movement.

Let's say I think that that's what really matters, regardless of the size of the body.

But I'm getting ahead of myself. So that was that was the idea that if you have extra brain compared to what you would in principle need to have to operate, your body would give you an advantage. And because that ratio, that extra amounts of brain is by far the largest in relative terms for humans that seem to single this out. So people were very happy with that answer for actually a couple of decades. And on top of that, we've recently had an explosion in numbers of studies on comparing genes across humans and chimpanzees and macaques.

That's the usual comparison to to see what is different about the human brain. But of course, you find that a number of genes are different. But the issue to me is that, well, wouldn't you expect to find a number of genes to be different anyway? Because after all, if you look at a human and at a chimpanzee and I don't tell you which is which, you're not going to have any any doubts. And that is because there are a number of genetic differences across the species.

So if you if there are, that doesn't. Right.

If so, if you have these obvious differences on the outside, I wouldn't expect anything less than there to be differences on the inside as well. Right. Right.

Yeah. I think there's a tendency in in neuroscience and in genetics, at least in the popular understanding of those fields, to think that you've sort of resolved a question just by showing, look, there are differences between activity in two regions of the brain or look, there are differences in the genetics and think, you know, well, that's the explanation for the question we were interested in. But often that doesn't really get you very far.

Exactly. I think that the the answer necessarily will be there are a number of differences, not just this or this or that, but in terms of our cognitive capabilities or abilities, really. And I think there's an important difference between those two words. Capacity or capability for me is what you're biologically endowed with, what you're literally physically capable of doing and what you turn those capabilities into. So what you actually do and that depends a lot on your effort and your opportunities.

Those are your abilities.

Well, I just wanted to explore the what we mean by cognitive capacities or abilities a little bit, because when we're making these cross species comparisons, it seems pretty difficult to to have a way to measure ability that is meaningful across species like for humans we can measure one way to measure people's cognitive abilities is with an IQ test. But you can't really give that same IQ test to a bird and to a chimpanzee and have a meaningful comparison there. Right. So how would how a neuroscientist interested in this question measure something like that?

You're absolutely right. That is a huge problem. So right now, the best I think that we can work with is, well, there is besides this general intuition that however you define or measure capabilities or abilities, we must come out on top because, well, we are the species studying other species and not the other way around, or at least we don't think so.

I'm sure that some undergraduates somewhere want to say that, no, humans don't come out on top, the species that come out on top of the ones that aren't killing each other and ruining their planet. Right. That's that's the true measure of cognitive ability.

I've been that undergraduate, although not based on that argument that I must be some other species that's not out there killing each other. I think that's that's one of the signs of dominance. Exactly. That you're cognitively capable enough to enforce your dominance over other individuals or other species. But anyway. Defining and measuring abilities or capabilities is a huge problem. I don't think we have a proper way of doing that yet. There is essentially there have been two approaches.

One is choosing one or two tasks that are general enough and simple enough that can be applied to widely different species and sticking to those tasks and trying to compare those species that has been done.

Although some of the people I talked to have criticisms to that or like how how easy is it or how valid is it really to to to apply to different species? Another approach is to be on the opposite, to the contrary, to be as general and inclusive as possible and add a mixed bag of cognitive tests on memory and learning and self-control and whatnot. But then the problem is that you're necessarily restricted to similar species. So that's an approach that has been used for non-human primates.

And in that in that case, what they found was that the best correlate with this general cognitive index across non-human primates is simply the size of your brain or the size of your cerebral cortex because you can't really dissociate one thing from the other. But anyway, the important part is it's not that relative measure of how large your brain is relative to how large it should be for the size of your body.

Right. Which is interesting because then that that finding wouldn't really hold up across very different species. So it's only sort of it only has validity within a sort of cluster of similar species.

Exactly. Which is which is, by the way, the very observation that got me started thinking that all brains must not be made the same way. I mean, at least not in terms of how many neurons go into the brains of different or similar sizes, because here are my favorite comparisons. If you take a chimpanzee brain and a cow brain, they both have similar size. It's about four hundred grams, which, by the way, is a very respectable brain size.

And then so if those two brains were made the same way, you would expect them to be made of similar numbers of neurons. And if neurons are the basic information processing units of the brain, then those two brains should have similar cognitive capabilities. And yet, like I said, one belongs to a cow, the other belongs to a chimpanzee. And this is where like we're just asking we should be concerned about what we really know about the cognitive abilities or capabilities of cows and chimpanzees.

Maybe cows have this really, really rich internal mental life while they're just grazing and they're doing deep philosophy and they're so good about it that they don't let us in on it.

The but I and this is this is where this is the difference between just being an observator, being an observer in the in the world and actually doing science. We can look at a cow in a chimpanzee and think, well, I'll stick with a chimpanzee. I think that the chimpanzee can do much more interesting and flexible and complex things than a cow can. But that's just your impression. And until you actually get down to collecting facts systematically and comparing them systematically and objectively, that's really just an intuition.

But the problem is right now, that still is the best that we can do. So we're badly in need of direct, systematic way of comparing cognitive capabilities across species.

I see. And so so the way that we got here was we were talking about ways that people in previous decades or previous generations of science had attempted to answer this question about the human advantage. So before we move on to talk about how you approached it, I just want to check and make sure does that roughly bring us up to speed or are there were there more modern scientists who had another theory before you came along that's worth mentioning?

It was essentially this combination of the human brain is larger than it should be for the size of our body. We also have different genes that are expressed in neurons and in synapses. Maybe that maybe the human brain has different it has a different connectivity, a different pattern of connectivity, which isn't really the case. I mean, you would have to get down to details to find something that is different about the connectivity of the human brain. And even that, I would still argue that is just a byproduct of having more neurons in the cerebral cortex, although that's still up for grabs.

But the other the other thing that seemed special about the human brain is that for its size, for its relative. Have size, it costs way too much energy, so the human brain represents about two percent of the mass of your whole body, so you would initially expected to cost about two percent of the energy that your body uses, but it costs 10 times as much. It's about twenty to twenty five percent of all the energetic cost of your whole body.

Is your brain alone?

And what is that ratio like in other species? Is it is it roughly proportional to body size?

It's more than proportional. So that was known already that the brain gram by gram, the brain is more expensive than the rest of your body, but at best, it costs eight to 10 percent of the total energy cost of the body in other species. So we we did seem to be an outlier in that regard. Interesting. OK, so that's I think that pretty well sets us up with the the mystery that you are trying to solve and previous stabs at it.

So do you want to tell our listeners what your innovation was that helped us get some traction? Sure.

So I think it started with just thinking that there was something fishy about this story, that all brains are made the same way like I because of comparisons like I just mentioned about A, because cows and chimpanzees or birds and other species birds have very small brains, and yet they're capable of doing things that monkeys are also capable of doing with brains, monkey brains, or typically, what, five to 10 times bigger than bird brain, the same.

So I realized that we actually did not know how many neurons, different brains were made off and how that compared across species. Basic questions like how many neurons does the human brain actually have? Does it have do we have we do we have fewer neurons then larger brains like an elephant brain or a whale brain. So my I realized that it was still possible that maybe the simplest answer for what is it that sets humans apart in their cognitive capabilities? What if that's simply the a large, absolute number of neurons in the brain as a whole or maybe in the cerebral cortex in particular that nobody else does?

And why had we not already? I would imagine, just naively, that neuroscientists would be would have jumped on the project of trying to count neurons in the human brain. Was that was there a block to that before or so?

That that that to me and I guess maybe this is one of the advantages of not belonging to this field. When I got started, I trained as a neurophysiologist. I trained in I got my Ph.D. recording neurons responding to visual stimuli in animals the same.

So I became interested in this this issue of what brains were made off and how they compared. When I started working at a science museum for the general public and I realized that 60 percent of college educated people in Brazil and in Rio accepted the notion that they only use ten percent of their brain and which is absolutely not true. You use your entire brain the whole time. You just use it in different ways and you can learn to use it more efficiently or better and solve problems more rapidly and so on.

But one of the possibilities behind that 10 percent myth was that when you open any textbook and you most likely see that the human brain is made of one hundred billion neurons and ten times more glial cells, so you see that neurons come up as come out as roughly 10 percent of your brain cells. So I started looking around for that number. It turned out that it didn't exist. And like you did, I I was surprised. I just expected this to be one of the most basic issues in neuroscience.

So if you want to figure out more complex, intricate things about brains, you would I thought you would start by figuring out what different brains were made off and what are the rules, let's say, for putting brains together. Right. Apparently, there was a simple reason why we didn't know those numbers, not even for the human brain, which was the lack of a technique of a method, really, that would allow you to estimate accurately numbers of neurons in the whole brain of different species, because the major difficulty is the the method available then, which relied on very, very precise, intricate, systematic microscopy.

Had the problem that the distribution of neurons in the brain is highly heterogeneous, so you can have a low density of neurons right here, and then you move two millimeters to the left and you find the density that's five times bigger in a different structure. So anyway, so so there was a technical difficulty behind that. And on top of that, there seemed to be this assumption that, well, our brains were made the same way. So why bother?

Whoever has a bigger brain should also have more neurons and of question. Next question.

You could just extrapolate up from this idea that, no, not even that you would have numbers because the numbers didn't exist, but that you could. So there was a consensus that larger brains are made of bigger neurons. So that's because you see the density of neurons decreases across the species that have been compared in the past. So you know that larger brains have bigger neurons.

And you simply you would you would just people would just assume that larger brains also have more neurons. So people in the field seem to be pretty content with that notion that, well, you take two brains. If they have the same size, they have similar numbers of neurons. A bird brain is smaller, so it must have fewer neurons. And whoever has the biggest brain wins has the most neurons. Right. So I had to come up with a different way to count neurons.

And that's what I called the brain soup.

You bring up soup. Tell me what that is. What's your recipe for my students? Tell me. I've killed apple juice for them because that's that's pretty much what it looks like. Oh, no, you you take fixed tissue.

It can be a whole brain. It can be any part of the brain that you can dissect, that you can cut apart from the others and study it separately. See, so you take that chunk of fixed tissue and what the bite fixation might mean, that the tissue has soaked in formaldehyde or formaldehyde.

The and that makes the membranes of the cell nuclei very, very, very sturdy and resistant to detergents and mechanical friction. So you take that fixed tissue and you you really dissolve it in a detergent solution. And to do that, we use glass. It's like a mortar and pestle. That's pretty much what it is, but in the shape of a tube. So you start with your chunk of tissue and in 10 to 20 minutes, depending on how large that the chunk of tissue is, you've turned it completely into soup.

And that soup now consists of all the cell nuclei that ones all the nuclei of the cells that once composed the tissue, except that now they're freed of the cell membrane. So you have these nuclei just floating in a suspension. And the beauty of having the nuclear now floating freely in a suspension is that you can just go first, you can round up the volume of the suspension so you know exactly how much volume you have. And once you've done that, you can just slosh it around just agitated.

And by doing that, you're making the nuclei become distributed evenly in the in the suspension.

And once you have that, you can just take four or five tiny little samples to the microscope and you can actually count with your eyes. You can you can count how many nuclei you see per given volume of the tissue. And because of the suspension and because you know what the total volume of the suspension was, it's just a simple extrapolation. And, you know, what was the total number of nuclei and therefore the total number of cells that you had in the tissue.

So it's it's pretty fast.

It's accurate. And that gives you the total number of cells. And now by using an antibody that so it's a protein, a molecule that binds specifically to the nuclei of neurons, it makes them red. So you go back to the microscope and now you count what fraction of all the nuclei actually are neurons. So that's another couple of hours. And after ten more minutes at the microscope, you now also have the total number of neurons that were in that.

Structure that she started with, and you can get this the same count for four glial cells and things like that, so the glial cells for now, what we work with is just by subtraction.

We take that number of neurons from the total number of cells that you had in the tissue. So that is technically the number of non neuronal cells in the tissue or other cells in the tissue.

But because the only non neuronal cells that are not glial are the cells that make up the capillaries and the brain. And because the volume of capillaries is very, very small, it's about two percent of the volume of the tissue.

We assume that. The vast majority of the non cells in the tissue are actually glial cells, so that's where we work with for now.

So what what did you find out? How close was the total number of neurons in the human brain and glial cells to what our best guess had been?

Oh, so the total number of neurons, what you find in textbooks is one hundred billion neurons, which I came to realize was an order of magnitude estimate and as an order of magnitude estimate, it was really good. So we found on average eighty six billion neurons.

And I like to point out that those are male Brazillian brains of people aged 50 to 70 years of, you know, psychology studies just say, you know, it's found that people react certain that way. They don't think it's found that 18 to 22 year old psychology students that Ivy League universities in the Northeast react this way. So good job. Yeah.

Thanks you all this this is I'm in the business of numbers and reporting what brains are made of and how I think that's important. So we have to be careful about what it is that we are comparing and for for a number of purposes. That is of course not enough.

We would like to know how that compares to to women younger and older. We actually one of my main interests right now is actually figuring out how much variation you find across individuals.

We know already that in mice, the those and these again were male mice exclusively to avoid for now the problem of sex differences. But we know that in mice of the same age, those animals with the bigger brains are not necessarily the ones that have the most neurons. So there is no across individuals. There's no correlation between the size of your brain or the size of the brain structure and how many neurons that structure has. So we're trying to find out, of course, whether that is also true for humans.

We know that it's also true for other species. But we need to look at humans and specifically the.

But anyway, the important thing is that for the purposes of what we were doing back then, which was comparing not individuals, but comparing species, this average of eighty six billion neurons, give or take a few for Brazilian males aged 50 to 70, was good enough that we could compare to other to other species, other primates, other non primates. So the number of neurons was fairly close to that order of magnitude estimate, although the people that some people try to tell me already six one hundred billion, who cares?

It's all the same.

Like, no, it's not. The difference is 14 billion neurons and that is more than an entire baboon brain and baboon brains are pretty big. But anyway, so the important thing about that estimate of eighty six billion neurons and not a hundred is that it puts us squarely with other primate brains.

So if you compare in terms of the relationship between the size of the brain and how many neurons it has, and actually also in terms of the relationship between the size of the body and how many neurons the brain that goes inside that body has, and that is true, provided that you are not looking at great apes, which we couldn't do back then simply because we didn't have tissue from great apes.

So that's what got start. That's what got me started thinking that maybe we've got it all backwards this whole time. Maybe it's not the human brain that is an oddity. Maybe it's not humans that are an evolutionary exception or oddity. Maybe it's the great apes that, for whatever reason, cannot have as large a brain as you would expect for their body size.

And and that that is a story that that we pursued and that I tell in the in the book, because it does it does turn out that now that we do have the data gorilla and the Uranga getting brains, they seem to be made in exactly the same way as other primate brains, including human brains are made. So with the very same relationship between the size of the brain and the number of neurons that applies to everyone else. What they wear, they do fall.

Out of the curve and by a lot is in this relationship between how many neurons you find in their brain and the size of their body. So indeed, they if you compare if you use every other primate as as as a basis for comparison, humans fall right where you would expect the generic primate to fall in terms of how many neurons in the brain for the size of their body. And you find that it is gorillas and orangutans that have that are, let's say, missing neurons for the size of the body that they have or else they have a much larger body for the number of neurons that they have in the brain.

See, and why is why is the size of the body adjustments relevant here, given that, as you mentioned earlier, most of the most of the brain isn't devoted to controlling the body? Why is that? Why are we adjusting for that?

Well, to begin with, because you you have to keep going with the existing arguments in the literature. If I want to make the point that I think things are different and actually the size of your body is not all that relevant, not in terms of how many neurons you have to you need to operate that body, then I actually have to do the proper comparison and show that, well, look, it's it's humans are not the species that fall out of the curve.

It's great apes. But really, the reason why I think it's this relationship between or or this oddity in the relationship between number of brain neurons and the size of the body is that once you realize that all of this costs a lot of energy and actually adding more neurons to the brain costs a lot more energy, that's when I started to to be suspicious that, well, maybe the reason why great apes are the odd ones out, that they don't have as many neurons as you would expect for the size of their body is that maybe they have they're limited by their diet.

They're constrained in such a way that they can either afford a very large body or a very large number of neurons, but they can't do both. And that does seem to be the case. That's that's what we found out when we when we did the math, comparing how much energy different primate species get from their natural diet. And on the other side of the scale, how much energy their bodies cost based on how large they are and how much energy their brains cost based on how many neurons they have.

Hmm. So would it be correct to say that that the takeaway here is that human cognitive abilities are are no greater than you would expect if you just took a primate brain and and didn't change its structure or the algorithms it was using, but just added more neurons to it?

That's exactly what I think. But but then the key question becomes, well, there's there's actually there's a number of questions, but one of them is, of course, how come humans are the only species that managed, the only primate species that managed to have that many neurons in the brain come back to that. But the other one still is, well, what about other animals? What about an elephant? Do we still have more neurons than an elephant?

And the question and the answer is yes and no, if you compare the whole human brain, we we our brain is one third the size of an elephant brain and it does have about one third the neurons that an elephant brain has.

But and this is a huge, huge but ninety eight percent of the neurons in an elephant brain or found in the cerebellum, which is it's not it's it's really not just a sensorimotor structure like people say in the in textbooks, but it probably does have a lot to do with a structure that elephants and elephants alone have, and that is enormously sensory and has a lot of motor precision that nobody else has.

And that's the trunk. If you you got to realize that the trunk of an elephant is pure muscle and sensory receptors, it can do very, very precise movement with infinite degrees of freedom because it has no joints. There is there's no articulation inside the inside the trunk.

But anyway, so when you compare the cerebral cortex alone and that's the part of the brain that we think is where the let's say, higher cognitive processing happens. So not only sensorimotor integration, but also Petrine finding and planning for the future and. Relating to to others, that's those are all functions of the cerebral cortex, so once you compare the cerebral cortex alone, what you find is that even though the human cerebral cortex is only half as large as an elephant's cerebral cortex, the human cerebral cortex does have three times as many neurons as the elephant cerebral cortex.

And actually, we do seem to have the most neurons in the cerebral cortex of any species on earth. That includes whales as well. Even the largest ones, which by our estimates only have as many neurons as you find in the in the elephant cerebral cortex. So second place is so so we have 16 billion neurons on average in the cerebral cortex. Second place is gorilla and orangutan. Chimpanzees have between six and seven million. Then you have the elephant with five point six whales probably have between three and four, maybe at best five billion.

And everybody else, even the other animals that have large brains like giraffes, they have fewer than one billion neurons in the cerebral cortex. So the reason for the difference between humans and everybody else is that we have a primate, typical brain and non primate species. They they do have fewer neurons in the same brain volume, more in the same volume of the cortex. And the reason is that when you are not a primate, as your brain gains neurons, the neurons do indeed become larger like people had suspected in the literature.

The difference is that. Primates don't follow that rule, primates, when primates diverged from other species in evolution, the the the rules for adding neurons to the brain changed. And now if you are primate, you gain more neurons. But the average neuron does not become any bigger, which allows you to fit more neurons into the same mouse.

Exactly.

So it actually I like to think that it allows your brain to not increase in size very rapidly as you gain more neurons.

Right.

See, so when you compare primate in a non primate, the way when you compare a primate brain with a non primate brain of a similar size, what you find is that the primate brain always has more neurons, especially in the cerebral cortex, than the non primate brain. And the larger the size of the brain, the larger this gap will be in. Primates always have more neurons than primates, which is what I call the primate advantage.

Got it. So the the the human advantage is basically it basically consists of, first, the primate advantage, being able to fit in more neurons without growing the brain to an unwieldy size. And then on top of that, just adding more neurons, which other primates didn't do essentially. Is that right? Yeah, exactly.

And which brings us back to that first question of why only humans? Why don't chimpanzees or gorillas, orangutans also have more neurons, especially gorillas, given that they do have the larger bodies that you would expect to come with more neurons? Right. So so that's where we started doing the math about how much energy these different species have available to, let's say, pay for a certain body mass in a certain number of neurons. And that's when we realized that gorillas have pretty much hit the energetic wall.

They they cannot feed for more than eight hours per day, which is how much time they spend foraging, which means actively looking for food and also eating.

The so they would they would need an extra one hundred and eighty kilocalories per day if they were to have as many neurons as you would expect them to have for the size of the body that they have. But it doesn't sound like a lot. I mean, you eat an apple and an apple, two apples would take care of one hundred and eighty calories.

But the problem is we think that it's not a lot because we have refrigerators in our homes now. And if the fridge goes empty, you just go down the street to your to a grocery store and you can get all the calories you need again.

And you've read food to be way more calorically dense than it used to be, right? Exactly.

I think that's the key thing about industrialized foods that get all the bad rap. But actually, they if you think that the original problem that our species solved was how to get enough calories, we're amazing.

We have this amazing possibility that no other species has of eating the two thousand calories that we need for the whole day. And just a single 15 minutes sitting at your favorite junk food restaurant.

See, so how you frame that as an achievement, as a grand achievement?

Well, that because it is just just think of think that a gorilla needs to spend eight hours per day to get the three thousand calories that that it needs. If by the same math, if we so ate like other primates do in the wild, we would have to spend over nine hours per day looking for food and eating.

That's that's how much time our ancestors would have had to spend per day just looking for food and eating food and and repeat all day long just to afford the body that they had and as many neurons as we have in the brain today. Now, once you realize that.

Well, let's just just to be clear, if you had to spend nine and a half hours per day foraging, looking for food, forget it, forget school, forget podcasts about how the human brain compares to to any other brain, we would not be doing any of this. We would still be out there just looking for food every single hour of the day.

So but so there's something I don't understand here. Feels naively like there's kind of a chicken and egg problem where, like, you have to be above a certain level of cognitive ability in order to come up with the innovations that allow you to make to make it easier to get calories every day. Exactly. But you also have to have enough calories to grow your brain or your number of neurons large enough to be smart enough to know how to do that.

So. Yep, exactly. Which is the answer. The answer.

And so this is this is where a little bit of knowledge about the human evolutionary history help solve the problem. Because the thing is, before our brains started to really increase rapidly in numbers of neurons, which is the feature, I think, that most sets us apart from everybody else.

Our ancestors not only had enough neurons already to use tools, which is something that chimpanzees do, gorillas and orangutans do, even some birds also do.

But they first our ancestors became bipedal.

And there's a lot that comes with being by becoming bipedal. And it starts it's it's not simply just having free limbs to carry, stuff to carry or tools much more than that.

Once you become bipedal, you can walking back walking costs much less energy due to remember seeing chimpanzees walk, they have that funky sideways gait with their knees splayed sideways. So chimpanzees are still quadruped.

So it costs them a lot of energy to walk the butt of a bipedal human ancestor would have used four times less energy than a chimpanzee to walk around, which means that you can actually extend your range of foraging. So that already gave them an advantage in terms of being able to find enough calories for the day. See, just because you can go farther in the day because it's easier to walk and walking costs less calories. So maybe that put them at a small advantage already compared to non bipedal every other primates.

On top of that, about three million years ago, or ancestors also learned to not just use natural. As tools, but actually to make their own tools. And this is where I I think technology comes in and plays a huge role in our biological evolution also because making tools is actually creating a technology, you you learn you create this method of using a naturally available object to shape another natural available, naturally available object into, say, a stone knife.

And now you have in your hand somebody something that nobody else does. And it's a tool that you can use to kill animals, carve meat crush.

So all these all these or things that you can do that start processing the food that you eat before you put it inside your mouth. And that goes by the name of cooking even before fire, even before you learn to use fire as a tool to modify even further the food that you eat. See? So what I think what starts setting us apart from other apes is this combination of a biological change, which is bipedality with this technology that other primates don't have.

And I think it's really interesting how this first this initially very simple technology, the making of your own tools, can actually feed back into your biology because it allows you to get more calories.

And then this whole thing just spirals up and very rapidly you could even see out of control, but not really, because the more technologies you have, the better you can modify your food, the more calories you get and therefore the more neurons that you can afford in your cerebral cortex, which gives you more capabilities. And so this is the important part. Where to this? So this is where it's important to separate between capabilities and abilities when as you become as human ancestors and human ancestors alone become capable of getting more calories.

So having having a larger brain with more neurons is no longer a liability. Much to the contrary, it now gives you this advantage of having more neurons to work with, which in principle gives you a cognitive advantage of having more cognitive capabilities. Now, as you use those capabilities to solve problems, which, by the way, is where it comes in really handy that you now have this cool technology of pre processing the food before you eat, because maybe the biggest advantage that comes from cooking your food is that it takes much less time to get all the calories that you need.

And you just try eating a raw carrot. It's it takes forever. It tastes horrible and it takes forever.

But as soon as you cook that that carrot and you don't need to do anything fancy to it, just toss it in the oven, just hold it over a fire, just roast. It not only becomes tasty, but you can now eat it, eat the whole thing in just a couple of minutes. So the really important thing about that is now you have free time to do much more interesting and challenging things with your. Capabilities that your neurons for afforded, you see.

So having more problems to solve actually helps you turn those biological capabilities into actual abilities.

And then if you have enough neurons to allow you to pass those abilities on to the next generation through teaching, then you really close the circle.

You're all set. You're ready to just spiral up and have those. That larger number of neurons there are now affordable actually actually translate into more cognitive abilities that you can pass on?

Well, we are now over time. So I'm going to wrap up the section of the podcast. But I think this has been a great a great walk through of the the mystery as it as it has existed about the human advantage and and what explanations previous generations have come up with and why those have been unsatisfactory. And then the technological innovation that allowed you to approach the question from a different angle and and what the implications of your answer have been. So that's so it's been great.

Thank you so much. OK, so now we'll move on to the rationally speaking pick.

Welcome back. Every episode, we invite our guest to introduce the rationally speaking pick of the episode that's a book or blog or movie or something that has influenced their thinking in some interesting way. So, Susanna, what's your pick for this episode?

I pick Catching Fire by Richard Wrangham. It's a book that enormously influenced my work. And that's where I first read about this idea that cooking while in his the way he tells it, he focus. He focuses on cooking with fire. But it doesn't matter. The important thing is this idea that transforming your food actually was really the watershed event in human evolution. So Richard Wrangham was the scientist who first proposed that the enormous and fast increase in brain size and human evolution actually started with the advent of cooking.

And this book is a delight to read. And I think anybody from any background can is able will be able to read it and also enjoy it a lot.

Excellent. Well, we will link both to your book, The Human Advantage, and also to your pick on the roughly speaking website. And once again, thank you so much. It's been wonderful having you as a guest.

Thank you, Julia. It was a pleasure to talk to you.

And before we close, I just want to remind all of our listeners to get your tickets for Nexus. That's the Northeast Conference on Science and Skepticism taking place this May 12th through 15th in New York City. Keynote speaker this year is the always excellent Richard Wiseman. Other featured speakers include Bill Nye, The Science Guy and the entire cast of the Skeptics Guide to the Universe. And of course, I'll be there taping a live show. Get your tickets now at NextG.

That's any Sea org. Hope to see you there. And now this concludes another episode of rationally speaking. Join us next time for more explorations on the borderlands between reason and nonsense. The rationally speaking podcast is presented by New York City skeptics for program notes, links, and to get involved in an online conversation about this and other episodes, please visit rationally speaking podcast Dog. This podcast is produced by Benny Pollack and recorded in the heart of Greenwich Village, New York.

Our theme, Truth by Todd Rundgren, is used by permission. Thank you for listening.