How Nature Makes A Complex Brain

A recent series of studies suggests that the brains of birds, reptiles and mammals all evolved independently — even though they share a common ancestor. That means evolution has found more than one way to make a complex brain, and human brains may not be quite as special as we think. To learn more about this, we talk to Fernando García-Moreno https://www.achucarro.org/people/fernando-garcia-moreno/about this series of studies https://www.science.org/doi/10.1126/science.adp3411he co-authored that came out in Science in February. Want to hear more about the complex road of evolution? Send us an email at [email protected] mailto:[email protected]. Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave http://plus.npr.org/shortwave. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

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• 0:00Speaker 00

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• 0:16Regina Barber?

  You're listening to Short Wave from NPR.

• 0:22Regina Barber?

  What do birds, mammals, and reptiles all have in common? We're amniotes, meaning we develop in a fluid-filled egg covered in a membrane. That allows us to develop outside of water, unlike, say, a fish. And that means we all have a common ancestor that branched out into other species that researchers think probably lived over 300 million years ago.

• 0:45Regina Barber?

  And was probably similar to an amphibian, with some key differences.

• 0:51Speaker 02

  But it already had some specializations, like a different skin, specialized lungs, or brains, et cetera.

• 0:59Regina Barber?

  Fernando Garcia Moreno is an evolutionary and developmental neurobiologist. He says for a long time, there's been a debate about how amniote brains, like birds and mammals, evolved, and what makes them similar.

• 1:14Regina Barber?

  One brain structure, called the pallium, has been seen as a comparable structure in birds, mammals, and reptiles.

• 1:21Speaker 02

  In every case, in all the species, the pallium is in charge of high task and high hierarchical tasks, such as cognitive processing, sensorial processing, motor control, also language, for instance, in the case of mammals and birds.

• 1:38Regina Barber?

  In mammals, the structure is near the top of the brain. It's sometimes called the cerebral cortex, and it includes an area called the neocortex, plus some other key structures.

• 1:48Speaker 02

  The hippocampus, which is in charge of memory, for instance, memory processing, or the amygdala, which is in charge of emotional processing.

• 1:57Regina Barber?

  Birds and reptiles don't have a neocortex. So some scientists say mammal brains are totally unique. They must have evolved completely separately from birds and reptiles. But other researchers say while birds and reptiles may not have a neocortex, they do have some of the same neurons, they're just in different places.

• 2:16Speaker 02

  So for some researchers, they thought that the same cell types appearing in the neocortex are located in a different manner, not in layers, but in nuclei, in birds, for instance. They tend to think this because all these neurons communicate each other in a quite similar fashion.

• 2:33Regina Barber?

  This side of the debate says maybe bird brains and mammal brains are more similar than they seem.

• 2:41Regina Barber?

  So Fernando and his lab try to figure out how these structures develop. And if that process could tell us anything about what makes our brains different from bird brains. So today on the show, how does nature make a brain? Why the phrase bird brains could be a misnomer? And why humans may not be as special as we think? I'm Regina Barber, and you're listening to Short Wave, the science podcast from NPR.

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• 4:22Regina Barber?

  Okay, so Fernando, we're talking about how your study found that a bird and mammal brains develop like through different processes. And so you're looking at these palaeums of birds, reptiles and mammals. And what did you find?

• 4:34Speaker 02

  We found that the pallium of all these structures develops in the embryo following different rules. So the areas in the brain in which these neurons are generated for the pallium, or the timing in which these neurons are generated, it is strictly different when we compare reptiles to mammals or to birds.

• 4:53Regina Barber?

  Let's go back into like how this evolutionary structure happened. Like what, what does the study show you?

• 5:00Speaker 02

  Yeah, we cannot research the brain of these animals who are extinct for 300 million years, but it allows us to hypothesize how the brain of these animals were. And we think that it shows us how the brain of the last common ancestor was organized in terms of pale neurons and pale circuit trees. So we think that the same ocular and circuits that we see today in different areas of the pallium were present in the last common ancestor, not present in, for instance, in the case of amphibians. But they're must be present in the last common ancestor because all amniotic species, mammals,

• 5:41Speaker 02

  reptiles and birds, evolved circuits from them. Definitely, this is, this were simpler circuits, much simpler than the ones that we can see today. But we think that the last common ancestor already had some of them in the pallium. So what we see is that this circuit, it is very relevant, its features, because it has been selected several times in evolution. And we think that for people trying to research connectomics and also to make artificial intelligence or neural, artificial neural networks, they should consider how this circuit acts because it is the most efficient and the optimal one selected by evolution. So we should not reinvent the wheel when nature is telling us this is the efficient circuit.

• 6:26Regina Barber?

  Wow. And then you, we were talking about this idea of, you know, these brains are, are different, but they're kind of doing similar things, this idea of convergent evolution. So like what, what is the process called convergent evolution?

• 6:43Speaker 02

  Convergent evolution means that two features in two independent animals have evolved separately.

• 6:50Speaker 02

  But they have reached the same feature, the same characteristics. So the classic example is the wings of bats, butterflies and birds. So when you research them, you can see that they develop in a completely different fashion. So bats are making the wings with their fingers, whereas birds, they are doing it with the whole arm, for instance, and insects, they are doing it with some different primordial in the embryo. So we know that they are convergent, because if we go to the common ancestor to these three animals, it was kind of a worm that didn't have any wing and didn't have even limbs. So we know that the wings of this species, they have evolved separately. They are not inherited from a common ancestor.

• 7:33Speaker 02

  But then, we know that they are fulfilling the same function, and therefore, they have a very similar structure. Because to fly, you need a wing which is in this fashion, with a particular surface, particular thickness or weight. So function, because it was common, you need to fly, dictated the shape of wings in all these species. In the case of the circuits, we consider they evolved in a convergent manner, because although they follow different evolutionary routes, they ended up generating circuits which are very similar.

• 8:07Regina Barber?

  If your studies are kind of pointing towards the development of brains being convergent evolution, why is that significant to the understanding of how our brains work?

• 8:18Speaker 02

  There is something quite relevant, which is intelligence, for instance, or the highest sensorial processing have appeared several times in evolution. So we are not just an example of something very unique and special. Intelligence is not such unique and special. We think that complex brain and complex cognitive tasks

• 8:41Speaker 02

  Have evolved separately several times because the circuits and the neurons in charge of them have evolved several times and separately. So for instance, birds, some birds can count.

• 8:52Regina Barber?

  Yeah, and some birds can talk, some birds can use tools.

• 8:55Speaker 02

  Yeah, exactly, and they are doing it with different parts of the pallium. Of course, the pallium is involved, but different parts of the pallium are in charge or making the sounds, for instance, or the motor control of the larynx and the tongue, this kind of thing. But the neurons and the areas of the pallium are different. So human intelligence or mammalian intelligence is not unique. Other species evolved intelligence and complex cognitive tasks through other neurons and different neurons and separately evolved.

• 9:26Regina Barber?

  Yeah, I mean, I I find it fascinating. You're you're basically saying that like, even though bird brains are different, the neurons are in different places, they're doing different things, they developed in different ways, they can still do similar tasks.

• 9:41Speaker 02

  Yeah.

• 9:42Regina Barber?

  But you're saying they're doing that and their intelligence is not the same intelligence we have.

• 9:47Speaker 02

  We think so because they are using different structures and different types of neurons to be this kind of intelligent. So bird brains are amazing, a couple of examples. So they have evolved to fly and therefore they have secondary specializations. And the most important in this case, and the pain in in the neck for us in the lab, is that they have reduced enormously the size of neurons, so they are tiny. In in this way, they can reduce the size of the brain and the weight of the brain. The whole brain and the whole body of birds is designed to weigh very low, so they can fly. The neurons in bird brains are tiny and they are thoroughly compacted, so they are, these

• 10:29Speaker 02

  are the most dense, in terms of neurons, the most dense brains in nature. But also, when you compare the number of neurons, which always or classically was considered a correlation to intelligence.

• 10:44Speaker 02

  Some birds, they have double the number of neurons of a primate of the same size.

• 10:49Regina Barber?

  Oh.

• 10:50Speaker 02

  So they have huge numbers of neurons. Definitely, these are very clever animals and they are differently clever to us.

• 10:57Regina Barber?

  And I know that you didn't study humans, but I'm curious if you think like, this research can tell us anything about whether there's something special about like the way the human brain developed.

• 11:12Speaker 02

  I have to say that I am the least anthropocentric person in biology.

• 11:18Regina Barber?

  You're like, boo, humans.

• 11:20Speaker 02

  Yeah, definitely. So I'm extending my low anthropocentric view. I am also very low mammal-centric view. So we see in the lab a lot of complexities in other brains. So I don't do research directly on human brains.

• 11:38Regina Barber?

  Okay.

• 11:39Speaker 02

  But I tend to think that in the last 25 years since I've been working in labs, there is a paper coming out every two, three years claiming that there is a specific feature to humans. It could be a cell type which only appears in humans or a circuit that is only developing in humans. But then, after three, four years, someone finds the same circuit or cell type or progenitor type in primates. And then someone else finds it five, 10 years later in mice. So in the end, me as the least anthropocentric researcher ever, I tend to think that our brain is special in quantitative features, but not in qualitative features. So we are a mammalian brain, definitely this is different to a bird brain.

• 12:23Speaker 02

  But we just have more of the same units. And there might be an emergent property coming up from this increase in the number. But I don't believe someone has convincingly so cell types which are specifically human or connections which are specifically human.

• 12:42Speaker 02

  In the case of birds, they can count and they can plan the future. As you said, they can make tools on their own. They can hide some...

• 12:54Regina Barber?

  Like food?

• 12:56Speaker 02

  Some food or something for the future, this kind of thing. So they can plan, but they are doing it with a different part of the pallium.

• 13:01Regina Barber?

  Wow. What do you hope people will take away from your study?

• 13:06Speaker 02

  The main takeaway is that humans are special, but so are birds and reptiles. So our brains are amazing. But bird brains are even as amazing, at least. We have neurons other species do not have. But the chicken, even the chicken, stupid chicken, they do have neurons that we don't have. So evolution have found so many different ways to generate complex brain, not just only one direct pathway from amphibians to humans. In this case, the tree of intelligence is just a tree. It's not just a single branch in the case of mammals.

• 13:44Regina Barber?

  Fernando, thank you so much for talking to us about bird brains.

• 13:47Speaker 02

  Of course, thank you.

• 13:52Regina Barber?

  This episode was produced by Rachel Carlson, edited by showrunner Rebecca Ramirez, and Tyler Jones checked the facts. Our audio engineer was Quacey Lee. Beth Donovan is our senior director, and Colin Campbell is our senior vice president of podcasting strategy. I'm Regina Barber. Thank you for listening to Short Wave from NPR.

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