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Evolution of Brains
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0:01
560 million years ago in what's now Canada, a strange wrinkly creature sat quietly in an ancient ocean. Stuck to the seafloor and surrounded by total darkness, this organism from the Ediarran period, didn't really have a lot going on. But did it have a brain? We often assume that having a brain, especially a big and complex one, is just better. that it's an evolutionary advancement that elevates us from sheer size and strength towards problem-solving smarts. I wonder where
0:33
we with our disproportionately large brains got that idea from. But plants and fungi, which grow bigger and live longer, seem to get by fine without them. So, if brains aren't strictly necessary for living long, large, and successful lives, then why do animals need them? [music] And when did they evolve? While we often think of brains as some kind of triumph over brawn, it turns out that those two things might not be mutually exclusive. And in fact, they've been linked far
1:02
longer than we might [music] imagine. Now, animal brains come in a variety of shapes and sizes. You've got octopuses whose brains are donut-shaped and surround their esophagus. So, if they eat a meal that's a bit too big, they risk giving themselves brain damage. or crocodile brains, which are around the size of a peanut, but still help them coordinate some of the most efficient and ruthless hunting strategies in the animal kingdom. Or even human brains, which make up 2% of our body mass, but
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use 20% of our energy. Running at a constant 20 watts, our brains are enough to power a light bulb, whether any bright ideas are forthcoming or not. But all of these brains have a common underpinning. They are all physical extensions of the basic nervous system serving as its centralized processing hub. A brain is mission control, the motherboard that makes sense of complex signals constantly being transmitted around the body. Nervous systems are made up of cells called neurons that
2:01
allow electrical signals to travel, carrying information throughout the body. Each neuron is capable of connecting with many others, forming a network and connecting cells, tissues, and organs that can sense with those that can do something about it. So naturally, before you have a brain, you need to have a nervous system. But as you can imagine, the spindly cells of a nervous system don't exactly fossilize well. Instead, scientists have to turn to animals alive today to give us a clue
2:28
about when they first got wired up. The main animal groups, vertebrates, arthropods, mllisks and myriad worms, all have nervous systems that are centralized around a brain of sorts, while others like nidarians, including corals and jellyfish, have a more distributed neural net throughout their bodies without a brain. And sponges don't appear to have any traditional nervous system at all. So, if we want to find when the first nervous system appeared, we need to look back to the
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origin of the most primitive animal groups. One way researchers do this is by using a technique called the molecular clock, which lets them use the genes of modern organisms combined with a predictable rate of mutation to trace their evolutionary origins. And this evidence suggests that some of the earliest creatures to have nervous systems first appeared by the Edidia period around 625 million years ago. In a way, this origin in the idi makes sense because before
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that, it's likely that any animal ancestors would have been microscopic, just bundles of cells that wouldn't have had much need or space for complex nervous [music] systems. But once you get into the idi, the genetic and fossil records are famously at odds. While the molecular clock may tell us that relatively advanced Nidarian ancestors are around, we don't see a whole lot of fossil evidence for them. The idi itself is dominated by bizarre creatures, some of which may have been related to
3:50
animals we know today, although it's not yet clear how. And the thing is, the idi organisms don't seem to do much at all. There's little to no evidence of them having sense organs or moving around. They just kind of sat there. So, if some of the idi creatures were animal ancestors and were some of the first to have nervous systems, then it's not clear what exactly they were doing with them. Regardless, something must have happened in the Idi which set the stage for a later explosion in brain
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evolution. And it was an explosion. The vast majority of animal groups with brains all appeared in a geological blink of an eye during the Cambrian explosion about 540 million years ago. Not only that, but it seems these animals came into the world with their brains already fully intact. Considering how little evidence there is for precursor nervous systems, we might not expect to find much fossil evidence of the first brains either. In fact, under most conditions, the brain is the first
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organ to break down after death. It's made exclusively of energy hungry cells and keeps its structure thanks to blood flow. So, when the blood and the energy stops, those cells start to eat themselves alive very quickly. But there is a surprisingly good record of brains among the early cambrian animals. Despite the fact that they're often small and softbodied, and it comes down to preservation. The same processes that allow for soft-bodied organisms to be fossilized in the first place also help
5:18
preserve their neural tissue. In Canada's Burgess Shale, for example, early animals were buried quickly by underwater mudslides, which stopped the organic matter from rotting away. The fossils are flattened, but early brains and neural tissues can occasionally be spotted as thin films of carbon running through their bodies. One of the earliest fossil brains comes from cardiodiction, a relative of the velvet worms found in southern hemisphere forest today. The fossils were
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discovered in 518 million-year-old deposits in China [music] in the 1980s. And a more recent study of these fossils revealed that they seem to have the entire nervous system preserved intact with knots of nerves along a multi-legged body plus a simple brain at the head end. Then there's carriakila which comes from the same group as cardiodiction that's been found in rocks from Greenland also dating to around 518 million years old. Here researchers have spotted a simple brain that connects its
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eyes and its claw-like frontal appendages. And then there's Stanley Carus, an animal related to the predator Anomalarus, known from 506 million-year-old Burgess Shale. In more than 80 fossils, researchers have identified a more complex brain, split into two distinct segments [music] connected to the animals three eyes and its front claw-like appendages, respectively. For comparison, modern arthropods have brains split into three segments, suggesting that ancient
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Stanley caris was 2/3 of the way to the arthropod's final form. Together, these fossils tell a story of brain evolution that seems to track with the so-called information revolution that followed [music] the Cambrian explosion of animal life. The evolution of eyes in early animals meant that there was suddenly a lot more information available to process, which drove the growth of neural processors, [music] aka brains. The way these brains would have acted is very similar to how we see brains
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working today. [music] As an animal senses something, the brain processes the information and then relays the information to allow the animal to act. And yet, the sudden appearance of such effective brains in the Cambrian comes as a bit of a surprise after the relative inactivity of the idiar. The common neural architecture that's shared among most animals, as well as the fact that these complex brains all appeared around the same time, makes it highly likely that brains originated just once
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among the early animal ancestors. So what happened to jumpstart brain evolution? What were those idi nervous systems doing that paved the way for such an explosion? And why make the leap from simple nervous systems to these centralized computers? Well, it helps to go back to the nervous systems earliest beginnings. Every [music] living thing needs to be able to sense and respond to things in its environment. Light, water, food, the competition. All of this is
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useful information if you want to stay alive. In single-sellled organisms, this is accomplished by simple chemical sensing on their outer surfaces, akin to our sense of smell or taste. And some colonial single cells called quanoflagulates evolved to use a primitive kind of electrical signaling too. So when the first multi-ellular organisms evolved, it's likely that this external sensing of the [music] environment became co-opted into internal sensing within the organism.
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The tried and tested electrochemical signaling systems were adapted to become the very first nervous systems, transmitting information as impulses throughout a simple body. But when animals started to get bigger, a change was needed to their passive sedentary lifestyles and the organic wiring that underpinned it. Because big bodies need more food and to find more to eat, the organisms in control of those bodies really need to move. Now, the other relative giants of the living world, the
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plants and the fungi, manage to move to get more food by simply growing towards it. But animals do things differently. They use muscles. The very first muscles probably started out as simple fibrous cells that could contract. Bundle enough of those together and you get a muscle tissue that contracts as one. And researchers have found some of the earliest muscle tissue in ancient organisms [music] as far back as the idi around 560 million years ago, including that wrinkly creature we met at the
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beginning of the episode. That guy is Hayutia, and it's thought to be a kind of nidarian related to modern corals and jellyfish. So, in the beginning, at least, the thing that set successful animals apart was their brawn. But the thing about muscles is that they need coordination. A body needs to contract its muscles in the right order to get motion in the direction you want. Otherwise, you just end up spasming and damaging yourself. And so before any kind of sensing or reacting comes into
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the equation, these early muscular creatures would have needed an internal processor just to coordinate their new body parts. For the sake of efficiency, it makes sense to put all of the relaying and feedback neurons close together, creating a knot of neural tissue. In other words, a brain. Which means that the earliest brains may have evolved not to help an animal process and react to its external environment, but to shape the internal actions of their new complicated bodies. So, it's
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no coincidence that the heia fossil is found at roughly the same time that genetic evidence points to the earliest nervous systems. It could be then that all through the quiet idiest brains were evolving in the background to help coordinate new muscular bodies. And heyia might have had a muscular body, but like the other idi organisms, it has no tracks or trace fossils. There's little to no evidence of it actually being able to move in a coordinated way yet. But by the dawn of
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the Cambrian, animal ancestors had got the hang of this movement thing. The very base of the Cambrian period is defined by a fossil burrow from a wormlike organism. And by the time things like eyes appeared, the brain architecture was already in place to provide processing power to efficiently sense and react. Through the Cambrian, even bigger bodies and more complex lifestyles like predation and social living only increased the demand for processing power. That additional
11:26
computational need was met with an increasing brain size. Fast forward 500 million years or so, and animals now rule the land, the sea, and the sky, powered by some of the most remarkable organic computers evolution could devise. But it's worth remembering that while we may consider brains to be superior to Braraw, without the appearance of Braun, we may never have got our brains in the first place.
12:04
Sponges may not have brains, but they are one of the oldest animal groups and have survived each of the big five mass extinctions. They may even have turned planet Earth into sponge world after the first ever mass extinction [music] at the end of the orivition period. Find out more in our episode, the second time sponges took over the world. and thinking this ontologist is a no-brainer. Addie, Annie and Eric Higgins, Carl Wolffold, Jackie Scott Rston, Jake Hart, John Davidson Ing,
12:31
[music] Juan M, Melanie Lamb, Carnival, Nico Robin, Raphael Hos, Tony Dy, and Steve. By becoming an Eonite at patreon.com/eons, you can get fun perks like a monthly digital puzzle of paleo art commissioned by Eons. And as always, thanks for joining me in the Annie and Erica Kiggins Studio. Subscribe at youtube.com/eons for more creature features.
13:00
[music] I feel like I should be reading the scripts on [music] an iPad and if I see that it's going bigger than the iPad, I should be like there not one period in the whole iPad. I don't think so. [laughter]
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