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Have you ever thought about sea sponges? They're some of the oldest and longest-lived creatures in the world. The oldest known sponge was at least 4,500 years old, and sponges themselves have been around for at least 750 million years. Clearly, they're doing something right, but whatever it is, they're doing it without a brain.
Sea sponges are among the simplest organisms in the world. They don't have any organs, not even neurons. But if you trace their evolutionary history back far enough, you'll find that before the brain evolved in our ancestors, only 18 cell types were needed to form a sea sponge. It had enough genes to form a brain, but the sea sponge didn't need one.
Despite its simplicity, the sea sponge has established itself firmly in place and feeds on the microorganisms that fall into it; it is also remarkable. Before the advent of sea sponges, most life on Earth consisted of single-celled organisms. For billions of years, this was the norm. And then life was stimulated to do something interesting.
Several cells merged, and soon after the comb jellies, sea sponges formed. As far as we know, sea sponges were likely the second multicellular organism in history. But comb jellies were the end point of evolution; nothing evolved from them. Everything evolved from the sponge, even us.
Our nervous system and brain evolved later. Perhaps they were present in earlier relatives, or perhaps the sponge simply laid the groundwork. But they proved, and continue to prove, that life can thrive without a brain, without a nervous system, without virtually anything at all. But how?
What organisms do not have a brain?
It may not surprise you to learn that most brainless creatures live in the sea. As we saw with sponges and ctenophores, multicellular life evolved in the sea, and most of it remained there, never developing the need for a brain. Most of these creatures are quite slow-moving or even permanently attached to one location.
Besides the two species we already know, starfish, jellyfish, sea cucumbers, corals, and mollusks also lack brains. All of them function perfectly well and can feed, reproduce, and live without the need for a brain. Sea anemones, nematodes, and oysters, as well as tapeworms and various other parasites, are also brainless. And, although it seems fairly obvious, plants and fungi also lack brains.
How can an organism survive without a brain?
So, you're an organism without a brain. How do you survive in this vast world? How does a brainless creature know where to find food? How does it avoid predators, extreme conditions, horrific accidents, and everything else your brain helps you cope with every day? For some creatures, it's a lot easier than you think.
All the creatures we've described are fairly simple organisms. They don't need to exert much effort to survive during the day. They eat, reproduce, and some do nothing else throughout their entire lives. When considering brainless organisms, consider how they live. A brain requires a lot of energy to function. Most of these organisms simply couldn't survive without a brain unless they significantly altered their lifestyle.
Most of these organisms have a branched system that functions like a nervous system, but isn't quite one. For example, starfish have a decentralized nervous system. They have a nerve ring, and nerve signals are transmitted from each individual tentacle via the radial nerve, but they function independently of each other.
The starfish moves using one of the tiny legs located at the base of its tentacle, and this movement is sensed by the other individuals, each of which independently decides to move along with it. So, there's no single brain deciding what to do; it's simply nerve signals working in concert, allowing the entire organism to move, feed, and do whatever else it needs to do.
Essentially, an organism doesn't need a brain to survive unless it's inherently complex. It's enough for the body to have at least some neural structure that sends signals, allowing its parts to perform their functions. That's quite sufficient. A life full of art, laughter, and music won't be rich, but a brain will help you get from point A to point B and provide you with sustenance.
What can a brainless creature do?
Jellyfish have a group of nerves called a nerve net or ring, which is distributed throughout the body rather than centralized like the brain, but in some ways functions much like a brain. It is capable of sensing changes in the environment, such as temperature, salinity, vibrations, and currents. These nerves allow the jellyfish to know where to move to avoid danger and pursue useful targets.
Clusters of nerve endings called rhopalia allow the jellyfish to sense light and maintain an upright position so it doesn't swim straight down all the time.
Even without a brain, jellyfish are capable of learning and adapting. Habituation and sensitization—when a person becomes accustomed to something or can't ignore it—is how, according to researchers, many creatures, such as jellyfish, have long learned. But associative learning, such as when a cat comes running upon hearing the sound of a can opener and associates the sound with dinner, was thought impossible for these simple creatures.
However, studies of jellyfish have shown that associative learning is possible even without a brain. Aquariums were painted to simulate distant mangroves, creating a contrast between light and dark roots that the sensitive eyes of Caribbean box jellyfish could detect.
At first, the jellyfish swam toward these contrasting colors, mistaking them for roots, and bumped into the aquarium walls. But they quickly learned to stay away from the colored roots, associating approaching them with the discomfort of bumping into the aquarium wall. For you, me, or even your cat, this might not be a problem, but for a creature without a brain? That's impressive.
Sea anemones demonstrated similar associative learning when exposed to light and electric shock. Over time, some learned to recognize light as a potential shock hazard and retracted only when exposed to light, but not the shock itself. This meant that, even without a brain, they learned to avoid one stimulus because it was associated with the other.
Other sea anemones, usually hostile to strangers, can learn, after repeated interactions, to recognize nearby sea anemones as genetic clones. They can recognize their own kind and not show hostility once they recognize them as the same creature.
Some jellyfish also exhibit a behavior that might seem mundane at first glance: sleep. We take it for granted that most creatures need to sleep at some point, but if a jellyfish doesn't have a brain, why would it sleep? Sleep isn't necessarily necessary for the body to rest, and while we don't fully understand the importance of sleep in the context of brain function, we do know that the brain requires sleep for a kind of reorganization, recharging, and "housekeeping." So why does the Cassiopeia jellyfish do this?
Researchers believe the cause of sleep is roughly the same as ours, but not as complex. This has led them to conclude that a primitive form of sleep may have existed even before the nervous system emerged biologically, hundreds of millions of years ago.
Slime molds—another fascinating and bizarre creature that can exist both as single-celled organisms and in clusters that form larger organisms—also lack a brain. Looking at them, you'd probably never doubt this because, as the name suggests, they look like slime. However, this slime has proven its ability to solve mazes.
If you place a slug at one end of a maze and food at the other, it will expand and fill all the paths, occupying all the dead ends. When approaching the food, it can lead the dead-end slugs back to the main body, leaving a trail that will serve as a signal to other slugs not to take that route. Instead, all slugs will follow the shortest route to the food once they realize all other paths are dead ends.
Even more amazingly, if one slime mold has learned to avoid something, it can pair up with another slime mold, and the new slime mold will learn from the old one to stay away from negative stimuli.
Slime molds were also able to solve the two-armed bandit problem, previously thought to be possible only for organisms with brains. Simply put, the two-armed bandit problem is a decision-making problem based on slot machines, or one-armed bandits. You're presented with two slot machines, and you need to decide which one to play to get the biggest payout.
The choice is based on the principle of exploration versus exploitation. If you can only use one hand at a time, you need to figure out which option is best: try one first, then the other, or settle on one in the hopes of maximizing the benefit. You don't know which option is right, so you need to make a decision based on your experience.
Decision-making isn't particularly complex, but for a brainless slime to demonstrate such an ability is astonishing. In an experiment, the slimes were offered two paths. One led to a valuable food item, the other to a less valuable one. Over time, the slime learned to adjust its path to only the valuable food item. In other words, it explored both options but chose the more profitable one.
Can a person survive without a brain?
We know that many creatures have brains far more primitive than humans. And we now know that some organisms lack a brain altogether. So what kind of brain does a human need for survival? Can a human live without a brain?
The man consulted a neurologist at the University of Marseille and underwent a brain scan. The neurologist was shocked to discover that the man's skull had virtually no brain tissue. The cavity was almost completely filled with cerebrospinal fluid, with very little tissue remaining. He was a 44-year-old civil servant. The test revealed that he had a below-average IQ, but he was perfectly capable of managing everyday activities independently. He lived a completely normal and, by all accounts, happy life.
The man was born with hydrocephalus. The fluid that normally surrounds the brain began to compress the brain tissue, taking up too much space. This is usually treated with a shunt, which allows the fluid to drain so the brain can expand back into its original space.
Some people with this condition experience severe neurological and cognitive impairment. But some, even with just 5% brain tissue, can live completely normal lives. In one case, a young man with just a millimeter of brain tissue inside his fluid-filled skull had an IQ of 126 and graduated with honors in mathematics.
People around the world have been seeking treatment at hospitals complaining of various problems, only to have their brain scans reveal a complete absence of the cerebellum. A woman named Michelle Mack was born with only the right side of her brain, which, according to doctors, appears to have taken over the function of repairing and rebuilding to compensate for the loss of the left side. She has lived a full life, graduated from high school, and is employed as a data entry specialist.
While this is certainly unusual and not ideal, in some cases, people can adapt to functioning with reduced or even very little brain tissue. As long as the brainstem remains intact and controls most autonomic functions, it appears that some people can continue to thrive even in the most extreme conditions.
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