The sea squirt is not an attractive animal. It goes by the charming Latin name of Ciona intestinalis (pillar of intestines) and, quite frankly, that is not a bad choice given the looks of an adult sea squirt. The sea squirt lives at the bottom of the sea. It spends its adult life attached to a single patch. It is a filter feeder, meaning that it passes water through its body and filters out useful particles. Water is taken in and expelled through tubular siphons. In between, the animal’s sack-like body filters nutrient particles out of the water. As a lifestyle, it is not very glorious, but it has kept sea squirts going for a good 518 million years.
The story of the sea squirt’s life cycle is often used to describe why animals have brains. The sea squirt doesn’t start life as a pillar of intestines. Its larva resembles a small tadpole, moving around the ocean floor by moving its tail. Once the larva is fully grown, it selects a nice spot on the sea floor and attaches itself there. The first thing it does in its new home is to consume its own brain! The brain, apparently, is no longer needed when the animal no longer moves around. The neuroscientist Daniel Wolpert jokingly likens this to an academic obtaining tenure: When you find a nice and cushy space to spend the rest of your days, you do not need a brain anymore. The point of the story is that the sea squirt’s brain, primitive though it is, is essential for movement. If you don’t move, you don’t need a brain. That makes some sense. After all, plants don’t move and plants don’t have brains.
The story of the sea squirt’s life cycle raises an interesting question: Why have a brain? Think about it. As humans, our young take the better part of two decades to develop, mostly because their brains are still maturing. A young gazelle, in contrast, is up and running with its parents 30 minutes after birth. Helpless as human babies are, at the time of birth their brain is already so big that giving birth is generally the most dangerous thing a woman of our species can do. Before the rise of modern medicine, women frequently died giving birth, something that is a rare occurrence for our smaller-brained chimpanzee cousins. At the other side of life, our brain is vulnerable to strokes and an array of neurodegenerative diseases. During life, the brain is expensive to maintain, gobbling up nine times as much energy as the average organ. In fact, our ancestors might have had to invent cooking to outsource food processing, just to be able to extract enough energy to feed our hungry brain. Why would we have such a slow to develop, energetically expensive, and fragile organ?
I will argue in this book that our brains evolved to help solve a particular challenge: that of obtaining nutrients that are distributed in space and time. At some point in evolution, life forms on one branch of the tree of life invested in a strategy of obtaining energy-rich nutrients that were hard to obtain but promised great reward. They invested in brains to help them overcome the obstacles of obtaining these energy-rich nutrients. Of course, this “investment” was not a conscious choice. The law of natural selection means that some variations in life forms proved successful enough to survive and reproduce. Some variants for which this happened to be the case were those able to obtain difficult to get, but energy-rich, nutrients distributed across space and time. Another word for that behavior is foraging. I propose that the brain is a foraging device. And I will argue that differences between different animal species’ brains largely reflect solutions to their foraging problems.
Does my theory hold in the case of the sea squirt that supposedly consumes its own brain when it settles down? It does. It is true that the sea squirt undergoes a substantial metamorphosis when it reaches the adult stage. It changes from a tadpole-like being into a creature with a sack-like body with two tubular siphons. During this metamorphosis, most of its brain cells do degenerate, although it is worth pointing out that there probably were only 177 neurons to begin with. But the sea squirt does maintain a kind of brain during its adult life, albeit one that is radically different from the one it possessed as a larva. One of the functions of this brain is to control the rate at which water is taken up and expelled, which depends on the food content of the water. If there are not enough nutrients in the water, the sea squirt will take in more water to compensate for the deficit. You could argue that, although the sea squirt itself doesn’t change location, it does affect movement of the water. Just as it did when it moved itself, it has a type of brain that controls the movement of muscles. Those muscles move with a very specific goal, to ensure that enough water flows through its body to filter out the right amount of nutritious particles. In both larva and adult stages, the brain controls movement for a specific purpose: to ensure that enough nutrients find their way into its body.
But we need more evidence. Animals are the only organisms we know of with brains. If our proposal is true, then somewhere in the evolution of animals something changed in their foraging strategies. That change coincided with [the] appearance of brains. Moreover, those brains must have provided an advantage to the foraging animal. In this chapter, we will examine each of these three arguments. First, we will chart the history of life on Earth, up until the first complicated animals. Then, we will have a look at when brains cells and brains appear. Finally, we will see if these brain cells were in some way special, helping their owners forage.
Excerpted from “The Fox, The Shrew and You: How Brains Evolved,” by Rogier Mars. Reprinted with permission from Princeton University Press. Copyright 2026.
