Brains evolve differently underwater

Cephalopods—squid, octopuses, cuttlefish, and nautiluses—are the most intelligent and agile of all mollusks, boasting extraordinary adaptations for predation, locomotion, and communication. Despite their distant evolutionary relationship to humans, their remarkable cognitive abilities are challenging conventional views on the evolution of complex brains, revealing unexpected pathways to intelligence.

Artwork by Sina Sommer

The brain is a product of evolution, who shaped and transformed it across the vast tree of animal life. Yet, it remains one of the greatest mysteries of our time, with its structure, chemistry, and functions being the object of countless unanswered questions.

As humans, anthropocentric as we are, we tend to view ourselves as the pinnacle of complexity, believing that there is nothing more fascinating than the human mind. However, while the Homo sapiens brain is undeniably remarkable, a group of marine animals continues to captivate scientists: the coleoid cephalopods—octopuses, squids, and cuttlefish. Despite being classified alongside mollusks such as clams and cockles (nothing against these two), coleoid cephalopods boast a nervous system comparable to that of vertebrates. These extraordinary creatures possess both a central brain and a peripheral nervous system capable of acting independently. For instance, if an octopus loses a tentacle, the detached limb retains its sensitivity to touch and can still move¹.

Octopuses display remarkable signs of intelligence. They are curious by nature, capable of remembering events, and can even recognize individual people—showing preferences for some over others. Recent research suggests they may even dream, as they change the color and texture of their skin while sleeping².How could an organism so distantly related to us in evolutionary terms—and so closely related to relatively simple creatures with minimal capacities—could develop such a complex nervous system?

Biologists are particularly fascinated by this question. To explore the mystery, they often focus on genes, the fundamental units of heredity. Evolutionary biologists use them to piece together how organisms adapt, evolve and interact with their environments, which is equivalent to solving a massive, ever-changing genetic puzzle.

Imagine our genetic makeup as a vast private library, where each book contains instructions for building and maintaining the organism. These books can be edited, new pages can be added and existing ones removed:  we call this gene modification. Traditionally, these large-scale genetic changes have been considered the primary drivers of evolutionary innovation. However, this view tells only part of the story. For genes to fulfill their functions, they must be expressed. This process involves doing copies of sections of these “books” (what we call mRNA) and transporting them to other parts of the cell, where they are read to produce proteins—the true functional units of the cell.

But this process is not as simple as flipping an on/off switch. It works more like a dimmer switch, allowing precise control over the “brightness” or intensity of a gene’s activity. In other words, instead of reading entire genetic “books,” sometimes only specific sections are used, creating an even more sophisticated system of regulation. On top of this, there is another layer of regulation called microRNAs (miRNAs). These are small molecules that act as regulators and possess the ability to determine whether the mRNA copies made from the DNA—like borrowed books—will ultimately be used or discarded. By binding to these copies, miRNAs can either block their translation into proteins or mark them for destruction.This additional layer of control allows cells to fine-tune gene expression with remarkable precision, responding swiftly to environmental changes or developmental cues.

Now, if we take a closer look at cephalopods, we find that they possess an unusually large family of these regulators, particularly concentrated in neural tissues, including the brain and peripheral nervous system. Moreover, these miRNAs are highly active during nervous system development, likely playing a pivotal role in this complex process. Using these different dimensions of gene regulation is how cephalopods have achieved a level of nervous system complexity that rivals that of vertebrates. Evidence of this lies in the fact that these regulatory molecules are highly conserved across evolutionary time, which strongly suggests that their interactions are not only functional but also critical for the development and operation of the octopus nervous system³. This remarkable conservation and specialization in neural tissues implies that miRNAs may be key drivers of the sophisticated behaviors and cognitive abilities observed in cephalopods. It challenges the notion that complex nervous systems are exclusive to vertebrates, highlighting an alternative evolutionary pathway to intelligence and adaptability.

Biologists use evolution as a central framework to understand biological processes, organizing their thinking around how traits and systems have developed over time. Since the establishment of Mendel’s laws, countless paradigms have emerged, providing us with an incredible wealth of knowledge about genetics. However, discoveries like this remind us that the activity of genes is influenced by far more than their basic sequence. DNA has often been described as the "book of life," a sort of instruction manual. Yet, rather than being a straightforward list, it may be better compared to a dynamic weather system—filled with intricate feedback loops and interdependencies that drive its function.

Rather than perceiving this complexity as messy or chaotic, we should marvel at how nature uses a diverse array of genetic tools to create similar forms of intelligence across different species. These insights deepen our appreciation for the adaptability inherent to the natural world.

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References

1. Fossati, S.M., Carella, F., De Vico, G., Benfenati, F., and Zullo, L. (2013). Octopus arm regeneration: Role of acetylcholinesterase during morphological modification. J. Exp. Mar. Biol. Ecol. 447, 93–99. https://doi.org/10.1016/j.jembe.2013.02.015.

2. Pophale, A., Shimizu, K., Mano, T., Iglesias, T.L., Martin, K., Hiroi, M., Asada, K., Andaluz, P.G., Van Dinh, T.T., Meshulam, L., et al. (2023). Wake-like skin patterning and neural activity during octopus sleep. Nature 619, 129–134. https://doi.org/10.1038/s41586-023-06203-4.

3. Zolotarov, G., Fromm, B., Legnini, I., Ayoub, S., Polese, G., Maselli, V., Chabot, P.J., Vinther, J., Styfhals, R., Seuntjens, E., et al. (2022). MicroRNAs are deeply linked to the emergence of the complex octopus brain. Sci. Adv. 8, eadd9938. https://doi.org/10.1126/sciadv.add9938.

This article was copy edited by Alicia Velázquez de Castro Esteve.

Meet the author: Maite Freire Delgado