Phylogeny of cortical-hippocampal projections reveals selective elimination of input from sensory regions
This study reveals that across six mammalian species spanning over 100 million years of evolution, direct unimodal and primary sensory inputs to the hippocampus were selectively eliminated as brain size increased, suggesting that hippocampal processing and cognition differ fundamentally across species due to these distinct anatomical trajectories.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Imagine your brain as a bustling, ancient city where different neighborhoods specialize in different jobs. Some districts are like raw material factories, taking in direct signals from your eyes, ears, and skin—these are the "sensory" zones. Other districts are like high-level command centers, where information from all those factories is mixed, matched, and turned into complex ideas, memories, and plans—these are the "transmodal" zones. In the middle of this city sits a tiny, crucial hub called the hippocampus. Think of it as the city's master librarian and archivist. Its job is to take the information flowing through the city, organize it, and store it so you can remember where you left your keys or recall a specific day from your childhood. For a long time, scientists wondered if this librarian's job description changed as the city itself grew from a small village into a sprawling metropolis. Did the librarian start receiving raw materials directly from the factories, or did they start relying only on the reports sent by the command centers? Understanding this helps us figure out why a rat might remember a maze differently than a human remembers a story, and how our brains evolved to handle such complex thoughts.
This paper takes a fascinating trip back in time, looking at six different mammalian species to see how the "roads" connecting the city's neighborhoods to the librarian's hub have changed over more than 100 million years of evolution. The researchers looked at the tenrec (a small, shrew-like creature), the rat, the cat, the marmoset (a tiny monkey), the macaque (a larger monkey), and humans. They wanted to see if the librarian in a small-brained animal gets direct deliveries from the sensory factories, and if that changes as the brain gets bigger.
The findings reveal a dramatic shift in how the brain is wired as it grows. In the smallest animals, like the tenrec and the rat, it's like the librarian has a direct phone line to almost every single factory in the city. Nearly the entire cortex (the outer layer of the brain) sends information straight to the hippocampus. However, as we move to larger animals like cats and monkeys, and finally to humans, something interesting happens: the direct lines from the sensory factories start to get cut. The paper suggests that as the brain gets bigger, it selectively eliminates the direct connections from the primary sensory areas (the raw data) and even the secondary sensory areas (the slightly processed data).
Instead of losing all connections, the librarian keeps the direct lines open only to the high-level command centers—the transmodal regions. In humans, the hippocampus is almost completely disconnected from the raw sensory inputs. It no longer receives direct reports from the visual or auditory factories. Instead, it only talks to the areas that have already done the heavy lifting of combining and interpreting that information. The authors suggest this isn't just a random pruning; it's a specific evolutionary trend. They found that the brain gets rid of the direct link to primary sensory areas first, and only later does it cut the link to the non-primary sensory areas. The only exception to this rule is the sense of smell (the piriform cortex), which seems to keep a direct line to the hippocampus even in humans, likely because it is physically right next door to the librarian's office.
So, what does this mean for how we think? The paper suggests that in small-brained animals, memory might be tightly tied to immediate, raw sensory details—like the exact texture of a surface or the specific frequency of a sound. But in humans, because the hippocampus is fed only by the high-level command centers, our memory operates on a different level. We aren't just storing raw sensory data; we are storing abstract, processed concepts. The authors propose that this evolutionary shift allowed humans to move from simple, sensory-based memory to complex, abstract thinking, like imagining the future or constructing detailed stories about the past. While the study doesn't prove exactly how this changes behavior in every single species, it strongly suggests that the "type of information" the hippocampus works with has fundamentally changed as our brains grew larger, turning the librarian from a raw data clerk into a master of abstract storytelling.
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