Mice grow bigger brains when given a stretch of human DNA

Scientists discovered a special human DNA “switch” that made mouse brains grow bigger. When this tiny piece of human DNA (called HARE5) was added to mouse embryos, the mice ended up with brains about 6.5% larger than normal by adulthood. How did it happen? The human DNA acted like a dial turning up a gene that helps build the brain. This dial caused the mice to produce more of the neuron-building cells (neural stem cells) that create brain neurons. In short, a little human DNA gave mice a slightly bigger brain boost: a clue to what makes human brains special.

A tiny DNA switch boosts brain growth

Think of HARE5 as a tiny genetic switch. It doesn’t code for a protein like typical genes; instead, it turns other genes on (that’s why scientists call it an enhancer). In mice, HARE5 normally cranks up a gene that helps neural cells grow. But the human version of HARE5 is extra effective; it’s like the volume knob goes higher. When researchers swapped in the human HARE5 for the mouse’s own version, the mouse’s brain grew larger than usual. This growth was most pronounced in the outer layer of the brain (the cortex), which got thicker because there were more neurons than normal. Why more neurons? The human HARE5 made the mouse’s neural stem cells (called radial glia) divide more and produce extra neurons compared to mice with mouse HARE5. It’s as if the human DNA told the mouse’s brain “build more brain cells!”

Importantly, the mice were healthy: just with a bit bigger brains. Scientists don’t yet know if these mice are any smarter or have better memory. (Having a bigger brain doesn’t always mean big changes in behavior.) Testing the mice’s learning and problem-solving in future studies will be exciting. Still, simply seeing a mouse grow a larger brain due to one human DNA switch is amazing. It’s direct evidence that small DNA tweaks can make a noticeable difference in brain development.

Clues to how our brains got so big

Human brains are much larger and more complex than those of our primate cousins. In fact, our cortex is about three times bigger than a chimpanzee’s. How did our ancestors’ brains grow so large since we split from chimps millions of years ago? Scientists are still piecing together the answer. The HARE5 findings give one piece of the puzzle: human-specific DNA changes can boost brain growth. HARE5 belongs to a set of DNA switches called human accelerated regions (HARs). HARs are stretches of genome that stayed almost the same in many mammals but changed rapidly in humans. That makes scientists think HARs might hold secrets to what makes us uniquely human.

Earlier research had already hinted at the power of these HAR switches. For example, a 2021 study screened hundreds of HARs for enhancer activity in human brain development, and found many of them active in building our cortex. Another study in 2025 mapped out how HAR DNA interacts with other genes during brain growth in humans versus chimps because insight into which genes they control was lacking. Those studies suggested HARs are key players, but HARE5’s experiment shows it in action: tweaking a single HAR can make a brain grow bigger. It’s a dramatic demonstration of evolution’s little tricks. Over time, tiny DNA changes add up. Each human is born with ~100–200 brand new mutations not seen in their parents, and some changes (like in HARs) might have helped our brains expand. Even our ability to use language might trace back to such changes; chimpanzees, for instance, show surprisingly complex communication (mixing calls in grammar-like ways), hinting that human language and big brains have deep evolutionary roots.

From evolution to modern health with SlothMD

Discoveries like HARE5 highlight how biology and evolution shape who we are. Understanding these genetic switches could one day help with brain health, perhaps shedding light on developmental brain disorders or inspiring new therapies. And while we marvel at how evolution crafted our big brains, we’re also using those brains to improve life today. Science isn’t just about the past; it’s also about innovation now. For example, healthcare is getting smarter with technology. Health AI is transforming healthcare by helping doctors and powering personal health apps. (SlothMD loves to explore how smart computers are changing healthcare and making life better for everyone!) By studying the secrets of our DNA and leveraging modern tech, we get a fuller picture of health and evolution together.