Around 470 million years ago, moss cells discovered something remarkable: they could divide in more than one direction. This single shift—from flat, two-dimensional growth to three-dimensional expansion—became the foundation for everything that makes Earth green. Without it, there would be no towering trees, no wildflowers dotting meadows, no shrubs sheltering wildlife. Life on land would have stayed small, simple, and utterly limited.
Now researchers at the University of Copenhagen have identified a protein that likely made this evolutionary leap possible. Named RAK1, it appears in moss species and seems to have been essential for enabling plants to grow upwards and sideways simultaneously. The discovery, published in New Phytologist, suggests that evolution sometimes doesn't invent entirely new solutions—it recombines existing tools in unexpected ways.
RAK1 is itself a fusion protein, formed when two previously separate proteins merged into one: a signaling protein called a kinase and an acetyltransferase. This combination gave moss cells a new superpower. By influencing how cells access and use energy, RAK1 allows cells to divide in multiple directions and form buds and shoots. Without it, cells get stuck.
The researchers made this clear through a simple but revealing experiment. They studied two versions of the same moss species: one with RAK1 present, one with it removed. The difference was stark. Moss cells lacking RAK1 divided poorly and formed defective buds—misshapen and unable to grow properly. With RAK1 intact, cells developed normally, creating the three-dimensional structure we recognize as plant life today.
"We observed that cells in the moss lacking RAK1 did not divide properly and formed defective buds," explains Cloe De Luxan Hernandez, a co-author of the study. "This shows that RAK1 may have been crucial for enabling the moss to grow efficiently."
This discovery reshapes how scientists understand plant evolution. Until now, researchers focused mainly on gene regulation—the idea that certain genes simply switched on and off at the right moments. But the University of Copenhagen team revealed something deeper: turning genes on and off alone is not enough. Plants also needed precise metabolic control. RAK1, it turns out, coordinates the balance of energy within cells during stem cell division and bud formation, essentially managing the internal chemistry that makes three-dimensional growth possible.
The implications ripple outward. Because moss represents some of Earth's earliest land plants, understanding how it developed three-dimensional growth illuminates a fundamental moment in planetary history. And because moss stem cells behave much like human stem cells—both depend on tightly controlled metabolism during growth and division—the discovery offers insights into how life itself manages the delicate balance required for development.
Associate Professor Eleazar Rodriguez, another author on the study, points to this broader significance: "Without the ability to grow in three dimensions, the landscape would look very different. We would not see trees and shrubs grow the way they do today. Life on land would likely have remained much more limited." The RAK1 protein, hidden inside the cells of ancient moss for hundreds of millions of years, quietly enabled everything that followed.