Scientists have discovered that simple sea slugs have nervous systems organized in much the same way as human brains, with similar modules for making decisions and controlling movement. Even though sea slugs lack bones, brains, and complex bodies compared to humans, their basic neural architecture mirrors ours, suggesting that this organizational plan evolved long ago in simple ancestral organisms. This finding helps us understand how complex brains evolved and reveals that nature has reused the same fundamental neural designs across hundreds of millions of years of evolution.