Simple single-celled organisms appeared across the planet approximately 3.5 billion years ago. By around 2.5 billion years ago, they’d increased in size and developed sophisticated internal structures that improved their ability to interact with their world. The transition set the scene for the proliferation of millions of complex life forms, including birds, beetles and baby’s breath. It constitutes one of the most important moments of the planet’s past, and yet, its particulars continue to stump scientists.