One reason I love writing about biology is that it has so many levels. Down at the molecular scale, proteins flop and twist. Higher up, cells crawl and feed and divide. They organize into animals and plants and other big organisms, which must obey their own rules in order to survive. For some organisms, a day is a lifetime. Others must weather centuries. When millions of organisms get together, they form ecosystems that wax and wane in ways that could not be predicted from lower levels. And over the course of generations, genes take on a new personality, no longer passive bits of code, but units of selection that can sweep across the planet and leap from species to species. As a science writer, I have the good fortune to be able to travel from level to level, meeting scientists who generously explain to me what they've learned about each one. They may be paleontologists digging up fish with fingers, or microbiologists studying selfless slime molds, or entomologists studying beetle horns, or anthropologists battling over a strangely shaped skull, or molecular biologists working out the wiring of genetic circuitry. I can't help noticing, though, that biologists who work at life's different levels often don't talk much to each other. I've spent some time puzzling over this, and the result is an essay I've written for the journal PLOS Computational Biology. The full text is available for free here. See you in 07.