Our planet is teeming with trees, grasses, bird and bees. It's also swarming with countless bacteria. All of it is life, and all of it reproduces to keep its kind alive. But how in the world did life begin in the first place? How did a mass of cells shift from an inert collection of organic molecules into a wiggling and sometimes even intelligent being?
The short answer is: We don't know exactly how life originated. There's the off chance that 4 billion years ago, aliens dropped off a few microbes and let them run wild. And of course, many religions have supernatural explanations for life's origins.
Many scientists think that life is a natural progression for planets that feature the necessary ingredients for biology, such as carbon, hydrogen, oxygen and other fundamental building blocks. With the right spark — say, a bolt of lightning — those cornerstones slowly develop into the cell walls and DNA that are suited to reproducible life. Researchers are continually running experiments like this in laboratories, hoping to zoom in on the formula for making life.
Try as they might, it's a mystery how those bits of non-living parts assembled themselves into actual living creatures. It could be that we are as yet still ignorant to the characteristics that truly define life. Or maybe we're blind to the principles of physics that really make life tick. No matter the case, the search for life's origins will undoubtedly continue for a long while.