Life is a complex chemical reaction. It’s what we see on the large scale when trillions of tiny chemical reactions are going on all at once – neurons firing, proteins being synthesized, sugar being broken down for energy, sensory cells lighting up when they encounter a smell or taste.
Living things, at their most basic level, are cells. The smallest living things on Earth are single-celled organisms like bacteria and amoebas. At higher levels, these individual units get joined together into larger and more complex combinations. Sponges, for example, are multi-celled organisms with a very simple body structure. At the other extreme, a redwood tree has an extremely complex body structure, with many specialized parts that each perform a specific function.
Despite being so different in their shape, size, and functions, all of these living things have a common molecular basis: they’re all based on DNA, which the cell uses to build amino acids into proteins. In all the life on earth, there are only 20 amino acids: just 20 “letters” in the alphabet of life, but they can be combined in so many different ways that they produce all the living poetry we see around us.