Earth's first living cell appeared nearly four billion years ago. We call it LUCA, the last universal common ancestor. We do not know much about LUCA, but every plant, every animal, and every one of us humans evolved from LUCA. We know this thanks to studies done with special proteins, which are large, complex molecules that play many critical roles in the body. Proteins do most of the work in cells and are required for the structure, function, and regulation of the body's tissues and organs contained in the cells of LUCA's descendants. Proteins are indispensable for every living being, as they are the molecules that do the things that keep everything alive. Every biological process necessary for life, like respiration, digestion, or reproduction, involves proteins in some way. And while many of these processes and their corresponding proteins vary between organisms—for example, the respiratory proteins of fishes are very different from those of elephants—some proteins are so important that everyone has to produce them. One of these crucial functions for life is the mechanism used to produce proteins, which is carried out by a bunch of proteins coupled with other molecules and collectively called the ribosome.

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