Ed Yong in Nautilus:
At first glance, a tree could not be more different from the caterpillars that eat its leaves, the mushrooms sprouting from its bark, the grass growing by its trunk, or the humans canoodling under its shade. Appearances, however, can be deceiving. Zoom in closely, and you will see that these organisms are all surprisingly similar at a microscopic level. Specifically, they all consist of cells that share the same basic architecture.
These cells contain a central nucleus—a command center that is stuffed with DNA and walled off by a membrane. Surrounding it are many smaller compartments that act like tiny organs, carrying out specialized tasks like storing molecules or making proteins. Among these are the mitochondria—bean-shaped power plants that provide the cells with energy.
This combination of features is shared by almost every cell in every animal, plant, fungus, and alga, a group of organisms known as “eukaryotes.”
Bacteria showcase a second, simpler way of building a cell—one that preceded the complex eukaryotes by at least a billion years. These “prokaryotes” always consist of a single cell, which is smaller than a typical eukaryotic one and bereft of internal compartments like mitochondria and a nucleus. Even though limited to a relatively simple cell, bacteria are impressive survival machines. They colonize every possible habitat, from miles-high clouds to the deep ocean. They have a dazzling array of biological tricks that allow them to cause diseases, eat crude oil, conduct electric currents, draw power from the Sun, and communicate with each other.
Still, without the eukaryotic architecture, bacteria are forever constrained in size and complexity. Sure, they have their amazing skill sets, but it’s the eukaryotes that cover the Earth in forest and grassland, that navigate the planet looking for food and mates, that build rockets to Mars.
The transition from the classic prokaryotic model to the deluxe eukaryotic one is arguably the most important event in the history of life on Earth. And in more than 3 billion years of existence, it happened exactly once.