This chapter describes the nested structures found inside a human or any other complex organism, comparable to Russian dolls. As we delve deeper into this structure, we also go further back in time. All animals, and also plants and many fungi, are multi-cellular, made up of trillions of cells that act in supremely close cooperation, mostly using chemical signalling mechanisms to coordinate their actions. The adoption of the multi-cellular habit has enabled a huge increase in the complexity of size and form of living things. The three kingdoms of plants, animals, and fungi, which have raised multi-cellularity to a high art form, and which contain virtually all the living things that can actually be seen with the unaided eye, all appear in force in the fossil record after the most recent Snowball Earth event.
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