The Advantages of Staying Together
Multicellularity evolved independently many times — in animals, plants, fungi, brown algae and elsewhere — which tells you the advantages are real and reachable by several routes.
The gains are size, specialisation and homeostasis. A single cell must do everything itself; a multicellular body can divide the labour, with each cell type doing one job well. It also escapes the surface-area-to-volume ceiling, since a large body built from small cells keeps every cell's own ratio favourable.
The costs are equally real. Cells must cooperate, which means suppressing their own reproduction for the good of the whole, and a lineage that stops cooperating is a cancer. Development must be coordinated, and only the germ line passes to the next generation, so most cells in a body are evolutionary dead ends.
Development starts from one cell and proceeds by three processes acting together:
| Process | What happens |
|---|---|
| Cell division | One cell becomes many |
| Differentiation | Cells become different types |
| Morphogenesis | Cells arrange into shapes |
Differentiation is the striking one: nearly every cell in a body carries the same genome, and they differ because they express different subsets of it. A liver cell and a neuron are running different programs off identical code.
Stem cells retain the ability to divide and to become other types. Totipotent cells can form a whole organism, pluripotent ones any body cell, and multipotent ones a limited family. Adult tissues keep multipotent stem cells for renewal, which is why skin and gut lining replace themselves continuously while neurons largely do not.
Growth from one cell is exponential: after divisions there are cells, so about 10 divisions give a thousand and 20 give a million. Reaching the roughly cells of an adult human needs only around 43 doublings.
Common pitfall: thinking differentiated cells have lost the genes they no longer use. They have not, the genome stays essentially complete, which is exactly why cloning from an adult cell works and why induced pluripotent stem cells can be made by switching expression back on.