The fossil record makes it clear that organisms started out as single-celled entities and developed into multicelled organisms as time passed. It's not hard to imagine the evolutionary pathway that led from one condition to the other:
1. A single-celled organism divided into two daughter cells.
2. Each daughter cell divided into two daughter cells.
3. Each of those daughter cells divided, and so on and so forth, until a whole bunch of single-celled organisms were going happily about their business.
4. One day, a mutation occurred, resulting in two daughter cells remaining attached instead of separating after cell division.
These two still-attached cells had a couple of advantages over individual cells — maybe they were harder to eat (because they were bigger), and maybe they had more access to food sources — so this particular mutation increased in frequency in the population.
This example seems fairly simple, but making a multicelled organism isn't as easy as assembling a collection of identical cells. In larger organisms, different cells have to become specialized to perform different functions. The cells in your own body, for example, are remarkably good at sharing resources, yet few of them do any of the reproducing; that task is left entirely to the tissues that produce eggs and sperm. Yet the specialization that makes complex functioning possible can also lead to conflicts between different cell lineages.
Cells can be selfish in a couple of ways:
i By not performing their appropriate functions: If all the cells want to be gonads, and none of them wants to be a thighbone, the community (the multicelled organism) breaks down. The only chance that one of your nose-hair cells has of getting its genes into the next generation is to play nice, do its job, and not suck up any more resources than necessary.
i By trying to hog all the resources: Every once in a while, a mutation in a cell can result in that cell's breaking ranks and going all out for the resources. Need a recognizable word for this phenomenon? Try cancer.
Part of the evolution of multicellularity — organisms growing from 4 cells to 8 cells to 16 cells to 32 to eventually billions of cells — seems to be the natural selection of traits that keep selfish cells in check. For the most part, things have gone swimmingly. Organisms have evolved from single-celled creatures to multicelled creatures of amazing complexity and variety that can, for example, write or read a book about evolution.
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