[outworlder]

> A really important fact about multicellular life is that each cell is genetically identical to any other cell in the organism.

Not necessarily. https://en.wikipedia.org/wiki/Chimera_%28genetics%29#Humans

Our cells DNA also drift over time due to localized mutations. What matters is that they should keep obeying all the chemical signalling.

> Otherwise, different cells will start competing with one another, until the relationship falls apart.

When that happens in complex multicellular organisms, it's usually called "cancer".