Cell Nucleus Structure

DNA chromosomes in a cell nucleus, in organisms that have a cell nucleus, are organized in one of two main ways (which have further minor variations within each type): neatly folded and crumpled into little balls. 

This is determined by a protein known as condensin II. Tweak this single switch and the chromosome organization flips. A given species generally does so one way or the other.

Many lines of evolution have flipped back and over between these configurations over their evolutionary history.

The story of this recent major discovery is told here. 

The primary paper related to the discovery (with a great many authors) is: 

Claire Hoencamp, et al., "3D genomics across the tree of life reveals condensin II as a determinant of architecture type." 372 (6545) Science 984 (2021). DOI: 10.1126/science.abe2218